Download Chapter 4: Forces and Newton`s Laws of Motion

Chapter 5: Forces and
Newton’s Laws of Motion
 Previously, we have studied kinematics, which
- describes the motion of an object (x, v, a)
- does not explain the cause of the motion
 Now, we begin the study of the second part of
mechanics – dynamics
- which does address the cause of motion
- that cause is a force, a push or pull
 Force, F, is a vector, has magnitude and direction
 How forces affect the motion of an object is described
by Newton’s Laws of Motion (Newtonian Mechanics)
 Objects are treated as point particles; in Chapter 10 we
will consider the shape of an object
Newton’s First Law of Motion
 An object at rest will remain at rest
 An object moving at a constant velocity will continue to
move at the constant velocity, unless acted upon by a net
force
 What does it mean?
- tendency for an object’s motion not to change
 Net force = the sum of all applied forces
   
 Fi  F1  F2  F3  0
i
- No effect on the motion
F3
F1
F2
According to the 1st law, zero velocity (at rest) is
equivalent to constant velocity
An object with a constant velocity does not require a
force to maintain its velocity
- forces act to change motion, not sustain (e.g., the
space shuttle)
- seems contrary to everyday experience
 Inertia – tendency for an object to remain at rest, or to
remain in motion with a constant velocity
- all objects have inertia
 Mass – a quantitative measure of inertia (a scalar)
- use symbol m - unit is kg (SI) or slug (British)
- more mass, means more inertia
- not equivalent to weight (a force)
Newton’s Second Law of Motion
 If there is a net force, there is a change in velocity (an
acceleration)



 Fi   F  ma
i
 1st law implies the 2nd law
 Meaning: if a net external force acts on an object of
mass m, it will be accelerated and the direction of the
acceleration will be in the same direction as the net

force
F1
F
 F
a
m
F3
F2
a
The Free Body Diagram (FBD)
 A schematic representation of an object and all
the external forces that act upon it
 Always draw in every problem
 From Newton’s 2nd law:


F  ma  0



Fgrav  Ftable  0


Ftable   Fgrav

Ftable

Fgrav
Book at
rest on
the table
Newton’s Third Law of Motion
 The first two laws deal with a single object and
the net forces applied to it
- but not what is applying the force(s)
 The third law deals with how two objects
interact with each other
 Whenever one object exerts a force on
a second object, the second object exerts
a force of the same magnitude, but
opposite direction, on the first object
Astronaut, ma

Fs

Fa
Space
station, ms
Third law says: force astronaut applies to space
station, Fs, must be equal, but opposite to force space
station applies to astronaut, Fa
FBD



Fs  Fa  F

Fa
aa 




Fa  ma aa  aa  Fa / ma   F / ma





Fs  ms as  as  Fs / ms  F / ms
Since
ma  ms  aa  as
Fundamental Types of Forces
1. Gravitational
2. Electromagnetic – (electric and magnetic)
3. Weak Nuclear
Electroweak
4. Strong Nuclear
We will only consider the first two
Gravitational Force
From our studies of free-fall motion and projectile
motion  gravity causes an object to accelerate in the
negative y-direction
y

a y   g yˆ
y

Apply the
second law
m

Fgrav  mg yˆ
 This is only an approximation which holds only near the
surface of the Earth (as g is only constant near the
surface). But a good approximation!
 We would like a more fundamental description of gravity
- g is an empirical number
- physicists don’t like empirical numbers
 This lead Newton to devise his Law of Universal
Gravitation
Chap. 12. Law of Universal
Gravitation (12.1,12.2)
 Every object in the Universe exerts an attractive force
on all other objects
 The force is directed along the line separating two
objects
 Because of the 3rd law, the force exerted by object 1
on 2, has the same magnitude, but opposite direction, as
the force exerted on 2 by 1
m1
 
F12 F21
r
m2


F12   F21
By 3rd law
where
Gm1m2
F12 
2
r
And G  Universal Gravitational Constant
= 6.67259 x 10-11 N m2/kg2
 G is a constant everywhere in the
Universe, therefore it is a fundamental
constant
 g is not a fundamental constant, but we
can calculate it. Compare:
F  mg
and
Gm1m2
F12 
2
r
Let m1 = ME = mass of the Earth,
m2 = m = mass of an object which is << ME,
r = RE , object is at surface of the Earth,
Set the forces equal to each other:
m
GM E m
mg 
RE2
ME
GM E
g
RE2
g
(6.67259x10
-11 Nm 2
kg 2
RE
)(5.9742x10 24 kg)
6
(6.378x10 m)
2
 9.80 sm2
 Weight
 mass
 Weight - the force exerted on an object by the Earth’s
gravity
F
= mg = W
 Mass is intrinsic to an object, weight is not
 From previous page, W=m(GME/RE2)
- your weight would be different on the moon
 Gravity is a very weak force, need massive objects
 Units of force F = ma [M][L/T2]
in SI - kg m/s2 = Newton, N
in BE - slug ft/s2 = pound, lb