1 chapter 4 work & energy dr. ali. 2 chapter outline work work power power energy and its...
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1
Chapter 4
Work & Energy
Dr. Ali
2
CHAPTER OUTLINE
Work Power Energy and Its Forms Kinetic Energy Potential Energy Conservation of Energy
3
WORK
Who is doing more work?
4
WORK
Work is defined as the product of the net force acting on a body and its displacement (in the direction of the force).
Work = force x parallel distance
W = F x d
Unit of Energy: 1 Joule = 1 J = 1 Nm = 1 kgm2/s2
5
WORK
Only the component of the applied force, F, in the direction of the motion of the lawn mower, FHorizontal, is used to do work on the lawn mower.
For work to be done, force and motion must be in the same direction
6
Example 1:
An object is moved with a force of 15 N across a horizontal surface. How much work is done if the object is moved 50 m?
F = 15 N
d = 50 m
W = ???W = 750 J
W = F x d
W = (15 N)(50 m)
7
Example 2:
650 J of work is done in moving a desk a horizontal distance of 5 m. How much force is used to move the desk?
WF =
dF = ???
d = 5 m
W = 650 J
650 J=
5 m
F = 130 N
8
Example 3:
How much work is done in lifting a 10 kg box 1.5 m off the floor?
m = 10 kg
d = 1.5 m
F = ???
W = ???W = 150 J
F = w
W = F x d
= m x g
F = 10 kg x 10 m/s2 = 100 N
= 100 N x 1.5 m
9
Example 4:
How much work is done while walking 5.0 m holding an object with mass of 3.0 kg?
No work is being done, since force and motion are not in the same direction.
10
POWER
Power is the rate at which work is done.
workPower=
time
SI Unit for Power: 1 watt = 1 J/s
11
Example 1:
A force of 150 N is used to push a motorcycle 10 m along a road in 20 s. Calculate the power in watts.
F = 150 N
d = 10 m
t = 20 s
P = ???P = 75 watts
150 N x 10 m=
20 s
12
Example 2:An 80 kg man runs up a flight of stairs 5.0 m high in 10 seconds. What is the man’s power output in watts?
W F x dP = =
t t
m = 80 kg
d = 5.0 m
t = 10 s
P = ???P = 400 watts
F = w
F = 80 kg x 10 m/s2 = 800 N
800 N x 5.0 m=
10 s
= m x g
13
Example 3:
A pump lifts 30 kg of water a vertical distance of 20 m each second. What is the power output?
W F x dP = =
t t
m = 30 kg
d = 20 m
t = 1 s
P = ???P = 6000 watts
F = w = m x g
F = 30 kg x 10 m/s2 = 300 N
300 N x 20 m=
1 s
14
Example 4:
A crane uses 750 kwatts of power to lift a car 0.5 m in 12 seconds. How much work is done?
P = 750 kwatts
d = 0.5 m
t = 12 s
W =??? W = 9000 kJ
W = P x t
= 750 kwatts x 12 s
15
Example 4:
A crane uses 750 kwatts of power to lift a car 0.5 m in 12 seconds. How much work is done?
WF =
dW = 9000 kJ
d = 0.5 m
F =??? F = 18000 kN
What force is used by the crane?
90000 kJ=
0.5 m
16
Example 5:
Which person does more work, 1 or 2?
Which person has greater power, 1 or 2?
1 2
F1 = F2 d1 = d2
W1 = W2
t1 > t2
P1 < P2
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ENERGY
Energy is the ability to do work
The SI units for energy is joules (J).
The brick and the hammer possess
energy and thus can do work on the nail
1J = 1 joule = 1 kgm2/s2
18
ENERGY
There are two types of energy here:
KineticPotential
Energy of motion
Stored energy
19
ENERGY
Energy can be converted from one type to another.
Kinetic E
Potential E
Kinetic & potential
20
KINETICENERGY
Kinetic energy is energy of motion.
21KE = m v
2
KE is a scalar quantity
(Note: Energy is never a vector)
Velocity has greater effect on KE than mass
21
Example 1:
What is the kinetic energy of a 60 kg girl on skis traveling at 20 m/s?
21KE = mv
2m = 60 kg
v = 20 m/s
KE = ???
KE = 12000 J
21= (60 kg)(20 m/s)
2
22
Example 2:
A sports car is moving at 4.0 m/s. If the mass of the car is 800 kg, how much KE does it have?
21KE = mv
2m = 800 kg
v = 4.0 m/s
KE = ???
KE = 6400 J
21= (800 kg)(4.0 m/s)
2
23
Example 3:
Two identical cars are moving, one with twice the velocity of the other. How much more kinetic energy does the faster car possess?
21KE = mv
2v2 = 2 v1
Faster car has 4 times greater KE
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POTENTIALENERGY
Potential energy is stored energy.
The compressed spring has potential
energy because when released it can do
work on the mass, m.
25
POTENTIALENERGY
Gravitational potential energy is energy of position.
Potential energy = Weight x height
PE = m g h
Low PE
High PE
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Example 1:
A mass of 100 kg is lifted a distance of 50 m. How much PE does it possess?
m = 100 kg
h = 50 m
PE = ??? PE = 50000 J
PE = m g h
= (100 kg) (10 m/s2) (50 m)
27
Example 2:
A 70-kg diver standing on a diving platform possesses 35000 J of PE. How high is the platform?
PEh =
mgm = 70 kg
h = ???
PE = 35000 J h = 50 m
2
35000 J=
(70 kg)(10 m/s )
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CONSERVATIONOF ENERGY
The sum of KE and PE in a system is constant, in the absence of friction.
KE + PE = constant
KE + PE = ETotal
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CONSERVATIONOF ENERGY
Energy cannot be created or destroyed
It may be transformed from one form to another
PE
KE
ETotal in a system remains constant.
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Example 1:
A 10-kg boulder rests at the edge of a 100-m cliff. How much PE does the rock possess?
m = 10 kg
h = 100 m
PE = ???PE = 10000 J
PE = m g h
= (10 kg) (10 m/s2) (100 m)
31
Example 1:The rock rolls off the cliff and falls to the bottom. How much KE does the rock possess at the bottom of the cliff?
KEbottom = PEtop = 10000 J
32
Example 1:
What speed does the rock have just before hitting the ground?
m = 10 kg
v = ???
KE = 10000 J
Thus, v = 45 m/s
21KE= mv
2
v2 = 2000 m2/s2
33
Example 2:
A 60-kg boy and his sled (neglect the mass of the sled) are at a 10 m high slope. How much PE do the boy and sled possess?
m = 60 kg
h = 10 m
PE = ???PE = 6000 J
PE = mgh
= (60 kg)(10 m/s2)(10 m)
34
Example 2:
What kinds of energy, and how much of each, do they possess halfway down the slope?
PE = KE = 3000 J
PE = KE
@ halfway
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THE END
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