Download Energy, Work and Power

Chapter
7
Energy, Work and Power
Energy,
Work &
Power
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
1. How does our body get
energy?
YUMMY!!!
From Food.
2. Where do cars get energy?
From Petrol
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Txbk pg 5
Energy, Work and Power
What is Energy???
Definition:
Energy is the capacity to do work.
Without Energy = no light
no electricity
no water from tap
no bus, no train!!!


Without Energy = CLASS WILL BE VERY QUIET!!!!!!
 can’t talk, can’t walk, can’t breath!!!
Without energy, no living, non-living things would
work.
SO!!!!!!!!!!! Without energy: Everything dies  no life.
THEME TWO:
NEWTONIAN MECHANICS
Any7body or system that
canWork
do work
Energy,
and Power
possess Energy.
Chapter
D = distance
moved in the
same direction
as force applied
Energy
Defined as:
capacity to do WORK
FxD
1 Joule = 1 Newton meter
1 J = 1 Nm
THEME TWO:
NEWTONIAN MECHANICS
7
Qui
z
Chapter
Energy, Work and Power
Which of the following are forms of energy?
a.
b.
c.
d.
e.
Sound
Nuclear
Elastic Potential
Chemical Potential
Joules
Is Energy MATTER?
NO! Energy does not occupy space
and has no mass.
THEME TWO:
NEWTONIAN MECHANICS
Forms
of Energy
Chapter 7
Energy, Work and Power
kinetic
Potential
(stored)
Gravitational
Energies in Action
thermal
ENERGY
light
Elastic
sound
chemical
nuclear
electrical
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Kinetic
Energy
Energy, Work and Power
Energy a body possess due to its motion:
1
Ek = KE = mv2
2
Ek = KE (J)
m = mass (kg)
v = speed of the body (ms-1)
THEME TWO:
NEWTONIAN MECHANICS
30-second Quiz 1
Chapter
7
Energy, Work and Power
Usain Bolt holds the Olympic record of 9.69s
for his 100-m race. Assuming his mass is 70kg.
What is the kinetic energy KE that he possess?
D 100

Velocity=
= 10.32 m/s
t
9.69
KE
= ½ mv2
= ½ (70) (10.32)2
= 3727 J = 3730 J (3 s.f.)
THEME TWO:
NEWTONIAN MECHANICS
30-second Quiz 2
Chapter
7
Energy, Work and Power
A car with mass of 2000 kg is travelling
with a speed of 5 km/h on PIE in a jam.
What is its kinetic energy KE?
Velocity= 5 km/h =
KE
5 1000 m
60  60 s
= ½ mv2
= ½ (2000) (1.389)2
= 1929 J = 1930 J (3 s.f.)
THEME TWO:
NEWTONIAN MECHANICS
= 1.389 m/s
Chapter
7
Energy, Work and Power
Gravitational
Potential Energy
Energy a body has due to its position.
Ep = PE = mgh
Ep = GPE (J)
m = mass (kg)
g = gravitational field strength (N/m)
h = height (m)
THEME TWO:
NEWTONIAN MECHANICS
30- second Quiz 3
Chapter
7
Energy, Work and Power
A
box of mass 20 kg is being pushed up a slope of
15m long with constant speed of 30 m/s as shown in
Figure.
a)What is the gain in gravitational potential energy?
m = 20
g = 10 N/kg
h = 5m
15 m
5m
PE
= mgh
= 20(10)(5)
= 1000J
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
Other Types of Energy
• Substances that can be burnt contain chemical
potential energy.
• Thermal energy of an object = total kinetic
energy of the atoms or molecules in the object.
*** Heat is the transfer of thermal energy from one
body to another.
• Molecular kinetic energy is known as internal
energy.
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
COE
THEME TWO:
NEWTONIAN MECHANICS
Energy, Work and Power
7
7.2 Conservation of
Energy
Chapter
Energy cannot be created or
destroyed in any process, but only
changes from one form to another or
transferred from one body to another
Total amount of energy CONSTANT
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
Pile-driver for
constructing buildings
Conservation of Energy
BEFORE
Gravitational
energy
THEME TWO:
AFTER
Kinetic
energy
Thermal
energy and
Sound
NEWTONIAN MECHANICS
Amount
of energy
no
change
Energy, Work and Power
7
Conservation of
Energy
Chapter
• Conversion between PE and KE
For example, in the roller coaster,
As carriages move downwards,
– PE  KE.
