Download PHY_101_NOTE_-REVISED

P HY 1 01 - M ec h a n i c s P HY 1 01 – M e c h a n i c s P H Y 1 01 – M ec h a n i c s P HY 1 0 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 – M e c h a n i c s P HY 10 1 - M ec h a n i c s
P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P H Y 1 01 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
PHY 101 – GENERAL PHYSICS I
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h(MECHANICS)
a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 - M e c h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n2015/16
i c s P SESSION
HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – Ajiboye,
M ec hY.,
a nBadmus
i c s P HYO.G.,10
1 O.A
– M ec h a n i c s P HY 10 1 –
& Alao
M ec h a n i c s P HY 1 0 1 - M ec h a n i c s P HY 101 - M ec h a n i c s P HY 1 0 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
M ec h a n i c s P HY 1 0 1 – Mec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – Mec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – Mec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 10 1 – M ec h a n i c s P HY 10 1 –
M ec h a n i c s P HY 1 0 1 – M ec h a n i c s P HY 1 01 - M ec h a n i c s P HY 1 01 –
Course Outline
• Measurement in Physics
• Space and Time
• Units and Dimension
• Kinematics
• Fundamental Laws of Mechanics
• Statics and Dynamics
• Work and Energy
• Conservation laws
WORK PLAN
Week 1
Week 2
Week 3
Week 4
Week 5
Week 6
Week 7
Week 8
Week 9
Week 10
Week 11
Week 13
Week 14
Week 15
REVISION
Page 1 of 43
1.0
MEASUREMENT
Physics is a science based upon exact measurement of physical quantities. Therefore
it is essential that student first becomes familiar with the various methods of
measurement and the units in which these measurements are expressed.
A unit is a value quantity or magnitude in terms of which other values, quantities or
magnitudes are expressed.
1.1
FUNDAMENTAL QUANTITIES AND UNITS
A fundamental quantity also known as base quantity is a quantity which cannot
be expressed in terms of any other physical quantity. The units in which the
fundamental quantities are measured are called fundamental units. In mechanics
(study of the effects of external forces on bodies at rest or in motion), the quantities
length, mass and time are chosen as fundamental quantities.
Fundamental Quantity
Length
Mass
Time
1.2
Fundamental Unit
Meter
Kilogram
Second
Unit Symbol
m
kg
S
SYSTEM OF UNITS
The following systems of units have been in use –
(i)
The French or C.G.S (Centimeter, Gramme, Second) System;
(ii) The British or F.P.S (Foot, Pound, Second) System;
(iii) The M.K.S (Metre, Kilogram, Second) System; and
(iv) The S.I. (International System of Units).
1.3
THE INTERNATIONAL SYSTEM OF UNITS (S.I.)
The S.I. is the latest version of the system of units and only system likely to be used
all over the world. This system consists of seven base or fundamental units from
which we can derive other possible quantities of science. They are
S.No Physical Quantity
Unit
Unit Symbol
1.
Length
Meter
M
2.
Mass
Kilogram
Kg
3.
Time
Second
S
Page 2 of 43
4.
Electric Current
Ampere
A
5.
6.
Temperature
Amount of
Substance
Luminous Intensity
Kelvin
Mole
K
mol
Candela
Cd
7.
1.4
CONCEPT OF DIMENSION
Dimension of a physical quantity simply indicates the physical quantities which
appear in that quantity and gives absolutely no idea about the magnitude of the
quantity. In mechanics the length, mass and time are taken as the three base
dimensions and are expressed by as letter [L], [M] and [T] respectively. Hence, a
formula which indicates the relation between the derived unit and the fundamental
units is called dimensional formula.
Example 1: Deduce the dimensional formula for the following physical quantities:
(a) Velocity, (b) acceleration, (c) force, (d) pressure, (e) work, and (f) power.
Solution:
(𝑎) 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =
[𝐿 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑙𝑒𝑛𝑔𝑡ℎ
=
= [𝐿𝑇 −1 ]
[𝑇 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑡𝑖𝑚𝑒
𝐻𝑒𝑛𝑐𝑒 𝑡ℎ𝑒 𝑑𝑖𝑚𝑛𝑒𝑠𝑖𝑜𝑛𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎 𝑓𝑜𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑤𝑖𝑙𝑙 𝑏𝑒 [𝑀0 𝐿𝑇 −1 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 [𝐿𝑇 −1 ]
(𝑏) 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =
=
= [𝐿𝑇 −2 ]
[𝑇 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑡𝑖𝑚𝑒
= [𝑀0 𝐿𝑇 −2 ]
(𝑐 ) 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒 = 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑚𝑎𝑠𝑠 × 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
= [𝑀] × [𝐿𝑇 −2 ] = [𝑀𝐿𝑇 −2 ]
(𝑑 ) 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐹𝑜𝑟𝑐𝑒 [𝑀𝐿𝑇 −2 ]
=
= [𝑀𝐿−1 𝑇 −2 ]
2
[
]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐴𝑟𝑒𝑎
𝐿
Students to attempt (𝑒) and(𝑓).
Example 2: Deduce the dimensional formula for (a) modulus of elasticity (Ƴ), and
(b) coefficient of viscosity(𝜂 ).
Page 3 of 43
Solution:
𝑆𝑡𝑟𝑒𝑠𝑠
𝐹𝑜𝑟𝑐𝑒⁄𝐴𝑟𝑒𝑎
(𝑎 ) Ƴ =
=
𝑆𝑡𝑟𝑎𝑖𝑛 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑙𝑒𝑛𝑔𝑡ℎ⁄𝑂𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
[𝑀𝐿𝑇 −2 ] × [𝐿]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒 × 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑙𝑒𝑛𝑔𝑡ℎ
=
=
[𝐿2 ] × [𝐿]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐴𝑟𝑒𝑎 × 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑙𝑒𝑛𝑔𝑡ℎ
= [𝑀𝐿−1 𝑇 −2 ]
(𝑏) 𝑡ℎ𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 (𝜂 ) 𝑜𝑓 𝑎 𝑙𝑖𝑞𝑢𝑖𝑑 𝑖𝑠 𝑑𝑒𝑓𝑖𝑛𝑒𝑑 𝑎𝑠 𝑡𝑎𝑛𝑔𝑒𝑛𝑡𝑖𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑝𝑒𝑟 𝑢𝑛
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑝𝑎𝑟𝑡.
𝐹
1
𝜂=
𝐴 (𝑑𝑉 ⁄𝑑𝑥 )
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐹𝑜𝑟𝑐𝑒 × 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝜂 =
𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐴𝑟𝑒𝑎 × 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦
[𝑀𝐿𝑇 −2 ] × [𝐿] [𝑀𝐿2 𝑇 −2 ]
= 2
=
= [𝑀𝐿−1 𝑇 −1 ]
[𝐿 ] × [𝐿𝑇 −1 ]
[𝐿3 𝑇 −1 ]
1.4
USES OF DIMENSIONAL EQUATIONS
(a)
To check the homogeneity of a derived physical equation (i.e. to check the
correctness of a physical equation).
Example 3:
Show that the following relation for the time period of a body executing
simple harmonic motion is correct.
𝑇 = 2𝜋√
𝑙
𝑔
where𝑙 and 𝑔 are the displacement and acceleration due to gravity
respectively.
Solution:
For the relation to be correct, the dimension of L.H.S must be equal to
dimension of R.H.S
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐿. 𝐻. 𝑆 = 𝑀0 𝐿0 𝑇1 𝑜𝑟 𝑇
[𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑙]1⁄2
[𝐿]1⁄2
[𝐿]1⁄2
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑅. 𝐻. 𝑆 =
=
=
[𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑔]1⁄2 [𝐿𝑇 −2 ]1⁄2 [𝐿]1⁄2 𝑇 −1
1
= −1 = 𝑇
𝑇
Since 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝐿. 𝐻. 𝑆 = 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑅. 𝐻. 𝑆, the relation is
dimensionally correct.
Page 4 of 43
(b)
To derive a relationship between different physical quantities.
Example 4:
If the frequency 𝑓 of a stretched string depends upon the length𝑙 of the string,
the tension 𝑇 in the string and the mass per unit length 𝜆 of the string.
Establish a relation for the frequency using the concept of dimension.
Solution:
𝑓 ∝ 𝑙 𝑥 𝑇 𝑦 𝜆𝑧
…
(1)
𝑥 𝑦 𝑧
𝑓 = 𝑘𝑙 𝑇 𝜆
… (2)
[𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑓] = [𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑙]𝑥 × [𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑇]𝑦 × [𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝜆]𝑧
[𝑀0 𝐿0 𝑇 −1 ] = [𝐿]𝑥 × [𝑀𝐿𝑇 −2 ]𝑦 × [𝑀𝐿−1 ]𝑧
[𝑀]0 [𝐿]0 [𝑇]−1 = [𝑀]𝑦+𝑧 [𝐿]𝑥+𝑦−𝑧 [𝑇]−2𝑦
𝐸𝑞𝑢𝑎𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑝𝑜𝑤𝑒𝑟𝑠
(3)
𝑦+𝑧 =0 …
𝑥+𝑦−𝑧 =0 …
(4)
−2𝑦 = −1 …
(5)
1
(6)
𝑓𝑟𝑜𝑚 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 (5),
𝑦=
…
2
𝑠𝑢𝑏𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑛𝑔 (6)𝑖𝑛𝑡𝑜 (3), 𝑤𝑒 ℎ𝑎𝑣𝑒
1
+𝑧 =0
2
1
𝑧=−
…
(7)
2
𝑠𝑢𝑏𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑛𝑔 (6) 𝑎𝑛𝑑 (7) 𝑖𝑛𝑡𝑜 (4)
1
1
𝑥 + − (− ) = 0
2
2
𝑥 = −1 …
(8)
𝑠𝑢𝑏𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑛𝑔 (6), (7) 𝑎𝑛𝑑 (8)𝑖𝑛𝑡𝑜 (2), 𝑤𝑒 ℎ𝑎𝑣𝑒
𝑓 = 𝑘𝑙 −1 𝑇 1⁄2 𝜆−1⁄2
1 𝑇
𝑓=𝑘 √
𝑙 𝜆
… (∗)
Equation (∗) is the required relation.
(c)
To derive the unit of a Physical Quantity
Page 5 of 43
Example 5:
The viscous drag F between two layers of liquid with surface area of contact A
in a region of velocity gradient 𝑑𝑣 ⁄𝑑𝑥 is given by
𝐹 = 𝜂𝐴 𝑑𝑣 ⁄𝑑𝑥
where𝜂 is the coefficient of viscosity of the liquid. Obtain the unit for 𝜂.
Solution:
𝐹 = 𝜂𝐴 𝑑𝑣 ⁄𝑑𝑥
𝑀𝑎𝑘𝑖𝑛𝑔 𝜂 𝑡ℎ𝑒 𝑠𝑢𝑏𝑗𝑒𝑐𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑜𝑟𝑚𝑢𝑙𝑎
𝐹 1
𝜂=
𝐴 𝑑𝑣 ⁄𝑑𝑥
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒 = [𝑀𝐿𝑇 −2 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 = [𝐿2 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 = [𝐿𝑇 −1 ]
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = [𝐿]
[𝑀𝐿𝑇 −2 ][𝐿] 𝑀𝐿2 𝑇 −2
𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 𝑜𝑓 𝜂 = 2
= 3 −1 = 𝑀𝐿−1 𝑇 −1
[𝐿 ][𝐿𝑇 −1 ]
𝐿𝑇
−1 −1
∴ 𝑈𝑛𝑖𝑡 𝑜𝑓 𝜂 = 𝑘𝑔𝑚 𝑠
1.5
LIMITATIONS OF DIMENSIONAL ANALYSIS
(i)
(ii)
(iii)
The method does not provide any information about the magnitude of
dimensionless variables and dimensionless constants.
