CP PHYSICS
WORK, POWER, AND ENERGY REVIEW - Answers
1.
Name: __________________________
____ period
What is work?
Work is done when a force is applied to an object and that object moves in the
direction that the force is applied. (W = Fd)
2.
What conditions must be met to do work on an object?
The object must move in the direction that the force is applied.
3.
What are some examples of doing work on an object?
Lifting, pushing, pulling, kicking, throwing, dropping objects
4.
Can carrying an object be said to be doing work on that object? NO (work done on you)
5.
When lifting an object, what force is used? object’s weight
6.
What is power?
Power is the rate at which work is done. (How fast you do work.)
7.
Two students who have the same mass run from the Maroon gym lobby to the commons
area traveling the exact same distance. The first student takes 42 seconds to travel the
distance while the second student takes 37 seconds.
a. Which student does more work? Explain.
Both students do the same amount of work—they apply the same amount of
force over the same distance.
b. Which student has more power? Explain.
The second student has more power because he completes the work in a
shorter period of time.
8.
Why does a machine make work easier to do?
It multiplies your force and/or changes the direction of your force.
9.
What factor does a machine multiply?
Force
10.
What force do you apply to the machine?
11.
What force does the machine apply to the object it moves?
12.
What is the resistance force always equal to?
the object’s weight
13.
What is the work you do on a machine called?
input work
14.
What is the work done by the machine called?
effort force
output work
resistance force
CP PHYSICS, WORK, POWER, AND ENERGY REVIEW - Answers, page 2
15.
Identify the following simple machines and give examples of each:
a.
levers:
long rigid bar with support called a fulcrum
ex. crowbars, car jacks
b.
pulleys:
special lever with grooved wheel over which a rope passes
ex. window shade pull, block and tackle, well, flag pole
c.
wheel and axles:
special lever consisting of a wheel attached to an axle
ex. pencil sharpener, egg beater
d.
inclined planes:
flat surface with one end higher than the other
ex. ramp
e.
wedges:
inclined plane with either one or two sloping sides
ex. door stops, knives, axes, scissors, chisels
f.
screws:
an inclined plane which is spiral
ex. screw
16.
Work is the transfer of energy by mechanical means.
17.
What is kinetic energy?
18.
What is potential energy?
19.
What is the change in kinetic energy equal to?
energy of motion
stored energy
The change in kinetic energy is equal to the net work done on an object.
20.
What happens to the kinetic energy of an object falling towards earth? What happens to
its potential energy as it falls?
The object’s kinetic energy increases, and its potential energy decreases.
21.
What happens to the kinetic energy of an object thrown up into the air? What will happen
to its potential energy?
The object’s kinetic energy decreases as it moves upward while its potential energy
increases. At every point, its kinetic energy plus its potential energy equals the
total energy.
22.
If an object has 1250 J of potential energy at the top of a tall building, how much kinetic
energy will it have just before it hits the ground if it falls?
1250 J
23.
If you do 156 J of work to lift an object to a shelf at the top of a bookcase, how much
gravitational potential energy will the object have?
156 J
CP PHYSICS, WORK, POWER, AND ENERGY REVIEW - Answers, page 3
24.
Why is the first hill of a roller coaster always the tallest?
The first hill must be the highest because it stores the energy for the rest of the
ride. If it is the highest, then a maximum amount of potential energy is stored.
25.
If both a compact car and F250 pickup truck travel at the same velocity, do they have
the same kinetic energy? Explain.
No. The pickup truck will have a greater kinetic energy because it has a greater
mass than the compact car.
26.
What happens to the kinetic energy of an object if the object’s mass doubles?
2x the original kinetic energy
27.
What happens to the kinetic energy of an object if the object’s velocity doubles?
4x the original kinetic energy
28.
A 45 kg box of books is lifted to a bookshelf 1.75 m above the floor. How much
work was done to lift the books?
F = 45 kg x 9.8
= 441 N
d = 1.75 m
W=?
29.
A 58 N force is used to push a laboratory cart down a hallway that is 23 m long.
How much work is done on the cart?
F = 58 N
d = 23 m
W=?
30.
W = Fd
= (58)(23)
W = 1334 J
If the cart in #29 was pushed down the hallway in 78 seconds, how much power was
demonstrated?
W = 1334 J
t = 78 s
P=?
31.
W = Fd
= (441)(1.75)
W = 772 J
P = W/t
= 1334/78
P = 17.10 W
A student pushes a 850 kg compact car for a distance of 25.0 m in 58.0 seconds.
How much power does the student display?
F = 850 kg x 9.8
= 8330 N
d = 25.0 m
t = 58.0 s
P=?
P = W/t = Fd/t
= (8330 x 25.0)/58.0
P = 3590.52 W
CP PHYSICS, WORK, POWER, AND ENERGY REVIEW - Answers, page 4
32.
A force of 235 N is used to lift a 410 N object with a pulley. What is the
mechanical advantage of the pulley?
Fe = 235 N
Fr = 410 N
MA = ?
33.
MA = Fr/Fe
= 410/235
MA = 1.74
A mover pushes a 1275 N piano up a ramp that has a height of 1.95 m. The mover
use a force of 288 N to push the piano up the 7.4 m long ramp.
a. What is the ramp’s mechanical advantage?
Fe = 288 N
Fr = 1275 N
MA = ?
MA = Fr/Fe
= 1275/288
MA = 4.43
b. What is the ramp’s ideal mechanical advantage?
de = 7.4 m
dr = 1.95 m
IMA = ?
34.
What is the efficiency of a inclined plane which required 2100 J of input work to
produce 1750 J of output work?
Wo = 1750 J
Wi = 2100 J
Efficiency = ?
35.
Efficiency = Wo/Wi x 100%
= (1750/2100) x 100%
Efficiency = 83%
What is the kinetic energy of a bullet with a mass of 0.015 kg that moves with a
velocity of 400.0 m/s?
m = 0.015 kg
v = 400.0 m/s
KE = ?
36.
IMA = de/dr
= 7.4/1.95
IMA = 3.79
KE = ½ mv2
= ½ (0.015)(400.02)
KE = 1200 J
Find the potential energy given to the 50.0 kg hammer of a pile driver when it is
raised 4.00 m.
m = 50.0 kg
g = 9.8 m/s2
h = 4.00 m
PE = ?
PE = mgh
= (50.0)(9.8)(4.00)
PE = 1960 J