As carriages move upwards,
– KE PE
Conservation_of_Energy_demolition_ball.wmv
THEME TWO:
NEWTONIAN MECHANICS
Energy, Work and Power
7
Conservation of
Energy
Chapter
KE = 0, PE = max
Both KE + PE
KE = 0, PE = max
KE = max,
PE = 0
Both KE + PE
Assume negligible
air resistance
THEME TWO:
NEWTONIAN MECHANICS
15 –second Quiz 4
Chapter
7
Energy, Work and Power
Conversion of energy
Which one of the following correctly describes the
energy conversion that occurs after a bungee
jumper jumps from the bridge to the instant when
the chord is extended to the maximum?
A. EPE  KE  GPE
B. GPE  KE  EPE
C. GPE  EPE  KE
D. KE  GPE  EPE
THEME TWO:
NEWTONIAN MECHANICS
ans B
EPE = elastic PE
KE = kinetic energy
GPE = gravitational PE
Electricity
for Singapore
Chapter 7
Energy, Work and Power
2) water turned into steam
under intense pressure.
steam
high
3) Turbine
pressure
4)
turned by
steam
Generator
steam
turbine
produces
electricity
air for
combustion
exhaust
gases
Oil or
gas
1) Fuel (oil
or natural
gas)
is burnt
THEME TWO:
water for
cooling
condenser
water
N E W T O N Water
I A N M runs
E C H Athrough
NICS
pipes to boiler
Energy, Work and Power
7
Conservation of
Energy
Chapter
• Worksheet 7 A Q3
• Text book Pg 127
• Try 7B Q 3 (3mins)
• Try 7B Q 4 (3mins)
THEME TWO:
NEWTONIAN MECHANICS
Chapter
1 –min Quiz 5i
7
Energy, Work and Power
An acrobat of mass 70 kg jumps down on to the
seesaw and lift his partner upward. (Assume
negligible air resistance and frictions Take g=10 N kg-1)
(i) Calculate the loss of gravitational potential
energy when the acrobat touches the see saw.
Loss of GPE = mgh
= 70 x 10 x 3
= 2100 J
THEME TWO:
NEWTONIAN MECHANICS
3m
1 –min Quiz 5ii
Chapter
7
Energy, Work and Power
An acrobat of mass 70 kg jumps down on to the
seesaw and lift his partner upward. (Assume
negligible air resistance and frictions Take g=10 N kg-1)
(ii) What is the speed of the acrobat just
before touching the see saw? loss in PE = 2100
Gain in KE
= loss in PE
½ mv2 = 2100
½ x 70 x v2 = 2100
v
THEME TWO:
= 7.75 m/s
NEWTONIAN MECHANICS
3m
1 –min Quiz 5iii
Chapter
7
Energy, Work and Power
An acrobat of mass 70 kg jumps down on to the
seesaw and lift his partner upward. (Assume
negligible air resistance and frictions. Take g=10 N kg-1)
(iii) Given that his partner has a mass of 60 kg,
how high would he reach?
By Conservation of Energy,
Amount of energy transferred to
partner = 2100 J
Gain in PEpartner
= Loss in KE
mgh = 2100
60 x 10 x h= 2100
h
= 3.5 m
THEME TWO:
NEWTONIAN MECHANICS
3m
Chapter
7
Energy, Work and Power
Green arrow
acceleration g
(no change)
Blue arrow Velocity
(highest at bottom,
lowest at top)
Brown arrow
Spring’s force when
stretched (greatest
at bottom)
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
GPE
GPE + KE
GPE + KE + Elastic PE
KE + Elastic PE (just
before max stretch)
Elastic PE (max stretch)
Green
Blue
arrow
T H E Marrow
E T W O : acceleration
N E W T O N I AgN M
ECH
A N I C S Velocity Brown arrow Tension
Energy, Work and Power
7
What have we covered so far??
Chapter
Give examples of the various form of energy
state the principle of the conservation of energy
Give the formula for calculating KE and GPE?
apply the relationships for KE and GPE to new
situations or to solve related problems
Next:
Apply relationship
Work Done = force x distance moved
in direction of force
to new situations or to solve related problems
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
7.3 Work
• Work is done when a force produces motion.
• Work = force × distance moved in the direction of
the force
• SI unit: joule (J).
W=F×d
Initial
position
F
d
Final
position
d must be in the direction that force F is applied
THEME TWO:
NEWTONIAN MECHANICS
10-second Quiz 6
Chapter
7
Energy, Work and Power
In which of the following cases is work done?
A.
B.
C.
D.
A person pushing a wooden box forward.
A person pushing the wall of a building.
A farmer carrying a bag of rice.
Two opposing teams of people
pulling a tug-of-war rope which
is stationary.
ans
WORK = F x D
THEME TWO:
NEWTONIAN MECHANICS
A
Chapter
7
Energy, Work
7.3 Work
and Power
• No work is done unless a force causes an object
to move in direction of applied force.
No work is
done!!