The method cannot be used if the quantities depend upon more than
three-dimensional quantities: M,L and T.
The method is not applicable if the relationship involves trigonometric,
exponential and logarithmic functions.
EXERCISE
1) Show on the basis of dimensional analysis that the following relations are
correct:
a. 𝑣 2 − 𝑢2 = 2𝑎𝑆, where 𝑢 is the initial velocity, 𝑣 is final velocity, 𝑎 is
acceleration of the body and 𝑆 is the distance moved.
b. 𝜌 = 3𝑔⁄4𝑟𝐺 where𝜌 is the density of earth, 𝐺 is the gravitational
constant, 𝑟 is the radius of the earth and 𝑔 is acceleration due to
gravity.
2) The speed of sound 𝑣 in a medium depends on its wavelength 𝜆, the young
modulus𝐸, and the density 𝜌, of the medium. Use the method of dimensional
analysis to derive a formula for the speed of sound in a medium. (Unit for
Young Modulus 𝐸: 𝑘𝑔𝑚−1 𝑠 −2 )
Page 6 of 43
2.0
VECTORS
Physical quantities can generally be classified as (i) Scalars and (ii) Vectors.
Scalars are physical quantities which possess only magnitude and no direction in
space. Examples are mass, time, temperature, volume, speed etc. On the other hand,
Vectors are physical quantities which have both magnitude and direction in space.
Examples are force, velocity, acceleration, etc.
2.1
RESOLUTION OF VECTORS
A two dimensional vector can be represented as the sum of two vectors. Consider
figure A above, the vector 𝐴⃗ can be expressed as
𝐴⃗ = 𝐴⃗𝑥 + 𝐴⃗𝑦 𝑜𝑟 𝐴⃗ = 𝐴𝑥 𝑖̂ + 𝐴𝑦 𝑗̂
…
2.1
Vectors 𝐴⃗𝑥 and 𝐴⃗𝑦 are called vector components of 𝐴⃗.𝑖̂and𝑗̂ are unit vectors along the
𝑥 axis and 𝑦 axis respectively. A unit vector is a vector that has a magnitude of
exactly 1 and specify a particular direction.
Let 𝜃 be the angle which the vector 𝐴⃗ makes with the positive 𝑥-axis, then we have
𝐴𝑥 = 𝐴 cos 𝜃 𝑎𝑛𝑑 𝐴𝑦 = 𝐴 sin 𝜃
…
𝑀𝑎𝑔𝑛𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑣𝑒𝑐𝑡𝑜𝑟 𝐴 = |𝐴⃗| = 𝐴 = √𝐴2𝑥 + 𝐴𝑦2
tan 𝜃 =
𝐴𝑦
𝐴𝑥
…
2.2
…
2.3
2.4
In three dimensions, a vector 𝐴⃗can be expressed as
Page 7 of 43
𝐴⃗ = 𝐴⃗𝑥 + 𝐴⃗𝑦 + 𝐴⃗𝑧 𝑜𝑟 𝐴⃗ = 𝐴𝑥 𝑖̂ + 𝐴𝑦 𝑗̂ + 𝐴𝑧 𝑘̂
Here, the magnitude is expressed as
|𝐴⃗| = 𝐴 = √𝐴𝑥2 + 𝐴2𝑦 + 𝐴𝑧2
2.2
…
…
2.5
2.6
ADDITION/SUBTRACTION OF VECTORS BY COMPONENTS
Example:
⃗⃗ = 𝑖̂ + 4𝑗̂ + 5𝑘̂ . Find the magnitude of (𝐴⃗ + 𝐵
⃗⃗)and (𝐴⃗ − 𝐵
⃗⃗).
If 𝐴⃗ = 2𝑖̂ + 3𝑗̂ + 4𝑘̂ and 𝐵
Solution:
⃗⃗) = (2𝑖̂ + 3𝑗̂ + 4𝑘̂ ) + (𝑖̂ + 4𝑗̂ + 5𝑘̂ ) = 3𝑖̂ + 7𝑗̂ + 9𝑘̂
(𝐴⃗ + 𝐵
⃗⃗) = (2𝑖̂ + 3𝑗̂ + 4𝑘̂ ) − (𝑖̂ + 4𝑗̂ + 5𝑘̂ ) = 𝑖̂ − 𝑗̂ − 𝑘̂
(𝐴⃗ − 𝐵
Hence their magnitudes will be
⃗⃗| = √(3)2 + (7)2 + (9)2 = √139
|𝐴⃗ + 𝐵
⃗⃗| = √(1)2 + (−1)2 + (−1)2 = √3
|𝐴⃗ − 𝐵
2.3
MULTIPLICATION OF VECTORS
(i)
Dot Product or Scalar Product
The scalar product of two vectors is defined as the product of the magnitude
of two vectors and the cosine of the smaller angle between them.
⃗⃗ = |𝐴⃗||𝐵
⃗⃗| cos 𝜃 = 𝐴𝐵 cos 𝜃 = 𝑆
𝐴⃗. 𝐵
where𝑆 is a scalar quantity.
Example:
⃗⃗ = 3𝑖̂ + 4𝑗̂ −
Find the angle between the two vectors 𝐴⃗ = 3𝑖̂ + 4𝑗̂ + 5𝑘̂ and 𝐵
5𝑘̂ .
Solution:
⃗⃗ = |𝐴⃗||𝐵
⃗⃗| cos 𝜃
𝐴⃗. 𝐵
⃗⃗ = 𝐴𝑥 𝐵𝑥 + 𝐴𝑦 𝐵𝑦 + 𝐴𝑧 𝐵𝑧 = 3 × 3 + 4 × 4 + 5(−5) = 0
𝐴⃗. 𝐵
|𝐴⃗| = √32 + 42 + 52 = √50
⃗⃗| = √32 + 42 + (−5)2 = √50
|𝐵
⃗⃗
𝐴⃗. 𝐵
0
cos 𝜃 =
=
=0
⃗⃗| √50 × √50
|𝐴⃗||𝐵
𝜃 = 90𝑜
Page 8 of 43
(ii)
Cross Product or Vector product
The vector product of two vectors is defined as a vector having a magnitude
equal to the product of the magnitudes of the two vectors and the sine of the
angle between them and is in the direction perpendicular to the plane
⃗⃗ are two vectors, then their vector
containing the two vectors. Thus if 𝐴⃗ and 𝐵
⃗⃗, is a vector 𝐶⃗ defined as
product (or cross product), written as 𝐴⃗ × 𝐵
⃗⃗ = |𝐴⃗||𝐵
⃗⃗|𝑠𝑖𝑛𝜃𝑛
𝐶⃗ = 𝐴⃗ × 𝐵
where𝜃 is the angle between the two vectors and 𝑛 is the unit vector
⃗⃗.
perpendicular to the plane of vectors 𝐴⃗ and 𝐵
Page 9 of 43
3.0
KINEMATICS
Kinematics is the study of the motion of objects without referring to what causes the
motion. Motion is a change in position in a time interval.
3.1
MOTION IN ONE DIMENSION/MOTION ALONG A STRAIGHT LINE
A straight line motion or one-dimensional motion could either be vertical (like that
of a falling body), horizontal, or slanted, but it must be straight.
3.1.1 POSITION AND DISPLACEMENT
The position of an object in space is its location relative to some reference point,
often the origin (or zero point). For example, a particle might be located at 𝑥 = 5𝑚,
which means the position of the particle is 5𝑚 in positive direction from the origin.
Meanwhile, the displacement is the change from one position 𝑥1 to another
position 𝑥2 . It is given as
∆𝑥 = 𝑥2 − 𝑥1
…
(3.1)
3.1.2 AVERAGE VELOCITY AND AVERAGE SPEED
The average velocity is the ratio of the displacement ∆𝑥 that occurs during a
particular time interval ∆𝑡 to that interval. It is a vector quantity and is expressed as:
∆𝑥 𝑥2 − 𝑥1
𝑣𝑎𝑣𝑔 =
=
….
(3.2)
∆𝑡
𝑡2 − 𝑡1
𝑜𝑟
∆𝑦 𝑦2 − 𝑦1
𝑣𝑎𝑣𝑔 =
=
….
(3.2)
∆𝑡
𝑡2 − 𝑡1
The average speed is the ratio of the total distance covered by a particle to the time.
It is a scalar and is expressed as:
𝑡𝑜𝑡𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑆𝑎𝑣𝑔 =
… (3.3)
∆𝑡
3.1.3 INSTANTANEOUS VELOCITY AND SPEED
The instantaneous velocity describes how fast a particle is moving at a given instant.
It is expressed as:
∆𝑥 𝑑𝑥
𝑣 = lim
=
…
(3.4)
∆𝑡→0 ∆𝑡
𝑑𝑡
Page 10 of 43
Speed is the magnitude of instantaneous velocity; that is, speed is velocity that has
no indication of direction either in words or via an algebraic sign. For example, a
velocity of +5𝑚/𝑠 or −5𝑚/𝑠 is associated with a speed of 5𝑚/𝑠.
Example 6:
The position of a particle moving on an 𝑥 axis is given by 𝑥 = 7.8 + 9.2𝑡 − 2.1𝑡 3 ,
with 𝑥 in meters and 𝑡 in seconds. What is its velocity at 𝑡 = 3.5𝑠? Is the velocity
constant, or is it continuously changing?
Solution:
𝑑𝑥 𝑑 (7.8 + 9.2𝑡 − 2.1𝑡 3 )
𝑣=
=
= 0 + 9.2 − (3)(2.1)𝑡 2
𝑑𝑡
𝑑𝑡
𝑣 = 9.2 − 6.3𝑡 2 (∗)
At 𝑡 = 3.5𝑠,
68𝑚
𝑠
𝑠𝑖𝑛𝑐𝑒 𝑒𝑞𝑛 (∗) 𝑖𝑛𝑣𝑜𝑙𝑣𝑒𝑠 𝑡, 𝑡ℎ𝑒 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑣 𝑑𝑒𝑝𝑒𝑛𝑑𝑠 𝑜𝑛 𝑡𝑖𝑚𝑒 (𝑡 )𝑎𝑛𝑑 𝑠𝑜 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠𝑙𝑦 𝑐ℎ𝑎𝑛𝑔𝑖𝑛𝑔.
