Download Review (Key): Work, Energy, Power

Name: ____________________
Physics 670
Date:____/____/____
Review (Key): Work, Energy, Power
Work
1. What are the two important variables for work? (hint: look at the equation!)
The important variables for work are distance and force.
2. Is work done in the following situations?
A. Is work done if you lift a 2 N book straight up 2 meters at a constant speed?
Yes, work is done because the force and the movement are in the
same direction.
B. Is work done if you pull a wagon 20 meters with a force of 10 N?
Yes, work is done because the force and the movement are in the
same direction.
C. Is work done if you spend 5 hours studying for a quiz?
No, work is not done; there is no movement.
D. Is work done if you carry a 5 N tray a horizontal distance of 10 meters at a constant
speed?
No, work is not done because the applied force is not in the same
direction as the motion of the tray.
E. Is work done if you push as hard as you can against a cement wall for two minutes
but it doesn’t move?
Yes, work is done because the force and the movement are in the
same direction.
3. How much work is done if you push a crate with a constant force of 20 N for 10
meters?
F = 20 N
W=F•d
d = 10 m
W = 20 • 10
W=?
W = 200 J
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4. How much work is done if you carry a 10 N box a horizontal distance of 25 meters?
No work is done because the force is not in the same direction as
movement of the box.
Power
5. Who is more powerful: someone who can do 100 J of work in 5 seconds or someone
who can do 100 J of work in 50 seconds? Explain.
Someone who can do 100 J of work in 5 seconds is more powerful
because he does the same work in less time (he is faster).
6. Who is more powerful: someone who can do 10 J of work in 10 seconds or someone
who can do 100 J of work in 10 seconds?
Someone who can do 100 J of work in 10 seconds is more powerful
because he does more work in the same amount of time.
7. What is your power output if you can do 600 J of work in 15 seconds?
W = 600 J
P =
t = 15 s
P =
P=?
!
W
t
600
15
P = 40 W
!
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Complete the table below:
Force
(N)
Data
Distance
(m)
Time
(s)
Work (J)
Show your work in this
column
Work and Answers
Work
Power (W)
(J)
Power
(W)
Show your work in this
column
W=F• d
Record
Final
Answer
Below
50 J
P = 50/2
25 W
P=W/t
Record
Final
Answer
Below
EX
10 N
5m
2s
W = 10 • 5
8.
20 N
5m
2s
W = 20 • 5
100 J
P = 100/2
50 W
9.
20 N
10 m
2s
W = 20 • 10
200 J
P = 200/2
100W
10.
40 N
5m
4s
W = 40 • 5
200 J
P = 200/4
50 W
Energy
11. What does kinetic energy depend upon? (choose between: speed or height)
Kinetic energy depends on the speed of the object.
12. What does potential energy depend upon? (choose between: speed or height)
Potential energy depends on the height of the object.
13. Can an object that is at rest have energy? Give an example to support your answer.
An object that is at rest can have energy. For example, a book at
rest on top of a table has gravitational potential energy.
14. What kind of energy does an apple that is hanging (at rest) in a tree have?
An apple that is hanging in a tree has gravitational potential energy.
15. What kind of energy does an apple that has fallen out of a tree have just before it hits
the ground?
The apple has kinetic energy just before it hits the ground.
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16. Which has more potential energy: a 25 N kitten that is sitting on the counter 1 m above
the ground or a 50 N cat who is also 1 m above the ground?
The 50 N cat who is 1 m above the ground has more potential
energy because he is heavier.
17. Which has more potential energy: a 50 N cat that is 1 m above the ground or a 50 N cat
who has climbed a tree and is 10 m above the ground?
The 50 N cat who is 10 m above the ground has more potential
energy because he is higher.
18. Find the equation for kinetic energy on your MCAS formula sheet and copy it into the
space below:
KE =
!
1
2
mv 2
19. Which has more kinetic energy: a mouse who is running at 2 m/s or a cat who is
running at 2 m/s?
The cat who is running at 2 m/s has more kinetic energy because he
has more mass.