Wall did not
move even
though force is
applied
THEME TWO:
wall
NEWTONIAN MECHANICS
No work is done!!
Chapter
7
THEME TWO:
Energy, Work
7.3 Work
NEWTONIAN MECHANICS
and Power
Chapter
7
Energy, Work
7.3 Work
and Power
F
d = 1.5 m
Fr = 6N
Initial
position
On horizontal plane,
Force required
to move the block
Hence,
THEME TWO:
Final
position
=
Force to
overcome friction
= 6 N
Work Done = F x D
(D in direction of F)
= 6 x 1.5 = 9.0 J
NEWTONIAN MECHANICS
Chapter
7
THEME TWO:
Energy, Work
7.3 Work
NEWTONIAN MECHANICS
and Power
Chapter
Energy,7
Work
2 min- Quiz
7
and Power
An object of mass 20 kg is pulled up a slope of 15m long
with a constant speed. The height of the slope is 5m. The
frictional force between the object and the slope is 30 N.
a)What is the work done to overcome friction?
b)What is the total work done in pulling the object up the
slope?
Rope
15 m
5m
Fr = 30N
THEME TWO:
NEWTONIAN MECHANICS
Chapter
Energy, Work
7 2 min- Quiz
7
and Power
a)What is the work done to overcome friction?
b)What is the total work done in pulling the object up
the slope?
Rope
15 m
a) WD to overcome friction
= 30 x 15
Fr = 30N
= 450 J
b) WD to lift 20 kg up 5 m = mgh
= 20 x 10 x 5 = 1000J
Total WD
5m
= WD to overcome friction + WD to lift 20 kg up 5m
= 450 + 1000 = 1450 J
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
Work
Try 7B Q 7 ans :
a) 2400 J,
b) 2400 J (explain)
c) KE = 1200 J
Those finished, do
Q 9: ans a) 0N, b) 100N, c) 150J, e) 50W
Q11: ans a) 2 ms-2 b) 60J
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work
7.3 Work
and Power
• Work is done when a force produces motion.
• SI unit: joule (J).
W=F×d
WD against
another force
eg, 1. Against gravity pull
2. Against elastic forces
3. Against friction etc
THEME TWO:
NEWTONIAN MECHANICS
WD to change
object speed
Chapter
7
Energy, Work
7.3 Work
and Power
W=F×d
WD against another force
1. against gravitational force on object
Final
position
F
h
Initial
position
Force to overcome gravity pull,
F = W = mg
WD against gravity pull = F x d
=Wxh
= mgh
W = mg
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work
7.3 Work
W=F×d
WD against another force
2. against elastic forces
WD to stretch spring
= Fe x extension d
Fe
THEME TWO:
NEWTONIAN MECHANICS
and Power
Chapter
7
Energy, Work
7.3 Work
W=F×d
WD against another force
3. against friction
WD to overcome friction = Fr x d
Final
position
Initial
position
F
Fr
d
THEME TWO:
NEWTONIAN MECHANICS
and Power
Chapter
7
Energy, Work
7.3 Work
and Power
W=F×d
WD to change speed of object
Work done on object to change its speed.
F
Frictionless
u m/s
d
KEi = ½ mv2
THEME TWO:
NEWTONIAN MECHANICS
v m/s
KEf = ½ mv2
Chapter
7
Energy, Work
7.3 Work
and Power
W=F×d
WD against
another force
eg, 1. Against gravity pull
2. Against elastic forces
3. Against friction etc
THEME TWO:
NEWTONIAN MECHANICS
WD to change
object speed
Chapter
1- min Quiz 8
7
Energy, Work and Power
A bullet of mass 50g was travelling at a speed of
200ms-1 before striking a sandbag. It travelled
through 20cm of the sandbag before stopping.
What was the total resistive force produced by
the sandbag?
Sand
bag
Conservation Of Energy,
Loss in KE = WD by bullet to move
through 20 cm of sandbag
½ mv2 = F x d
½ (0.05) (200)2 = F x (0.20)
F = 5000 N
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
Work
Go through Wksht 7B
Q11: ans a) 2 ms-2 b) 60J
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 7.4
Energy, Work and Power
Power
70kg
70kg
Height
risen 10
m
Walking leisurely
Took 30 secs
Chased by dog
Took 1 sec
Feel more tired running upstairs
compared to walking upstairs.
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 7.4
Energy, Work and Power
Power
Defined as:
Rate of work done or
Rate of energy conversion
Refers to how fast work is done
or how fast energy is converted
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 7.4
Energy, Work and Power
Power
70kg
70kg
Walking leisurely
Took 30 secs
WD = F x d
= mg x h
= 7000 J
Height
risen 10
m
SAME WD
But running
took less
time then
walking!