𝑣 = 9.2 − 6.3(3.5)2 = −
3.1.4 ACCELERATION
When a particle’s velocity changes, the particle is said to undergo acceleration. The
average acceleration𝑎𝑎𝑣𝑔 over a time interval ∆𝑡 is
∆𝑣 𝑣2 − 𝑣1
𝑎𝑎𝑣𝑔 =
=
…
3.5
∆𝑡
𝑡2 − 𝑡1
where the particle has velocity 𝑣1 at time 𝑡1 and then velocity 𝑣2 at time 𝑡2 . The
instantaneous acceleration (or simply acceleration) is the derivative of velocity
with respect to time:
𝑑𝑣
𝑎=
… 3.6
𝑑𝑡
Equation 3.6 can also be written as
𝑑
𝑑 𝑑𝑥
𝑑2 𝑥
(
)
(
)
𝑎=
𝑣 =
= 2 …
3.7
𝑑𝑡
𝑑𝑡 𝑑𝑡
𝑑𝑡
In words, the acceleration of a particle at any instant is the second derivative of its
position 𝑥(𝑡) with respect to time.
Example 7:
A particle’s position on the 𝑥-axis is given by 𝑥 = 4 − 27𝑡 + 𝑡 3 , with 𝑥 in meters and
𝑡 in seconds. (a) find the particle’s velocity function 𝑣(𝑡) and acceleration 𝑎(𝑡). (b) Is
there ever a time when 𝑣 = 0?
Page 11 of 43
Solution:
(𝑎 )
(𝑏 )
3.2
𝑑𝑥
𝑑
= (4 − 27𝑡 + 𝑡 3 ) = −27 + 3𝑡 2
𝑑𝑡 𝑑𝑡
𝑑𝑣
𝑑
𝑎=
= (−27 + 3𝑡 2 ) = 6𝑡
𝑑𝑡 𝑑𝑡
0 = −27 + 3𝑡 2
𝑡 2 = 27⁄3 = 9
𝑡 = 3𝑠
𝑣=
MOTION IN TWO AND THREE DIMENSIONS
The concept of position, velocity and acceleration for motion along a straight line is
similar to that two and three dimension, but more complex.
3.2.1 Position and Displacement
The position vector 𝑟⃗ is used to specify the position of a particle in space. It is
generally expressed in the unit vector notation as
𝑟⃗ = 𝑥𝑖̂ + 𝑦𝑗̂ + 𝑧𝑘̂ …
(3.8)
where 𝑥𝑖̂, 𝑦𝑗̂ and 𝑧𝑘̂ are the vector components of 𝑟⃗, and the coefficients 𝑥, 𝑦, and 𝑧
are its scalar components.
If a particle moves from point 1 to point 2 during a certain time interval, the position
vector changes from 𝑟⃗1 to 𝑟⃗2 , then the particle’s displacement is
∆𝑟⃗ = 𝑟⃗2 − 𝑟⃗1
…
(3.9)
Example 8:
Page 12 of 43
The position vector of a particle is initially 𝑟⃗1 = (−3𝑚)𝑖̂ + (2𝑚)𝑗̂ + (5𝑚)𝑘̂ and then
later is 𝑟⃗2 = (9𝑚)𝑖̂ + (2𝑚)𝑗̂ + (8𝑚)𝑘̂. What is the particle’s displacement ∆𝑟⃗ from
𝑟⃗1 to 𝑟⃗2 ?
Solution:
∆𝑟⃗ = 𝑟⃗2 − 𝑟⃗1 = [9 − (−3)]𝑖̂ + [2 − 2]𝑗̂ + [8 − 5]𝑘̂
∆𝑟⃗ = (12𝑚)𝑖̂ + (3𝑚)𝑘̂
Example 9:
A rabbit runs across a parking lot. The coordinates of the rabbit as a function of time
𝑡 are: 𝑥 = −0.3𝑡 2 + 7.2𝑡 + 28 and 𝑦 = 0.22𝑡 2 − 9.1𝑡 + 30 with 𝑡 in seconds and 𝑥
and 𝑦 in meters. At 𝑡 = 15𝑠, what is the rabbit’s position vector 𝑟⃗ in unit vector
notation and as a magnitude and an angle?
Solution:
𝑟⃗ = 𝑥𝑖̂ + 𝑦𝑗̂
At 𝑡 = 15𝑠
𝑥 = −0.3(15)2 + 7.2(15) + 28 = 68.5𝑚
𝑦 = 0.22(15)2 − 9.1(15) + 30 = −57𝑚
∴ 𝑟⃗ = (68.5𝑚)𝑖̂ + (−57𝑚)𝑗̂
The magnitude of the position vector 𝑟⃗ is
𝑟 = √𝑥 2 + 𝑦 2 = √(68.5)2 + (−57)2 = 89.11𝑚
The angle of 𝑟⃗ is
𝑦
−57
) = −39.76𝑜
𝜃 = tan−1 = tan−1 (
𝑥
68.5
3.2.1 Average Velocity and Instantaneous Velocity
If a particle moves through a displacement ∆𝑟⃗ in a time interval ∆𝑡, then its average
velocity𝑣⃗𝑎𝑣𝑔 is
∆𝑟⃗ ∆𝑥𝑖̂ + ∆𝑦𝑗̂ + ∆𝑧𝑘̂ ∆𝑥
∆𝑦
∆𝑧
𝑣⃗𝑎𝑣𝑔 =
=
=
𝑖̂ +
𝑗̂ + 𝑘̂
…
3.10
∆𝑡
∆𝑡
∆𝑡
∆𝑡
∆𝑡
When we speak of the velocity of a particle, we usually mean the particle’s
⃗⃗. It is expressed as
instantaneous velocity 𝒗
𝑑𝑟⃗ 𝑑𝑥
𝑑𝑦
𝑑𝑧
𝑣⃗ =
=
𝑖̂ +
𝑗̂ + 𝑘̂ = 𝑣𝑥 𝑖̂ + 𝑣𝑦 𝑗̂ + 𝑣𝑧 𝑘̂
…
3.11
𝑑𝑡 𝑑𝑡
𝑑𝑡
𝑑𝑡
Example 10:
Page 13 of 43
Find the velocity 𝑣⃗ at time 𝑡 = 15𝑠 of the rabbit in example 9 in unit vector notation
and as a magnitude and angle.
Solution:
The components of the velocity are𝑣𝑥 and 𝑣𝑦
𝑑𝑥
𝑑
𝑑
𝑣𝑥 =
= (𝑥) = (−0.3𝑡 2 + 7.2𝑡 + 28) = −0.6𝑡 + 7.2
𝑑𝑡 𝑑𝑡
𝑑𝑡
𝐴𝑡 𝑡 = 15𝑠, 𝑣𝑥 = −0.6(15) + 7.2 = −1.8𝑚/𝑠
𝑑𝑦
𝑑
𝑑
𝑣𝑦 =
= (𝑦) = (0.22𝑡 2 − 9.1𝑡 + 30) = 0.44𝑡 − 9.1
𝑑𝑡 𝑑𝑡
𝑑𝑡
𝐴𝑡 𝑡 = 15𝑠, 𝑣𝑦 = 0.44(15) − 9.1 = −2.5𝑚/𝑠
𝑣⃗ = 𝑣𝑥 𝑖̂ + 𝑣𝑦 𝑗̂
𝑣⃗ = (−1.8𝑚/𝑠)𝑖̂ + (−2.5𝑚/𝑠)𝑗̂
𝑀𝑎𝑔𝑛𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑣⃗ = 𝑣 = √𝑣𝑥2 + 𝑣𝑦2 = √(−1.8)2 + (−2.5)2 = 3.08𝑚/𝑠
𝑣𝑦
−2.5
) = 54.25𝑜
𝜃 = tan−1 ( ) = tan−1 (
𝑣𝑥
−1.8
3.2.2 Average Acceleration and Instantaneous Acceleration
The average acceleration is defined as the change in velocity from 𝑣⃗1 to 𝑣⃗2 in a time
interval ∆𝑡. It is mathematically expressed as
𝑣⃗2 − 𝑣⃗1 ∆𝑣⃗
𝑎⃗𝑎𝑣𝑔 =
=
…
3.12
∆𝑡
∆𝑡
The instantaneous acceleration 𝑎⃗ (or acceleration) is the limit of average velocity as
∆𝑡 approaches zero. It is expressed mathematically as
∆𝑣⃗ 𝑑𝑣⃗
𝑎⃗ = lim
=
…
3.13
∆𝑡→0 ∆𝑡
𝑑𝑡
We note that
𝑑𝑣⃗
𝑑
𝑑
𝑎⃗ =
= (𝑣⃗ ) = (𝑣𝑥 𝑖̂ + 𝑣𝑦 𝑗̂ + 𝑣𝑧 𝑘̂ )
𝑑𝑡 𝑑𝑡
𝑑𝑡
𝑑𝑣𝑦
𝑑𝑣𝑥
𝑑𝑣𝑧
𝑎⃗ =
𝑖̂ +
𝑗̂ +
𝑘̂
𝑑𝑡
𝑑𝑡
𝑑𝑡
𝑎⃗ = 𝑎𝑥 𝑖̂ + 𝑎𝑦 𝑗̂ + 𝑎𝑧 𝑘̂
3.2.3 Constant Acceleration
Page 14 of 43
In many types of motion, the acceleration is either constant or approximately so.
The following equations describe the motion of a particle with constant
acceleration:
s/n
Equation
1.
𝑣 = 𝑣𝑜 + 𝑎𝑡
1
2.
𝑠 = 𝑣𝑜 𝑡 + 𝑎𝑡 2
2
3.
𝑣 2 = 𝑣𝑜2 + 2𝑎𝑠
1
4.
𝑠 = (𝑣𝑜 + 𝑣 )𝑡
2
1
5.
𝑠 = 𝑣𝑡 − 𝑎𝑡 2
2
Note that these equations are only applicable if
acceleration is constant.
EXERCISE:
A driver spotted a police car and he braked from a speed of 100𝑘𝑚/ℎ to a speed of
80𝑘𝑚/ℎ during a displacement of 88𝑚, at a constant acceleration. (a) What is that
acceleration? (b) How much time is required for the given decrease in speed.
Example 11:
Find the acceleration 𝑎⃗ at time 𝑡 = 15𝑠 of the rabbit in example 10 in unit vector
notation and as a magnitude and angle.
Solution:
𝑑𝑣𝑥
𝑑
𝑑
= (𝑣𝑥 ) = (−0.6𝑡 + 7.2) = −0.6𝑚𝑠 −2
𝑑𝑡
𝑑𝑡
𝑑𝑡
𝑑𝑣𝑦
𝑑
𝑑
𝑎𝑦 =
= (𝑣𝑦 ) = (0.44𝑡 − 9.1) = 0.44𝑚𝑠 −2
𝑑𝑡
𝑑𝑡
𝑑𝑡
(
𝑎⃗ = −0.6𝑚𝑠 −2 )𝑖̂ + (0.44𝑚𝑠 −2 )𝑗̂
𝑚𝑎𝑔𝑛𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑎⃗ = 𝑎 = √(−0.6)2 + (0.44)2 = 0.74𝑚𝑠 −2
𝑎𝑦
0.44
) = −36.25𝑜
𝜃 = tan−1
= tan−1 (
𝑎𝑥
−0.6
𝑎𝑥 =
EXERCISE
A particle with velocity 𝑣⃗𝑜 = −2𝑖̂ + 4𝑗̂ in meters per second at 𝑡 = 0 undergoes a
constant acceleration 𝑎⃗ of magnitude 𝑎 = 3𝑚𝑠 −2 at an angle 𝜃 = 130𝑜 from the
positive direction of the 𝑥 axis. What is the particle’s velocity 𝑣⃗ at 𝑡 = 5𝑠, in unit
vector notation and as a magnitude and an angle?
