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Honors Physics
Unit 5 Review Guide – Work and Energy
Test Format
 10 multiple choice problems (2 points each)
 5 short problems (6 points each)
 1 explanation problem (10 points)
 1 bar chart problem (10 points)
 2 medium problems (15 pts each)
Topics Covered
 Energy
o Various types of energy
o Kinetic energy
o Gravitational potential energy
 Choosing a baseline
o Elastic potential energy
 Work
o Relationship with force and distance
o Perpendicular forces do no work
o Sign of the work
 Work – Energy
o Connection between work and kinetic energy
o Doing work  more KE
o Negative work  less KE
 Conservation of Mechanical Energy
o No frictional forces  KE + PE conserved
o Identifying initial and final states
Practice Problems
Multiple Choice:
1. Which of the following situations is there positive work being done?
a. lifting an object
b. dropping an object
c. pushing an object
d. all of the above
2. If I were to double the velocity of an object, what happens to its kinetic energy?
a. doubles
b. quadruples
c. halves
d. quarters
3. I throw a ball into the air. When does its potential energy reach a maximum?
a. when it leaves my hand
b. at the top of the trajectory
c. when it reaches the ground d. potential energy is same everywhere
4. Two people throw an object from the same height with the same speed. Object A is thrown up and
object B is thrown down. Which one is moving faster right before it hits the bottom?
a. Object A
b. Object B
c. same time
d. depends on the masses of the object
5. A person swings on a pendulum. What is the correct explanation for the height getting slightly
smaller and smaller during each swing?
a. You keep turning potential energy into kinetic energy in each successive swing thereby losing
height each time
b. Gravity is doing negative work on the system, thereby causing a decrease in kinetic energy
and therefore a decrease in height
c. There is friction and/or air resistance present which causes a small loss of mechanical energy
during each swing.
d. IT’S A LIE. Mechanical energy is always conserved and the pendulum will swing at the same
height FOREVER.
Short Problems:
6. A 2 kg object is 10 meters in the air and moving to the left at 1 m/s. What is its potential and
kinetic energies?
7. A platform diver has a mass of 50 kg. She steps off a diving board that is elevated to a height of
10 meters above the water. How much kinetic energy will the diver have right before she hits the
water?
8. A toy gun uses a spring with a spring constant of 40 N/m that is compressed 0.15 meters. How
fast can it launch a 0.5 kg projectile?
9. Calculate the work required lift a 2.5-kg object to a height of 6.0 meters.
10. A 10 kg block is sliding along a rough ground with a constant force of friction of 100 N. If it starts
with a speed of 20 m/s, how far does the block travel before stopping?
Explanation Problem
11. Describe the energy transformations that occur while an object is loaded onto a spring which is
compressed and then fired up into the air before landing back on the ground.
Bar Chart Problem
12. Draw a bar chart representing the energies at the various stages of the object described in the
previous problem. The actual numerical values are not required, just the relative sizes of the bars. Draw
the bar charts for the kinetic energy, elastic potential energy and gravitational potential energy at the
very beginning, the top of the motion and right before it hits the ground
Medium Problems:
13. A ball with a mass of 2 kg starts at rest 3 meters above the ground and then slides down a smooth
curved track before going back up the other side
a. (2 pts) What is the initial kinetic energy?
b. (3 pts) What is the initial potential energy?
c. (6 pts) What is the speed of the ball at the bottom of the track?
d. (4 pts) How far up the other side of the track does the ball travel?
14. A 800 kg car travelling at a constant speed, slams on its breaks and comes to a stop over a distance
of 25 m. The force of friction between the car and the road is a constant 2000 N.
a. (4 pts) What is the work done by friction while the car is stopping?
b. (6 pts) Using the work-kinetic energy theorem, determine the initial speed of the car.
c. (5 pts) If the initial speed of the car were twice as fast as you got in part b), what would be
the new stopping distance of the car?
Practice Problems Key
1.
2.
3.
4.
5.
D
B
B
C
C
6. PE = 196 J, KE = 1 J
7. PE = 4900 J so the final KE = 4900 J
8. v = 1.34 m/s
9. W = 147 J
10. d = 20 m
11. Starts with elastic potential energy because spring is compressed
Turns into kinetic energy as it leaves the spring
Kinetic energy turns into potential energy as the object gains height until reaching a maximum
Potential energy turns back into kinetic energy as the objects falls to the ground
12.
KE
PEg
PEe
Initial
13. a.
b.
c.
d.
Top
Ground
0
58.8 J
v = 7.67 m/s
goes back up to same height = 3 m
14. a. W = -fd = -50,000 J
b. v = 11.2 m/s
c. stopping distance would quadruple = 100 m