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Energy Unit Review
On a separate sheet of paper write full solutions to the following problems.
1. How much work must be done to lift a 20 kg sack of potatoes vertically 6.5 m?
[1.3 x 103 J]]
2. A 100kg cart is moving at 2.3m/s. A machine has been designed to apply a force
of 1200N over a distance of 0.52m as this cart passes by. How fast will the car
now move?
3. The motor of an elevator can produce 2200 W of power. The elevator has a mass
of 1100 kg complete with contents. How long will it take to raise the elevator 10
m?
[49.0 s]
4. A car, starting from rest, experiences a driving force of 2000 N and frictional
forces of 600 N. If the car is 1500 kg what is its speed after traveling 20 m?
[6.11 m/s]
5. A sprinter generates 1000 W of power while accelerating from rest. If the sprinter
has a mass of 65 kg and accelerates for 2.5 s determine the final speed of the
sprinter.
[8.77 m/s]
6. At the moment when a shotputter releases a 5.00 kg shot, the shot is 2.00 m above
the ground and traveling at 15.0 m/s. It reaches a maximum height of 8.00 m
above the ground and then falls to the ground. Assume that air resistance is
negligible.
a. What was the potential energy of the shot as it left the hand, relative to the
ground?
b. What was the kinetic energy of the shot as it left the hand?
c. What was the total mechanical energy of the shot as it left the hand?
d. What was the total mechanical energy of the shot as it reached its
maximum height?
e. What was the potential energy of the shot at its maximum height?
f. What was the kinetic energy of the shot at its maximum height?
g. What was the kinetic energy of the shot just as it struck the ground?
[98.0 J, 563 J, 661 J, 661 J, 269 J, 392 J, 661 J]
Energy Unit Review
7. Some children go tobogganing on an icy hill. They start from rest at the top of the
hill as shown in the diagram. The toboggan and children have a combined mass
of 90 kg. If friction is small enough to be ignored, determine:
a. The total mechanical energy of the toboggan at A.
b. The speed of the toboggan at B.
c. The speed of the toboggan at C.
A
C
10 m
3.0 m
B
[8.8 x 103 J, 14 m/s, 12 m/s]
8. Several friends use a simple rope and pulley to raise a tree house from the ground
into a tree. The mass of the tree house is 150 kg. By pulling together, the friends
manage to exert an average force of 1.6 x 103 N as they raise the tree house a
distance of 3.2 m above the ground.
a. Find the work done in the raising the tree house.
b. How much “useful work” is done?
c. What is the efficiency of the rope and pulley is raising the tree house?
d. Suggest why the efficiency of this simple machine is not 100%.
[5.1 x 103 J, 4.7 x 103 J, 92%]
9. A container factory uses 370 W motor to operate a conveyor belt that lifts
containers from one floor to another. To raise 250 kg a vertical distance of 3.6 m,
the motor runs for 45 s.
a. Determine the useful energy output.
b. How much energy does the motor use?
c. What is the efficiency of the motorized conveyor system?
[8.8 x 103 J, 1.7 x 104 J, 52%]
10. After 2.0 kg of mercury gained 2.52 x 104 J of heat energy, its final temperature
was 130 C. What was its initial temperature?
[40 C]
11. When doing an experiment to find the specific latent heat of fusion of liquid
nitrogen, you find that 100kJ of heat must be removed to freeze 3.8kg of nitrogen
at its freezing point. What value will you get for the specific latent heat of fusion
of nitrogen?
Energy Unit Review
12. A 0.20 kg piece of copper at 375.0oC is submerged in 0.50 kg of water at 18.0oC
to be cooled quickly. Determine the final temperature of the copper and water.
13. How much heat does a refrigerator freezer have to remove from 0.10 kg of water
at 22.0oC to make ice at -4.0oC?
Application Questions:
1) How is the human body an energy transformation machine? Explain using
specific types of energy.
2) Two students move two identical boxes onto a platform. If one pushes it up a
50m frictionless ramp and the other lifts it, who does more work? Consider the
energy of the box. How would your answer change if there was friction.
3) How does a roller coaster take advantage of conservation of energy to function?
Explain.
4) How does a dam/solar panel/wind turbine transform energy?
5) An arrow launched straight up stops at the top, a student points and says “look, it
has no energy, the law of conservation of energy is wrong”. Explain to the
student their misunderstanding. When the arrow falls down again, it reaches the
height it was released moving at the same speed it was released at. Explain why
this makes sense.