𝐴𝑛𝑠: 𝑣⃗ = −12𝑖̂ + 16𝑗̂(𝑚/𝑠), 𝑣 = 19 𝑚⁄𝑠 , 𝜃 = 127𝑜
3.3 PROJECTILE MOTION
Page 15 of 43
A projectile is a particle which moves in a vertical plane with some initial velocity
𝑣⃗𝑜 but its acceleration is always the free fall acceleration 𝑔⃗ which is downward.
Example of a projectile is a golf ball. The motion of a projectile is called projectile
motion.
Consider the figure below.
When an object is projected at an angle to the horizontal, as shown in figure above,
the following should be noted:
(i)
the horizontal component of the velocity 𝑣𝑜𝑥 is constant;
(ii) the vertical component of the velocity 𝑣𝑜𝑦 changes;
(iii) the vertical component of the velocity is zero at the highest point;
(iv) the vertical acceleration 𝑎𝑦 = −𝑔 throughout the motion, while the horizontal
acceleration 𝑎𝑥 is zero.
In projectile motion, the horizontal motion and the vertical motion are independent
of each other; that is, neither motion affects the other.
Vertical motion:
Initial velocity 𝑣𝑜𝑦 = 𝑣𝑜 sin 𝜃, acceleration 𝑎𝑦 = −𝑔
Let 𝐻 be the maximum height reached. At maximum height 𝑣𝑦 = 0.
2
𝑈𝑠𝑖𝑛𝑔 𝑣𝑦2 = 𝑣𝑜𝑦
+ 2𝑎𝑦 𝐻
2
0 = (𝑣𝑜 sin 𝜃) + 2(−𝑔)𝐻
0 = 𝑣𝑜2 𝑠𝑖𝑛2 𝜃 − 2𝑔𝐻
Page 16 of 43
𝑣𝑜2 𝑠𝑖𝑛2 𝜃
𝐻=
…
3.14
2𝑔
Let the time taken to reach the maximum height be 𝑡.
𝑈𝑠𝑖𝑛𝑔 𝑣𝑦 = 𝑣𝑜𝑦 + 𝑎𝑦 𝑡
0 = 𝑣𝑜 sin 𝜃 + (−𝑔)𝑡
0 = 𝑣𝑜 sin 𝜃 − 𝑔𝑡
𝑣𝑜 sin 𝜃
𝑡=
…
3.15
𝑔
When the object landed on the ground, the vertical displacement 𝐻 = 0. Let 𝑇 be the
total time of flight.
1
𝑈𝑠𝑖𝑛𝑔 𝐻𝑦 = 𝑣𝑜𝑦 𝑡 + 𝑎𝑦 𝑡 2
2
1
0 = 𝑣𝑜 sin 𝜃 𝑇 + (−𝑔)𝑇 2
2
𝐷𝑖𝑣𝑖𝑑𝑖𝑛𝑔 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑏𝑦 𝑡, 𝑤𝑒 ℎ𝑎𝑣𝑒
1
0 = 𝑣𝑜 sin 𝜃 − 𝑔𝑇
2
𝑔𝑇
= 𝑣𝑜 sin 𝜃
2
2𝑣𝑜 sin 𝜃
𝑇=
= 2𝑡 … 3.16
𝑔
Horizontal Motion:
Initial velocity 𝑣𝑜𝑥 = 𝑣𝑜 cos 𝜃 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡,𝑎𝑥 = 0
To obtain the range (horizontal displacement) of the projectile, we use
1
𝑥 − 𝑥𝑜 = 𝑣𝑜𝑥 𝑡 + 𝑎𝑥 𝑡 2
2
The range 𝑅 of the projectile is
1
𝑅 = 𝑣𝑜𝑥 𝑇 + 𝑎𝑥 𝑡 2
2
𝑆𝑖𝑛𝑐𝑒 𝑎𝑥 = 0, 𝑡ℎ𝑒𝑛
𝑅 = 𝑣𝑜𝑥 𝑇
2𝑣𝑜 sin 𝜃
)
𝑅 = (𝑣𝑜 cos 𝜃) (
𝑔
𝑣𝑜2 (2 sin 𝜃 cos 𝜃)
𝑅=
𝑔
𝑟𝑒𝑐𝑎𝑙𝑙, 2 sin 𝜃 cos 𝜃 = sin 2𝜃
𝑣𝑜2 sin 2𝜃
𝑅=
..
3.17
𝑔
Page 17 of 43
The maximum value of 𝑅 is
𝑣𝑜2
𝑔
and it occurs when sin 2𝜃 = 1, or 2𝜃 = 90𝑜 , ∴ 𝜃 =
45𝑜 . Therefore the maximum range is obtained if the object is projected at an angle
45𝑜 to the horizontal.
Example 12:
A pirate ship is 560𝑚 from a military island base. A military cannon (large gun on
wheels) located at sea level fires balls at initial speed 𝑣𝑜 = 82𝑚/𝑠. (𝑎) At what
angle𝜃 from the horizontal must a ball be fired to hit the ship? (𝑏) How far should
the pirate ship be from the cannon if it is to be beyond the maximum range of the
cannonballs?
Solution:
(𝑎 )
𝑣𝑜2 sin 2𝜃
𝑔
𝑔𝑅
sin 2𝜃 = 2
𝑣𝑜
𝑔𝑅
9.8
×
560
) = sin−1 0.816 = 54.68𝑜
2𝜃 = sin−1 ( 2 ) = sin−1 (
2
𝑣𝑜
82
54.68𝑜
𝜃=
2
𝜃 = 27.34𝑜
𝑅=
𝑣𝑜2 sin 2𝜃
(𝑏 )
𝑅=
𝑔
𝑇ℎ𝑒 𝑟𝑎𝑛𝑔𝑒 𝑅 𝑖𝑠 𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑎𝑡 𝜃 = 45𝑜
822 × sin(2 × 45𝑜 )
𝑅=
9.8
𝑅 = 686𝑚
∴ 𝑇ℎ𝑒 𝑠ℎ𝑖𝑝 𝑠ℎ𝑜𝑢𝑙𝑑 𝑏𝑒 𝑏𝑒𝑦𝑜𝑛𝑑 686𝑚 (𝑖. 𝑒. 𝑅 > 686𝑚)𝑓𝑜𝑟 𝑡ℎ𝑒 𝑠ℎ𝑖𝑝 𝑡𝑜 𝑏𝑒 𝑠𝑎𝑓𝑒.
Example 13:
A projectile shot at an angle of 60𝑜 above the horizontal strikes a building 25𝑚 away
at a point 16𝑚 above the point of projection. Find the magnitude and direction of
the velocity of the projectile as it strikes the building.
Page 18 of 43
Solution:
Horizontal motion
Vertical motion
1
𝑈𝑠𝑖𝑛𝑔 𝑥 = 𝑣𝑜𝑥 𝑡 + 𝑎𝑥 𝑡 2
2
𝑟𝑒𝑐𝑎𝑙𝑙 𝑎𝑥 = 0, 𝑣𝑜𝑥 = 𝑣𝑜 cos 𝜃
𝑥 = 𝑣𝑜 cos 𝜃 𝑡
𝑥
25
50
𝑡=
=
=
𝑣𝑜 cos 𝜃 𝑣𝑜 × cos 60 𝑣𝑜
1
𝑈𝑠𝑖𝑛𝑔 𝑦 = 𝑣𝑜𝑦 𝑡 + 𝑎𝑦 𝑡 2
2
𝑟𝑒𝑐𝑎𝑙𝑙 𝑎𝑦 = −𝑔, 𝑣𝑜𝑦 = 𝑣𝑜 sin 𝜃
1
𝑦 = 𝑣𝑜 sin 𝜃 𝑡 − 𝑔𝑡 2
2
50
1
502
16 = 𝑣𝑜 × sin 60𝑜 × ( ) − × 9.8 × 2
𝑣𝑜
2
𝑣𝑜
12,250
16 = 43.3 −
𝑣𝑜2
𝑣𝑜 = 21.18𝑚/𝑠
50
50
=
= 2.36𝑠
𝑣𝑜 21.18
= 𝑣𝑜 cos 𝜃 = 21.18 × cos 60𝑜 = 10.59𝑚/𝑠
= 𝑣𝑜 sin 𝜃 = 21.18 × sin 60𝑜 = 18.34𝑚/𝑠
𝑈𝑠𝑖𝑛𝑔 𝑣 = 𝑣𝑜 + 𝑎𝑡
𝑣𝑥 = 𝑣𝑜𝑥 + 𝑎𝑥 𝑡 = 10.59 + (0)𝑡 = 10.59𝑚/𝑠
𝑣𝑦 = 𝑣𝑜𝑦 + 𝑎𝑦 𝑡 = 18.34 + (−9.8) × 2.36 = −4.79𝑚/𝑠
𝑁𝑜𝑤, 𝑡 =
𝑣𝑜𝑥
𝑣𝑜𝑦
Page 19 of 43
𝑣 = √𝑣𝑥2 + 𝑣𝑦2 = √(10.59)2 + (−4.79)2 = 11.62𝑚/𝑠
−4.79
) = −24.330
𝜃 = tan−1 (
10.59
EXERCISE
1.
A projectile leaves the ground at an angle of 60𝑜 to the horizontal. Its kinetic
energy is 𝐸. Neglecting air resistance, find in terms of 𝐸 its kinetic energy at
1
the highest point of the motion. (Answer: 𝐸)
4
2.
An object is released from an aeroplane which is diving at an angle of 30𝑜
from the horizontal with a speed of 50𝑚/𝑠. If the plane is at a height of
4000𝑚 from the ground when the object is released, find
(a)
the velocity of the object when it hits the ground.
(b) the time taken for the object to reach the ground.
Page 20 of 43
4.0
DYNAMICS
Dynamics is a branch of mechanics which deals with the forces that give rise to
motion. Just as kinematics describes how objects move without describing the force
that caused the motion, Newton’s laws of motion are the foundation of dynamics
which describes the motion and force responsible for the motion.
4.1
FORCE
Force is that which changes the velocity of an object. Force is a vector quantity. An
external force is one which lies outside of the system being considered.
4.1.1 TYPES OF FORCES
Contact Force is that in which one object has to be in contact with another to exert
a force on it. A push or pull on an object are examples of contact force.
Tension ⃗𝑻⃗ is the force on a string or chain tending to stretch it.
⃗⃗𝑵 is the force which acts perpendicular to a surface which supports
Normal force 𝑭
an object.
⃗⃗⃗⃗⃗of an object is the force with which gravity pulls downward upon it. It is
Weight 𝑾
⃗⃗⃗⃗ = 𝑚𝑔⃗. It is equal to the gravitational force on the body.
given as 𝑊
Frictional force ⃗⃗
𝒇is the force on a body when the body slides or attempts to slide
along a surface and is always parallel to the surface and directed so as to oppose the
motion of the body.