20. Which has more kinetic energy: a 5 kg cat who is walking at 1 m/s or a 5 kg cat who is
running at 3 m/s?
The cat who is moving at 3 m/s has more kinetic energy because he
is moving faster.
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MCAS Questions: The following questions are taken from past MCAS exams. Please
circle the correct answer for multiple choice questions.
21. (2004, grade 9, #17)
A skateboarder travels from location 1 to location 4 as shown below
At which location does the skateboarder have the most kinetic energy and the least
potential energy?
A. 1
B. 2
C. 3
D. 4
22. (2005, grade 8, #9)
The diagram below represents a diver’s motion from the top of a high diving board into
a pool of water.
At which labeled point does the diver have the least potential energy?
A. 1
B. 2
C. 3
D. 4
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23. (2004, grade 9, #12)
At a weightlifting competition, two competitors lifted the same weight to the same
height. The second competitor accomplished the lift in 2 seconds faster than the first
competitor. This demonstrated that the second competitor had more
A. energy than the first.
B. inertia than the first.
C. power than the first.
D. work than the first.
24. (2004, grade 9, #2)
An archer pulls back the bowstring to prepare to shoot an arrow as shown below.
She uses an average force of 40 N, moving the bowstring 0.2 m. How much energy is
stored in the bow?
A. 8 J
B. 16 J
C. 24 J
D. 36 J
25. (2005, grade 9, #19)
A man ran on a treadmill for 1,800 seconds. At the end of his run, the treadmill
indicated his energy output as 240,000 J. What average power did he generate?
A. 666 W
B. 133 W
C. 66 W
D. 7 W
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26. (2006, grade 9, #44)
An astronaut drops a 1.0 kg object and a 5.0 kg object on the Moon. Both objects fall a
total distance of 2.0 m vertically. Which of the following best describes the objects
after they have fallen a distance of 1.0 m?
A. They have each lost kinetic energy.
B. They have each gained the same amount of potential energy.
C. They have each lost the same amount of potential energy.
D. They have each gained one-half of the maximum kinetic energy.
27. (2006, grade 9, #24)
The figure below shows a wagon that moves from point X to point Y.
Which of the following best describes the wagon’s change in energy as it coasts from
point X to point Y?
A. The wagon has the same kinetic energy at point Y and a point X.
B. The wagon has more kinetic energy at point Y than at point X.
C. The wagon has the same gravitational potential energy at point Y and at point X.
D. The wagon has more gravitational potential energy at point Y than at point X.
28. (2006, grade 9, #31)
Which of the following describes the mechanical energy of a cart at rest at the top of a
steep hill?
A. The cart has no mechanical energy.
B. The cart’s mechanical energy is all kinetic.
C. The cart’s mechanical energy is all potential.
D. The cart’s mechanical energy is half potential and half kinetic.
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The next item is an OPEN RESPONSE QUESTION.
BE SURE TO ANSWER AND LABEL ALL PARTS OF THE QUESTION.
Show all your work (diagrams, tables, or computations) in the space
provided.
• If you do the work in your head, explain in writing how you did the work.
29. (2005, grade 9, #8)
The figure below shows a tennis ball bouncing from point 1 to point 5.
•
•
The tennis ball bounces up from point 1 as shown in the figure to a maximum height
labeled as point 2. The ball then bounces a few times. Neglect any horizontal motion.
A. Describe the kinetic and gravitational potential energy changes of the ball that occur
between points 1 and 2.
At point 1, there is more kinetic energy than potential energy. At
point 2, there is a lot of potential energy. The kinetic energy of the
ball at point 1 is transformed into potential energy at point 2.
B. Describe the kinetic and gravitational potential energy changes of the ball that occur
between points 2 and 3.
Most of the potential energy at point 2 is transformed into kinetic
energy at point 3.
C. Compare the kinetic energy of the ball at points 3 and 4.
The ball has more kinetic energy at point 3 than at point 4.
D. The tennis ball has less energy at point 5 than it had at point 3. Explain what
happened to the energy the ball had at point 3.
Some of the energy is being transformed into other forms of
energy, such as heat and sound.
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