Chased by dog
Took 2 secs
WD = F x d
= mg x h
= 7000 J
Work is done
N E W T Oslower
N I A N M E C H A N I C SWork is done faster
THEME TWO:
Chapter
7 7.4
Energy, Work and Power
Power
70kg
70kg
Height
risen 10
m
Walking leisurely
Took 30 secs
WD = 7000 J
SAME WD
But running
took less
time then
walking!
Work is done slower
Chased by dog
Took 2 secs
WD = 7000 J
Work is done faster
Running guy has more power!
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 7.4
Energy, Work and Power
Power
Defined as:
Rate of work done or
Rate of energy conversion
Power =
=
work done
time taken
Fxd
t
=
energy change
time taken
 in Energy
=
t
SI unit : watt (W)
• Other units: Joule per second (J s−1)
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 1-
min Quiz 9
Energy, Work and Power
A man of mass 60 kg takes 1 min to run up a flight
of stairs from X to Y as shown. What is his working
power?
A.60 J
B.80 J
ANS: C
C.60 W
D.80 W
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7 1-
min Quiz 9
Energy, Work and Power
A man of mass 60 kg takes 1 min to run up a flight
of stairs from X to Y as shown. What is his working
power?
Height h = 6 m, m= 60 kg, t = 1 min = 60s
Gain in GPE = mgh
= 60 x 10 x 6
= 3600 J
t = 60 s
THEME TWO:
Power = Δ in Energy =
Time
3600
60
=
60 W
NEWTONIAN MECHANICS
Chapter
71-
min Quiz 10
Energy, Work and Power
A girl weighing 400N runs up a flight of stairs of
vertical height 5 m in 4 s. What is her gain in
potential energy and the power developed?
Gain in GPE
THEME TWO:
Power developed
A. 1600 J
400 W
B. 2000 J
500 W
C. 16 000 J
4000 W
D. 20 000 J
5000 W
NEWTONIAN MECHANICS
ANS:B
Chapter
71-
min Quiz 10
Energy, Work and Power
A girl weighing 400N runs up a flight of stairs of
vertical height 5 m in 4 s. What is her gain in
potential energy and the power developed?
Height h = 5 m, W= 400 N, t = 4s
Gain in GPE = mgh
= 400 x 5
= 2 000J
t=4s
Power = Δ in Energy =
Time
THEME TWO:
NEWTONIAN MECHANICS
2000
4
= 500W
Chapter
71-
min Quiz 11
Energy, Work and Power
A windmill is used to raise water from a well. The
depth of the well is 5 m. The windmill raises 200 kg
of water every day.
What is the useful power extracted from the wind?
Height h = 5m, mass = 200 kg
0.116 W
WD in raising the water = mgh
= 200 x 10 x 5
= 1 x 104J
Everyday t = 24 x 60 x60 = 8.64 x 104 s
Work Done
1 x 104
Power =
=
Time
8.64 x 104 = 0.116 W
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
Efficienc
y
Useful Energy Output
Efficiency =
x 100%
Energy Input
THEME TWO:
NEWTONIAN MECHANICS
Work and Power
77.3 WorkEnergy,
and
Power
Try Worksheet
7C
Chapter
•
• Q3 ans: a) 200J, b) 20 W
• Q6 ans: 7.2 x 105 J, b) 3.6 x 104 W
Those finished can try
• Q8
• Q4 ans: a) 32J, b) 4 m/s, c) 1600 N
THEME TWO:
NEWTONIAN MECHANICS
7
Challenge
Chapter
Energy, Work and Power
Where did all these energy come
from?
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
SUMMARY
Energy, Work and Power
Conservation
of Energy
FxD
is defined as
ENERGY (J)
is defined as the
Capacity to do:
WORK (J)
is in the forms of
is related to
Others:
Thermal
Magnetic
Nuclear
etc
PE
KE
Change in Energy
Work Done
Time
Time
defined as
Energy body
has due to
its state,
shape or
position
GPE = mgh
THEME TWO:
is related to
Energy
body has
due to its
motion
KE = ½mv2
NEWTONIAN MECHANICS
OR
is defined as
POWER (W)
Chapter
7
Energy, Work and Power
Green arrow
acceleration g
(no change)
Blue arrow Velocity
(highest at bottom,
lowest at top)
Brown arrow
Spring’s force when
stretched (greatest
at bottom)
THEME TWO:
NEWTONIAN MECHANICS
Chapter
7
Energy, Work and Power
GPE
GPE + KE
GPE + KE + Elastic PE
KE + Elastic PE (just
before max stretch)
Elastic PE (max stretch)
Green
Blue
arrow
T H E Marrow
E T W O : acceleration
N E W T O N I AgN M
ECH
A N I C S Velocity Brown arrow Tension