Other important forces include gravitational
electromagnetic force, nuclear force.
force
or
simply
gravity,
4.1.2 NET FORCE
The net force is the vector sum of all force vectors that act on an object. It is
expressed as:
𝑛
𝐹⃗𝑛𝑒𝑡 = ∑ 𝐹⃗𝑖 = 𝐹⃗1 + 𝐹⃗2 + ⋯ + 𝐹⃗𝑛
…
4.1
𝑖=1
In Cartesian components, the net force are given by
Page 21 of 43
𝑛
𝐹⃗𝑛𝑒𝑡,𝑥 = ∑ 𝐹𝑖,𝑥 = 𝐹⃗1,𝑥 + 𝐹⃗2,𝑥 + ⋯ + 𝐹⃗𝑛,𝑥
𝑛
𝑖=1
𝐹⃗𝑛𝑒𝑡,𝑦 = ∑ 𝐹𝑖,𝑦 = 𝐹⃗1,𝑦 + 𝐹⃗2,𝑦 + ⋯ + 𝐹⃗𝑛,𝑦
𝑖=1
…
4.2
𝑛
𝐹⃗𝑛𝑒𝑡,𝑧 = ∑ 𝐹𝑖,𝑧 = 𝐹⃗1,𝑧 + 𝐹⃗2,𝑧 + ⋯ + 𝐹⃗𝑛,𝑧
𝑖=1
4.2
NEWTON’S LAWS OF MOTION
4.2.1 Newton’s First Law
If no net force acts on a body (i.e. ⃗𝑭⃗𝒏𝒆𝒕 = 𝟎), then the body’s velocity cannot
change; that is, the body cannot accelerate. If it was moving, it will remain in
motion in a straight line with the same constant velocity.
4.2.2 Newton’s Second Law
⃗⃗𝒏𝒆𝒕 acts on an object with mass 𝒎, the force will cause an
If a net external force, 𝑭
⃗⃗, in the same direction as the force. Mathematically, the law is
acceleration, 𝒂
expressed as:
⃗⃗𝒏𝒆𝒕 = 𝒎𝒂
⃗⃗
𝑭
…
4.3
Which may be written in Cartesian components as
𝐹⃗𝑛𝑒𝑡,𝑥 = 𝑚𝑎𝑥 , 𝐹⃗𝑛𝑒𝑡,𝑦 = 𝑚𝑎𝑦 ,
𝐹⃗𝑛𝑒𝑡,𝑧 = 𝑚𝑎𝑧
…
4.4
4.2.3 Newton’s Third Law
The forces that two interacting objects exert on each other are always exactly
equal in magnitude and opposite in direction:
𝐹⃗1→2 = −𝐹⃗2→1 …
4.5
Example 14:
Three forces that act on a particle are given by 𝐹⃗1 = (20𝑖̂ − 36𝑗̂ + 73𝑘̂ )𝑁, 𝐹⃗2 =
(−17𝑖̂ + 21𝑗̂ − 46𝑘̂ )𝑁 𝑎𝑛𝑑 𝐹⃗3 = (−12𝑘̂ )𝑁. Find their resultant (net) vector. Also
find the magnitude of the resultant to two significant figures.
Solution:
𝐹⃗𝑛𝑒𝑡,𝑥
𝐹⃗𝑛𝑒𝑡 = 𝐹⃗1 + 𝐹⃗2 + 𝐹⃗3
= [20𝑖̂ + (−17𝑖̂)]𝑁 = 3𝑖̂𝑁
Page 22 of 43
𝐹⃗𝑛𝑒𝑡,𝑦 = [−36𝑗̂ + 21𝑗̂]𝑁 = −15𝑗̂𝑁
𝐹⃗𝑛𝑒𝑡,𝑧 = [73𝑘̂ − 46𝑘̂ − 12𝑘̂ ]𝑁 = 15𝑘̂ 𝑁
𝐹⃗𝑛𝑒𝑡 = [3𝑖̂ − 15𝑗̂ + 15𝑘̂ ]𝑁
𝐹 = |𝐹⃗𝑛𝑒𝑡 | = √32 + (−15)2 + 152 = √459 = 21𝑁
Example 15:
Five coplanar forces act on an object as shown in the figure below. Find the resultant
of the forces. The magnitude of the forces are: 𝐹1 = 15𝑁, 𝐹2 = 19𝑁, 𝐹3 = 22𝑁, 𝐹4 =
11𝑁, 𝐹5 = 16𝑁.
Solution:
𝐹⃗1 = ⃗⃗⃗
𝐹1,𝑥
𝐹⃗𝑛𝑒𝑡 = 𝐹⃗1 + 𝐹⃗2 + 𝐹⃗3 + 𝐹⃗4 + 𝐹⃗5
+ ⃗⃗⃗
𝐹 = 𝐹 cos 60𝑜 𝑖̂ + 𝐹 sin 60𝑜 𝑗̂
1,𝑦
1
1
𝐹⃗2 = ⃗⃗⃗
𝐹2,𝑥 𝑖̂ = 𝐹2 𝑖̂
⃗⃗⃗ 𝑗̂ = −𝐹 𝑗̂
𝐹⃗3 = −𝐹
3,𝑦
3
𝑜
𝑜
⃗⃗⃗
⃗⃗⃗
̂
̂
⃗
𝐹4 = −𝐹4,𝑥 𝑖 − 𝐹4,𝑦 𝑗 = −𝐹4 cos 30 𝑖̂ − 𝐹4 sin 30 𝑗̂
⃗⃗⃗ + ⃗⃗⃗
𝐹⃗5 = −𝐹
𝐹5,𝑦 = −𝐹5 cos 45𝑜 + 𝐹5 sin 45𝑜
5,𝑥
𝑜
𝑜
𝐹⃗1 = 15 cos 60 𝑖̂ + 15 sin 60 𝑗̂ = 7.5𝑖̂ + 12.99𝑗̂
𝐹⃗2 = 19𝑖̂
𝐹⃗3 = −22𝑗̂
𝑜
𝑜
𝐹⃗4 = −11 cos 30 𝑖̂ − 11 sin 30 𝑗̂ = −9.53𝑖̂ − 5.5𝑗̂
𝑜
𝑜
𝐹⃗5 = −16 cos 45 + 16 sin 45 = −11.31𝑖̂ + 11.31𝑗̂
𝐹⃗𝑛𝑒𝑡,𝑥 = (7.5 + 19 − 9.53 − 11.31)𝑖̂ 𝑁 = 5.66𝑖̂ 𝑁
𝐹⃗𝑛𝑒𝑡,𝑦 = (12.99 − 22 − 5.5 + 11.31)𝑗̂ 𝑁 = −3.2𝑗̂ 𝑁
Page 23 of 43
𝐹⃗𝑛𝑒𝑡 = √(5.66)2 + (−3.2)2 = 6.5𝑁
Example 16:
A mass of 55𝑘𝑔 was suspended with two ropes as shown in the figure below. What
is the tension in each rope if 𝜃 = 45𝑜 ?
Solution:
𝐹⃗𝑛𝑒𝑡 = ∑ 𝐹⃗𝑖 = 𝑀(𝑎⃗) = 𝑀(0) = 0
⃗⃗1𝑥 + 𝑇
⃗⃗2𝑥 = (𝑇1 cos 𝜃 − 𝑇2 cos 𝜃)𝑖̂ = 0
∑ 𝐹⃗𝑥 = 𝑇
⃗⃗1𝑦 + 𝑇
⃗⃗2𝑦 − 𝑊
⃗⃗⃗⃗ = [𝑇1 sin 𝜃 + 𝑇2 sin 𝜃 − 55(9.8)]𝑗̂ = 0
∑ 𝐹⃗𝑦 = 𝑇
𝑁𝑜𝑡𝑒, 𝑏𝑜𝑡ℎ 𝑟𝑜𝑝𝑒𝑠 𝑠𝑢𝑝𝑝𝑜𝑟𝑡 𝑡ℎ𝑒 𝑙𝑜𝑎𝑑 𝑒𝑞𝑢𝑎𝑙𝑙𝑦
⃗⃗1 = 𝑇
⃗⃗2 = 𝑇
⃗⃗
𝑇
𝑇 cos 𝜃 − 𝑇 cos 𝜃 = 0
𝑇 sin 𝜃 + 𝑇 sin 𝜃 − 539 = 0
2𝑇 sin 𝜃 = 539 = 0
539
𝑇=
2 × sin 45
𝑇 = 381𝑁
Page 24 of 43
Example 17:
A cord holds stationary a block of mass 𝑚 = 15𝑘𝑔 on a frictionless plane that is
⃗⃗ on the block
inclined at angle 𝜃 = 27𝑜 . (𝑎)What is the magnitude of the tension 𝑇
⃗⃗ on the block from the plane? (𝑏)If the cord is
from the cord and the normal force 𝑁
cut, so that the body slides down the plane, calculate the acceleration of the body.
Solution:
(𝑎)
The free body diagram is sketched as
We note from Newton’s 2nd law, 𝐹⃗𝑛𝑒𝑡 = 𝑚𝑎⃗
⃗⃗ + 𝑁
⃗⃗ + 𝐹⃗𝑔 = 𝑚𝑎⃗
𝑇
…
(1)
𝑎⃗ = 0 (𝑠𝑖𝑛𝑐𝑒 𝑡ℎ𝑒 𝑓𝑜𝑟𝑐𝑒𝑠 𝑎𝑟𝑒 𝑖𝑛 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚)
⃗⃗ + 𝑁
⃗⃗ + 𝐹⃗𝑔 = 0
𝑇
…
(2)
𝑠𝑜𝑙𝑣𝑖𝑛𝑔 𝑖𝑛 𝑡ℎ𝑒 𝑐𝑎𝑟𝑡𝑒𝑠𝑖𝑎𝑛 𝑐𝑜𝑜𝑟𝑑𝑖𝑛𝑎𝑡𝑒𝑠, 𝑤𝑒 ℎ𝑎𝑣𝑒
𝑥:
𝑇 + 0 − 𝐹𝑔 sin 𝜃 = 0
𝑇 = 𝐹𝑔 sin 𝜃 = 𝑚𝑔 sin 𝜃
𝑇 = 15 × 9.8 × sin 27𝑜 = 66.74𝑁
Page 25 of 43
𝑦:
0 + 𝑁 − 𝐹𝑔 cos 𝜃 = 0
𝑁 = 𝐹𝑔 𝑐𝑜𝑠𝜃 = 𝑚𝑔 cos 𝜃
𝑁 = 15 × 9.8 × cos 27𝑜
𝑁 = 130.98𝑁
⃗⃗ 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑏𝑙𝑜𝑐𝑘, 𝑠𝑜 𝑡ℎ𝑎𝑡 𝑡ℎ𝑒 𝑏𝑙𝑜𝑐𝑘 𝑠𝑙𝑖𝑑𝑒𝑠 𝑎𝑙𝑜𝑛𝑔 𝑥 𝑎𝑥𝑖𝑠
(𝑏)𝐶𝑢𝑡𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑐𝑜𝑟𝑑 𝑟𝑒𝑚𝑜𝑣𝑒𝑠 𝑓𝑜𝑟𝑐𝑒 𝑇
∴ 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 (2)𝑏𝑒𝑐𝑜𝑚𝑒𝑠
0 + 0 − 𝐹𝑔 sin 𝜃 = 𝑚𝑎
−𝑚𝑔 sin 𝜃 = 𝑚𝑎
𝑎 = −𝑔 sin 𝜃
𝑎 = −9.8 × sin 27𝑜
𝑎 = −4.45 𝑚/𝑠 2
5.0
KINETIC ENERGY, WORK, AND POWER
5.1 KINETIC ENERGY
The kinetic energy 𝐾 is energy associated with the state of motion of a particle and
is defined as:
1
𝐾 = 𝑚𝑣 2
…
5.1
2
where 𝑚, is the mass of the particle and 𝑣 is the speed of the particle which is well
below the speed of light. Its S.I. unit is the joule (J).
Example 18:
A car of mass 1310𝑘𝑔 being driven at a speed limit of 24.6 𝑚/𝑠 has a kinetic energy
of:
1
1
𝐾𝑐𝑎𝑟 = 𝑚𝑣 2 = (1310)(24.6)2 = 4.0 × 105 𝐽
2
2
5.2 WORK
Work 𝑊 is the energy transferred to or from an object by means of a force acting on
the object. Energy transferred to the object is positive work, and energy transferred
from the object is negative work. It is a scalar quantity.
The work done by a force can be defined as the product of the magnitude of the
displacement and the component of the force in the direction of the displacement.
The unit of work in S.I. is the Joule (J).
Consider the figure below.
Page 26 of 43
Work can be expressed mathematically as
𝑊 = 𝐹𝑥 cos 𝜑
… (5.2)
𝑜𝑟
(5.3)
𝑊 = 𝐹⃗ . 𝑥⃗
…
Equation (5.3) is especially useful for calculating the work when 𝐹⃗ and 𝑥⃗ are given
in unit vector notations.
Consider the figure below.
If a force displaces the particle through a distance 𝑑𝑟, then the work done 𝑑𝑊 by the
forced is
𝑑𝑊 = 𝐹⃗ . 𝑑𝑟⃗
…
(5.4)
The total work done by the force in moving the particle from initial point 𝑖 to final
point 𝑓 will be
𝑓
𝑓
𝑊 = ∫ 𝐹⃗ . 𝑑𝑟⃗ = ∫ 𝐹𝑑𝑟 cos 𝜃
𝑖
…
(5.5)
𝑖
Page 27 of 43
5.3
WORK-KINETIC ENERGY THEOREM
The work-kinetic energy theorem states that “when a body is acted upon by a force
or resultant force, the work done by the force is equal to the change in kinetic
energy of the body.”
Proof:
𝑓
𝑓
𝑊 = ∫ 𝐹⃗ . 𝑑𝑟⃗ = ∫ 𝐹𝑑𝑟 cos 𝜃
𝑖
𝑖
𝐹𝑜𝑟𝑐𝑒 𝐹⃗ 𝑖𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑑𝑟⃗, ∴ 𝜃 = 0
𝑓
𝑓
𝑊 = ∫ 𝐹𝑑𝑟 = ∫ 𝑚𝑎𝑑𝑟
𝑓
𝑖
𝑖
𝑓
𝑑𝑣
𝑑𝑣 𝑑𝑟
) 𝑑𝑟
𝑊 = ∫ 𝑚 ( ) 𝑑𝑟 = ∫ 𝑚 (
𝑑𝑡
𝑑𝑟 𝑑𝑡
𝑖
𝑖
𝑓
𝑓
𝑓
𝑑𝑣
𝑊=∫ 𝑚
𝑣𝑑𝑟 = ∫ 𝑚𝑣𝑑𝑣 = 𝑚 ∫ 𝑣𝑑𝑣
𝑑𝑟
𝑖
𝑖
𝑖
2
2
2 𝑓
𝑣
𝑣
𝑣𝑖
𝑓
𝑊 = 𝑚[ ] = 𝑚[ − ]
2 𝑖
2
2
1
1
𝑊 = 𝑚𝑣𝑓2 − 𝑚𝑣𝑖2 …
(5.6)
2
2
𝑊 = 𝐾𝑓 − 𝐾𝑖
𝑊 = ∆𝐾
…
(5.7)
5.4
POWER
Power is the rate of doing work. If an amount of work ∆𝑊 is done in a small interval
of time ∆𝑡, the power 𝑃 is
∆𝑊 𝑑𝑊
𝑃 = lim
=
…
(5.8)
∆𝑡→0 ∆𝑡
𝑑𝑡
Power is a scalar quantity and its unit is 𝐽/𝑠 which is also called the watt (𝑊).
For a constant force
𝑃=
𝑑𝑊
𝑑𝑟⃗
= 𝐹⃗ .
= 𝐹⃗ . 𝑣⃗
𝑑𝑡
𝑑𝑡
(5.9)
𝑠𝑖𝑛𝑐𝑒 𝑡ℎ𝑒 𝑖𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑣⃗ =
If 𝐹⃗ acts at an angle 𝜃to 𝑣⃗, then
𝑃 = 𝐹⃗ . 𝑣⃗ = 𝐹𝑣 cos 𝜃
…
𝑑𝑟⃗
𝑑𝑡
(5.10)
Page 28 of 43
Example 19:
A body moves with velocity 𝑣⃗ = (5𝑖̂ + 2𝑗̂ − 3𝑘̂ )𝑚/𝑠 under the influence of a
constant force 𝐹⃗ = (4𝑖̂ + 3𝑗̂ − 2𝑘̂ )𝑁. Determine (𝑎) the instantaneous power; (𝑏)
the angle between 𝐹⃗ and 𝑣⃗.
Solution:
(𝑏 )
⃗⃗
(𝑎 )
𝑃𝑜𝑤𝑒𝑟 = 𝐹⃗ . 𝑉
= (4𝑖̂ + 3𝑗̂ − 2𝑘̂ ). (5𝑖̂ + 2𝑗̂ − 3𝑘̂ ) = 20 + 6 + 6 = 32𝑊
𝑃 = 𝐹𝑉 cos 𝜃
𝑃
𝜃 = cos −1 [ ]
𝐹𝑣
𝐹 = |𝐹⃗ | = √42 + 32 + (−2)2 = √29
𝑉 = |𝑣⃗ | = √52 + 22 + (−3)2 = √38
𝑃
32
] = 15.43𝑜
𝜃 = cos −1 [ ] = cos −1 [
𝐹𝑣
√29 × √38
Example 20:
A crate slides across an oily parking lot through a displacement 𝑑⃗ = (−3𝑚)𝑖̂ while a
steady wind pushes against the crate with a force 𝐹⃗ = (2𝑁)𝑖̂ + (−6𝑁)𝑗̂. (𝑎)
Calculate the work done by the wind, (𝑏) if the crate has a kinetic energy of 10𝐽 at
the beginning of the displacement 𝑑⃗, what is the kinetic energy at the end of 𝑑⃗?
Solution:
(𝑎 )
𝑊 = 𝐹⃗ . 𝑑⃗ = (2𝑖̂ − 6𝑗̂). (−3𝑖̂) = (2𝑖̂ − 6𝑗̂). (−3𝑖̂ + 0𝑗̂) = −6𝐽
(𝑏 )
𝑊 = ∆𝐾 = 𝐾𝑓 − 𝑘𝑖
𝐾𝑓 = 𝑊 + 𝐾𝑖 = −6 + 10 = 4𝐽
EXERCISE
A constant force 𝐹⃗ = (5𝑖̂ + 3𝑗̂ − 2𝑘̂ )𝑁 moves a particle from position 𝑟⃗1 =
(2𝑖̂ − 𝑗̂ + 4𝑘̂ )𝑚 to a position 𝑟⃗2 = (3𝑖̂ + 5𝑗̂ + 𝑘̂ )𝑚. Calculate the work done by the
force. [𝑨𝒏𝒔 = 𝟐𝟗𝑱]
Page 29 of 43
6.0
POTENTIAL ENERGY
Potential energy 𝑈 is the energy that can be associated with the configuration (or
arrangement) of a system of objects that exerts forces on one another.
Consider a tennis ball thrown upward as shown in figure 6.1 below.
As the ball is thrown upward, the gravitational force does negative work on it by
decreasing its kinetic energy. As the ball descends, the gravitational force does
positive work on it by increasing its kinetic energy. Thus the change in potential
energy is defined to equal the negative of work done. That is
(6.1)
∆𝑈 = −𝑊
…
Recall that
𝑥𝑓
𝑊 = ∫ 𝐹 (𝑥)𝑑𝑥
𝑥𝑓
𝑥𝑖
∴ ∆𝑈 = − ∫ 𝐹 (𝑥)𝑑𝑥
…
(6.2)
𝑥𝑖
6.1 GRAVITATIONAL POTENTIAL ENERGY
Gravitational potential energy is the potential energy associated with a system
consisting of Earth and a nearby particle.
𝑦𝑓
𝑦𝑓
𝑦𝑓
𝑦
𝑓
∆𝑈 = − ∫ 𝐹 (𝑦)𝑑𝑦 = − ∫ (−𝑚𝑔)𝑑𝑦 = 𝑚𝑔 ∫ 𝑑𝑦 = 𝑚𝑔[𝑦]𝑦𝑖 = 𝑚𝑔(𝑦𝑓 − 𝑦𝑖 )
𝑦𝑖
𝑦𝑖
∆𝑈 = 𝑚𝑔∆𝑦
…
Usually, reference point 𝑦𝑖 is taken as zero.
∆𝑈 = 𝑚𝑔𝑦
…
𝑦𝑖
(6.3)
(6.4)
Page 30 of 43
Example 21:
A mass of 2𝑘𝑔 hangs 5𝑚 above the ground. What is the gravitational potential
energy of the mass-earth system at (𝑖) the ground, (𝑖𝑖) a height of 3𝑚 above the
ground, and (𝑖𝑖𝑖) a height of 6m.
Solution:
(𝑖) 𝑈 = 𝑚𝑔(𝑦𝑓 − 𝑦𝑖 ) = 2 × 9.8 × (5 − 0) = 98𝐽
(𝑖𝑖) 𝑈 = 𝑚𝑔(𝑦𝑓 − 𝑦𝑖 ) = 2 × 9.8 × (5 − 3) = 39.2𝐽
(𝑖𝑖𝑖) 𝑈 = 𝑚𝑔(𝑦𝑓 − 𝑦𝑖 ) = 2 × 9.8 × (5 − 6) = −19.6𝐽
6.2 ELASTIC POTENTIAL ENERGY
Elastic potential energy is the energy associated with the state of compression or
extension of an elastic object. For a spring that exerts a force 𝑓 = −𝑘𝑥 when its free
end has displacement 𝑥, the elastic potential is
𝑥𝑓
𝑥𝑓
𝑥𝑓
∆𝑈 = − ∫ 𝐹𝑑𝑥 = − ∫ (−𝑘𝑥)𝑑𝑥 = 𝑘 ∫ 𝑥𝑑𝑥
𝑥𝑖
2 𝑥𝑓
𝑥𝑖
𝑥
1
1
= 𝑘 [ ] = 𝑘𝑥𝑓2 − 𝑘𝑥𝑖2
2 𝑥
2
2
𝑥𝑖
…
(6.5)
𝑖
6.3 CONSERVATION OF MECHANICAL ENERGY
We recall from equation (5.7)
(6.6)
𝑊 = ∆𝐾
…
Also, we recall from equation (6.1)
(6.7)
𝑊 = −∆𝑈
…
Combining equations (6.5) and (6.6)
∆𝐾 = −∆𝑈
𝐾2 − 𝐾1 = −(𝑈2 − 𝑈1 )
Rearranging
(6.8)
𝐾2 + 𝑈2 = 𝐾1 + 𝑈1
…
In words,
𝑠𝑢𝑚 𝑜𝑓 𝐾. 𝐸 𝑎𝑛𝑑 𝑃. 𝐸 𝑓𝑜𝑟
𝑡ℎ𝑒 𝑠𝑢𝑚 𝑜𝑓 𝐾. 𝐸 𝑎𝑛𝑑 𝑃. 𝐸 𝑓𝑜𝑟 𝑎𝑛𝑦
(
)=(
)
𝑎𝑛𝑦 𝑠𝑡𝑎𝑡𝑒 𝑜𝑓 𝑎 𝑠𝑦𝑠𝑡𝑒𝑚
𝑜𝑡ℎ𝑒𝑟 𝑠𝑡𝑎𝑡𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚
Equation (6.8) is the conservation of mechanical energy equation.
The principle of conservation of mechanical energy states that: In an isolated
system, the kinetic energy and potential energy can change, but their sum, the
mechanical energy 𝐸𝑚𝑒𝑐 of the system cannot change. That is
(6.9)
∆𝐸𝑚𝑒𝑐 = ∆𝐾 + ∆𝑈 = 0 …
Page 31 of 43
7.0
CENTER OF MASS
The center of mass of a body or a system of bodies is the point that moves as though
all of the masses were concentrated there and all external forces were applied there.
We define the position of the center of mass (com) of the two-particle separated by
a distance 𝑑 in figure 7.1 as:
𝑚2
(7.1)
𝑥𝑐𝑜𝑚 =
𝑑
…
𝑚1 + 𝑚2
We arbitrarily chose the origin of the 𝑥 axis to coincide with the particle of mass 𝑚1 .
If the origin is located farther from the particles, the position of the center of mass is
calculated as:
𝑚1 𝑥1 + 𝑚2 𝑥2
(7.2)
𝑥𝑐𝑜𝑚 =
…
𝑚1 + 𝑚2
Generally, for 𝑛 particles along 𝑥 axis
𝑛
𝑚1 𝑥1 + 𝑚2 𝑥2 + 𝑚3 𝑥3 + ⋯ + 𝑚𝑛 𝑥𝑛
1
(7.3)
𝑥𝑐𝑜𝑚 =
= ∑ 𝑚𝑖 𝑥𝑖
…
𝑀
𝑀
𝑖=1
Page 32 of 43
where 𝑀 = 𝑚1 + 𝑚2 + 𝑚3 + ⋯ + 𝑚𝑛 .
If the particles are distributed in three dimensions, the center of mass must be
identified by three coordinates.
𝑛
𝑛
𝑛
1
1
1
(7.4)
𝑥𝑐𝑜𝑚 = ∑ 𝑚𝑖 𝑥𝑖 , 𝑦𝑐𝑜𝑚 = ∑ 𝑚𝑖 𝑦𝑖 , 𝑧𝑐𝑜𝑚 = ∑ 𝑚𝑖 𝑧𝑖
…
𝑀
𝑀
𝑀
𝑖=1
𝑖=1
𝑖=1
Example 22:
Three particles of mass 𝑚1 = 1.2𝑘𝑔, 𝑚2 = 2.5𝑘𝑔, and 𝑚3 = 3.4𝑘𝑔 form an
equilateral triangle of edge length 𝑎 = 140𝑐𝑚. Where is the center of mass of this
three-particle system?
Solution:
The distance 𝑦 can be obtained using Pythagoras theorem
1402 = 702 + 𝑦32
𝑦32 = 1402 − 702 = 19600 − 4900 = 14700
𝑦3 = √14700 = 121𝑐𝑚
𝑚1 𝑥1 + 𝑚2 𝑥2 + 𝑚3 𝑥3 (1.2)(0) + (2.5)(140) + (3.4)(70)
𝑥𝑐𝑜𝑚 =
=
= 83𝑐𝑚
𝑀
7.1
𝑦𝑐𝑜𝑚 =
𝑚1 𝑦1 + 𝑚2 𝑦2 + 𝑚3 𝑦3 (1.2)(0) + (2.5)(0) + (3.4)(121)
=
= 58𝑐𝑚
𝑀
7.1
Page 33 of 43
8.0
COLLISION
A Collision is an isolated event in which two or more bodies exert relatively strong
forces on each other for a relatively short time.
8.1 ELASTIC AND INELASTIC COLLISION
A collision is said to be elastic if the total kinetic energy of the system of two
colliding bodies is unchanged by the collision (i.e. conserved). Whereas, if the kinetic
energy of the system is not conserved, such a collision is called an inelastic
collision.
In every day collision such as collision of two cars, kinetic energy is not conserved,
since some kinetic energy will be lost in form of heat and sound.
8.2 LINEAR MOMENTUM
In physics, momentum 𝑝⃗ is defined as the product of an object’s mass and its
velocity:
(8.1)
𝑝⃗ = 𝑚𝑣⃗
…
By Newton’s second law of motion, momentum is related to force by
𝑑𝑝⃗
(8.2)
𝐹⃗ =
…
𝑑𝑡
8.3 LAW OF CONSERVATION OF LINEAR MOMENTUM
The law states that: In a closed, isolated system, the linear momentum of each
⃗⃗ of the system
colliding body may change but the total linear momentum 𝒑
cannot change, whether the collision is elastic or inelastic.
(8.3)
𝑃⃗⃗ = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑟 𝑃⃗⃗𝑖 = 𝑃⃗⃗𝑓 …
where𝑖 and 𝑓 denote initial and final respectively.
8.4
ELASTIC COLLISIONS IN ONE DIMENSION
Consider the collision of two masses 𝑚1 and 𝑚2 along 𝑥 axis.
Page 34 of 43
By conservation of linear momentum, we have
(8.4)
𝑚1 𝑣1𝑖 + 𝑚2 𝑣2𝑖 = 𝑚1 𝑣1𝑓 + 𝑚2 𝑣2𝑓
…
Also, kinetic energy is conserved for elastic collision. So, we have
1
1
1
1
2
2
2
2
(8.5)
𝑚1 𝑣1𝑖
+ 𝑚2 𝑣2𝑖
= 𝑚1 𝑣1𝑓
+ 𝑚2 𝑣2𝑓
…
2
2
2
2
Equations 8.4 and 8.5 can be used simultaneously if there are two unknowns.
Example 23:
A block of mass 𝑚1 = 1.6𝑘𝑔, moving to the right with a speed of 4 𝑚⁄𝑠 on a
frictionless, horizontal track, collides with a spring attached to a second block, of
mass 𝑚2 = 2.1𝑘𝑔, that is moving to the left with a speed of 2.5 𝑚⁄𝑠. The spring
constant is 600 𝑁⁄𝑚. For the instant when 𝑚1 is moving to the right with a speed of
3 𝑚⁄𝑠, determine
(a)
the velocity of 𝑚2 and
(b) the distance, 𝑥, that the spring is compressed.
Solution:
(a)
(b)
𝑚1 𝑣1𝑖 + 𝑚2 𝑣21 = 𝑚1 𝑣1𝑓 + 𝑚2 𝑣2𝑓
(1.6)(4) + (2.1)(−2.5) = (1.6)(3) + (2.1)(𝑣2𝑓 )
𝑣2𝑓 = −1.74 𝑚⁄𝑠
𝑇𝑜 𝑑𝑒𝑡𝑒𝑟𝑚𝑖𝑛𝑒 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝑥, 𝑤𝑒 𝑎𝑝𝑝𝑙𝑦 𝑡ℎ𝑒 𝑐𝑜𝑛𝑠𝑒𝑟𝑣𝑎𝑡𝑖𝑜𝑛 𝑜𝑓
𝑒𝑛𝑒𝑟𝑔𝑦 𝑝𝑟𝑖𝑛𝑐𝑖𝑝𝑙𝑒
1
1
1
1
1
2
2
2
2
𝑚1 𝑣1𝑖
+ 𝑚2 𝑣2𝑖
= 𝑚1 𝑣1𝑓
+ 𝑚2 𝑣2𝑓
+ 𝑘𝑥 2
2
2
2
2
2
𝑠𝑢𝑏𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑔𝑖𝑣𝑒𝑛 𝑣𝑎𝑙𝑢𝑒𝑠, 𝑤𝑒 ℎ𝑎𝑣𝑒
𝑥 = 0.173𝑚
Page 35 of 43
9.0
UNIFORM CIRCULAR MOTION
A particle is said to be in uniform circular motion if it travels around a circle or a
circular arc at a constant (uniform) speed.
Figure 9.1 shows the relation between the velocity and acceleration vectors at
different stages during uniform circular motion.
The following points must be noted:
(i)
Although the speed is uniform, the velocity (a vector) changes in direction,
therefore, a particle in uniform circular motion accelerates.
(ii) The velocity vector 𝑣⃗ is always directed tangent to the circle in the direction
of the motion.
(iii) The acceleration is always directed radially inward, therefore the acceleration
is called centripetal acceleration.
(iv) The magnitude of the centripetal acceleration is:
𝑣2
(9.1)
𝑎=
…
𝑟
where𝑟 is the radius of the circle and 𝑣 is the speed of the particle.
(v)
The particle travels the circumference of the circle in time
2𝜋𝑟
(9.2)
𝑇=
…
𝑣
where𝑇 is called the period of revolution.
EXERCISE
Prove that the centripetal acceleration is 𝑎 =
𝑣2
𝑟
.
Page 36 of 43
Solution:
𝑣⃗ = 𝑣𝑥 𝑖̂ + 𝑣𝑦 𝑗̂
𝑣⃗ = (−𝑣 sin 𝜃)𝑖̂ + (𝑣 cos 𝜃)𝑗̂
𝑦
𝑥
𝐹𝑟𝑜𝑚 𝑑𝑖𝑎𝑔𝑟𝑎𝑚, sin 𝜃 = , cos 𝜃 =
𝑟
𝑟
𝑦
𝑥
𝑣⃗ = (−𝑣 ) 𝑖̂ + (𝑣 ) 𝑗̂
𝑟
𝑟
𝑑𝑣⃗
𝑑
𝑣 𝑑𝑦
𝑣 𝑑𝑥
) 𝑖̂ + (
) 𝑗̂
𝑎⃗ =
= (𝑣⃗ ) = (−
𝑑𝑡 𝑑𝑡
𝑟 𝑑𝑡
𝑟 𝑑𝑡
𝑑𝑦
𝑑𝑥
𝑁𝑜𝑡𝑒
= 𝑣𝑦 = 𝑣 cos 𝜃
𝑎𝑛𝑑
= 𝑣𝑥 = 𝑣 sin 𝜃
𝑑𝑡
𝑑𝑡
𝑣
𝑣
𝑎⃗ = (− 𝑣 cos 𝜃) 𝑖̂ + ( 𝑣 sin 𝜃) 𝑗̂
𝑟
𝑟
2
2
𝑣
𝑣
√
|
|
𝑎 = 𝑎⃗ = (− 𝑣 cos 𝜃) + ( 𝑣 sin 𝜃)
𝑟
𝑟
2
2
𝑣2
𝑣2
= √( ) (𝑐𝑜𝑠 2 𝜃) + ( ) (𝑠𝑖𝑛2 𝜃)
𝑟
𝑟
2
2
𝑣2
𝑣2
𝑣2
2
2
√
√
(
)
= ( ) 𝑐𝑜𝑠 𝜃 + 𝑠𝑖𝑛 𝜃 = ( ) =
𝑟
𝑟
𝑟
2
𝑣
∴𝑎=
𝑟
Page 37 of 43
10.0 ROTATION
10.1 RIGID BODY
A rigid body is a body is a body that can rotate with all its part locked together and
without a change in its shape.
10.2 ROTATION
Rotation occurs when every point of a rigid body moves in a circle whose center lies
on the axis of rotation, and every point moves through the same angle during a
particular time interval.
10.3 ANGULAR POSITION (𝜃)
Angular position 𝜃 is the angle of a reference line relative to a fixed direction, say 𝑥.
𝑠
𝜃 = (𝑟𝑎𝑑)
… (10.1)
𝑟
where𝑠 is the length of arc and 𝑟 is the radius of the circle. 𝜃is measured in radians
(𝑟𝑎𝑑).
10.4 ANGULAR DISPLACEMENT
A body undergoes angular displacement if it changes angular position from 𝜃1 to 𝜃2 .
It is expressed as:
(10.2)
∆𝜃 = 𝜃2 − 𝜃1
…
10.5 ANGULAR VELOCITY AND SPEED
The average angular speed𝜔𝑎𝑣𝑔 is the ratio of the angular displacement ∆𝜃 to time
∆𝑡 of a body undergoing rotation. It is expressed as:
∆𝜃
𝜔𝑎𝑣𝑔 =
…
10.3
∆t
⃗⃗⃗⃗ of the body is:
The instantaneous angular velocity 𝝎
Page 38 of 43
∆𝜃 𝑑𝜃
(10.4)
=
…
∆𝑡→0 ∆𝑡
𝑑𝑡
The magnitude of the body’s angular velocity is the angular speed 𝜔.
𝜔
⃗⃗ = lim
10.6 ANGULAR ACCELERATION
The average angular acceleration is expressed as
∆𝜔 𝜔2 − 𝜔1
𝛼𝑎𝑣𝑔 =
=
… (10.5)
∆𝑡
𝑡2 − 𝑡1
The instantaneous angular acceleration 𝛼 of a body is expressed as:
∆𝜔 𝑑𝜔
(10.6)
𝛼 = lim
=
…
∆𝑡→0 ∆𝑡
𝑑𝑡
10.7 RELATING THE ANGULAR AND LINEAR VARIABLES
Since for a rigid body, all particles make one revolution in the same amount of time,
they all have the same angular speed 𝜔. When a rigid body rotates, each particle in
the body moves in its own circle about that axis.
Recall from equation 10.1.
(10.7)
𝑇ℎ𝑒 𝑷𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝑆 = 𝜃𝑟
…
𝑑𝑆
𝑑
𝑑𝜃
𝑺𝒑𝒆𝒆𝒅 𝑣 =
= (𝜃𝑟) = 𝑟
= 𝑟𝜔
𝑑𝑡 𝑑𝑡
𝑑𝑡
(10.8)
𝑣 = 𝑟𝜔
…
where𝑣 is the linear speed and 𝜔 is the angular speed.
A rigid body in rotational motion will have two forms of acceleration.
𝑑𝑣
𝑑
𝑑𝜔
(1)𝑻𝒂𝒏𝒈𝒆𝒏𝒕𝒊𝒂𝒍 𝒂𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏 𝑎𝑡 =
= (𝜔𝑟) = 𝑟
= 𝑟𝛼
…
𝑑𝑡 𝑑𝑡
𝑑𝑡
𝑣 2 (𝑟𝜔)2
(2)𝑹𝒂𝒅𝒊𝒂𝒍 𝒂𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏𝑎𝑟 =
(10.10)
=
= 𝑟𝜔2 …
𝑟
𝑟
(10.9)
10.8 KINETIC ENERGY OF ROTATION
1
We cannot apply the formula 𝑘 = 𝑚𝑣 2 to rigid body such as compact disc because,
2
the kinetic energy of its center of mass is zero (𝑚 = 0 𝑎𝑡 𝑡ℎ𝑒 𝑐𝑒𝑛𝑡𝑒𝑟).
Instead, we treat the compact disc as a collection of particles with different speeds.
Page 39 of 43
1
1
1
𝑚1 𝑣12 + 𝑚2 𝑣22 + 𝑚3 𝑣32 + ⋯
2
2
2
1
𝑘 = ∑ 𝑚𝑖 𝑣𝑖2
2
𝑠𝑖𝑛𝑐𝑒 𝑣 = 𝜔𝑟
1
𝑘 = ∑ 𝑚𝑖 (𝑟𝑖 𝜔)2
2
1
𝑘 = (∑ 𝑚𝑖 𝑟𝑖2 ) 𝜔2
2
The quantity in parenthesis is called the rotational inertia or moment of inertia I
of the body with respect to the axis of rotation.
1
(10.11)
∴ 𝑘 = 𝐼𝜔2 …
2
where
𝑘=
𝐼 = ∑ 𝑚𝑖 𝑟𝑖2
…
(10.12)
10.9 PERPENDICULAR AXES THEOREM
Statement of the theorem: The moments of inertia of any plane lamina about an
axis normal to the plane of the lamina is equal to the sum of the moments of inertia
about any two mutually perpendicular axis passing through the given axis and lying
in the plane of the lamina. That is,
(10.13)
𝐼𝑧 = 𝐼𝑥 + 𝐼𝑦
…
Proof:
The position vector of particle with mass 𝑚𝑖 in the 𝑥𝑦 plane is:
𝑅⃗⃗𝑖 = 𝑥𝑖 𝑖⃗ + 𝑦𝑖 𝑗⃗
So that, the moment of inertia of particle 𝑚𝑖 is 𝑚𝑖 𝑅𝑖2 .
∴ 𝑇ℎ𝑒 𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝑖𝑛𝑒𝑟𝑡𝑖𝑎 𝑜𝑓 𝑎𝑙𝑙 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑎𝑏𝑜𝑢𝑡 𝑧 𝑎𝑥𝑖𝑠 𝑖𝑠
Page 40 of 43
𝑛
𝑛
𝐼𝑧 = ∑ 𝑚𝑖 𝑅𝑖2 = ∑ 𝑚𝑖 (𝑥𝑖2 + 𝑦𝑖2 )
𝑖=1
=
𝑖=1
𝑛
∑ 𝑚𝑖 𝑥𝑖2
𝑖=1
𝑛
+ ∑ 𝑚𝑖 𝑦𝑖2
𝑖=1
𝐼𝑧 = 𝐼𝑥 + 𝐼𝑦
11.0 GRAVITATION
Gravitation is the tendency of bodies to move toward each other. Isaac Newton
proposed a force law that we call Newton’s Law of gravitation.
11.1 NEWTON’S LAW OF GRAVITATION
The Law states that: Every particle attracts any other particle with a
gravitational force whose magnitude is given by
𝐺𝑚1 𝑚2 𝑁𝑒𝑤𝑡𝑜𝑛′ 𝑠 𝐿𝑎𝑤 𝑜𝑓
(11.1)
(
) …
𝐹=
𝑟2
𝐺𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛
where𝑚1 and 𝑚2 are the masses of the particle, 𝑟 is the distance between them, and
𝐺 is the gravitational constant with a value 𝐺 = 6.67 × 10−11 𝑁. 𝑚2 /𝑘𝑔2 .
11.2 SUPERPOSITION PRINCIPLE
This is a principle that says the net force on a particle is the vectorial summation
of forces of attraction between the particle and individual interacting particles.
Mathematically, the principle is expressed as
𝑛
𝐹⃗1,𝑛𝑒𝑡 = ∑ 𝐹⃗1𝑖 = 𝐹⃗12 + 𝐹⃗13 + 𝐹⃗14 + ⋯ + 𝐹⃗1𝑛
…
(11.2)
𝑖=2
if there are 𝑛 interacting particles. Here, 𝐹⃗1,𝑛𝑒𝑡 is the net force on particle 1.
For extended object, we write the principle as
𝐹⃗1 = ∫ 𝑑𝐹⃗
…
(11.3)
Example 24:
Page 41 of 43
The figure shows an arrangement of three particles. 𝑚1 = 6𝑘𝑔, 𝑚2 = 𝑚3 = 4𝑘𝑔, and
distance 𝑎 = 2𝑐𝑚. What is the magnitude of the net gravitational force 𝐹⃗1𝑛𝑒𝑡 that
acts on particle 1 due to other particles.
Solution:
By superposition principle
𝐹⃗1𝑛𝑒𝑡 = 𝐹⃗12 + 𝐹⃗13
𝐹⃗12 = 𝐹12 𝑖̂ + 𝐹12 𝑗̂ = 𝐹12 𝑖̂
𝐹⃗13 = 𝐹13 𝑖̂ + 𝐹13 𝑗̂ = −𝐹13 𝑗̂
𝐺𝑚1 𝑚2 (6.67 × 1011 )(6)(4)
𝐹12 =
=
= 4 × 10−6 𝑁
2
2
(0.02)
𝑎
𝐺𝑚1 𝑚2 (6.67 × 1011 )(6)(4)
𝐹13 =
=
= 1 × 10−6 𝑁
(2𝑎)2
(0.04)2
𝐹⃗12 = 𝐹12 𝑖̂ = 4 × 10−6 𝑁𝑖̂
𝐹⃗13 = −𝐹13 𝑗̂ = −1 × 10−6 𝑁𝑗̂
𝐹⃗1𝑛𝑒𝑡 = 𝐹⃗12 + 𝐹⃗13 = 4 × 10−6 𝑁𝑖̂ + (−1 × 10−6 𝑁𝑗̂)
𝐹1𝑛𝑒𝑡 = √(𝐹12 )2 + (−𝐹13 )2
𝐹1𝑛𝑒𝑡 = √(4 × 10−6 )2 + (−1 × 10−6 )2
𝐹1𝑛𝑒𝑡 = 4.1 × 10−6 𝑁
Page 42 of 43
EXERCISE
(1)
Five particles are arranged as shown in the figure above. 𝑚1 = 8𝑘𝑔, 𝑚2 =
𝑚3 = 𝑚4 = 𝑚5 = 2𝑘𝑔, 𝑎 = 3𝑐𝑚,and 𝜃 = 30𝑜 . What is the gravitational force
𝐹⃗1𝑛𝑒𝑡 on particle 1 due to the other particles.
(2)
What must be the separation between a 5.2𝑘𝑔 particle and a 2.4𝑘𝑔 particle
for their gravitational attraction to have a magnitude of 2.3 × 10−12 𝑁?
(3)
In the figure below, two spheres of mass 𝑚 and a third sphere of mass 𝑀 form
an equilateral triangle. The net gravitational force on that central sphere from
the three other spheres is zero. (𝑎) What is 𝑀 in terms of 𝑚? (𝑏) If we double
the value of 𝑚4 , what is the magnitude of the net gravitational force on the
central sphere?
Page 43 of 43