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Energy Review Questions
1. Work is being done on the textbook when a student
a. sits in a chair holding a 0.15kg textbook.
b. lifts a 0.15kg textbook from the floor to a desk.
c. walks across the classroom holding a 0.15kg textbook.
d. leans against a 0.15kg textbook that is sitting on a desk.
2. The engine of a car exerts a force of 452N when moving at a constant velocity. If
the car maintains a constant velocity for a distance of 52.0km,
a. what is the work done against friction? [2.35x107 J]
b. what is the net work done? [0.00 J]
3. What is the work done on a 50.0kg object to accelerate if from rest to 5.00m/s
along a 3.00m horizontal frictionless surface? [625 J]
4. A student pulls on a box with a force of 95.0N using a rope that makes an angle
of 35.0o with the floor. What work is done in moving the box a distance of
26.0m? [2.02x103 J]
5. What is the potential energy stored in a spring with a spring constant of 125N/m
when it is compressed 0.100m? [0.625 J]
6. A force of 125N is needed to stretch a spring 6.30cm. Calculate the force
needed to compress the same spring 2.50cm. [49.6 N]
7. When a 5.0kg mass is hung from a spring, it stretches 11cm. How much elastic
energy is stored in the spring when the mass is hung from it? [2.7 J]
8. Calculate the kinetic energy of a 925kg car travelling at a speed of 90.0km/h.
[2.89x105 J]
9. An object travelling at a certain speed has a kinetic energy of Ek. What is the
new kinetic energy when the speed is tripled and the mass is halved? [9/2 Ek]
10. A 27.0kg object has 190J of potential energy. Calculate the height of the object.
[0.717 m]
11. A heavy object was dropped from some distance above the ground. When it
reached the ground, it had a speed of 10.0m/s. From what height was the object
dropped? [5.10 m]
12. A 2.0kg object is thrown vertically downward at a speed of 5.0m/s from a height
of 12.0m above the ground. Calculate the speed of the object as it hits the
ground. [16 m/s]
13. A heavy object is lifted vertically from the ground at a constant speed of 1.2m/s
for 2.5s and then it is dropped. At what speed does the heavy object hit the
ground? [7.7 m/s]
14. A roller coaster is travelling along a frictionless track as
shown in the diagram. If the speed of the roller coaster at
point A is 3.0m/s and the speed at point B is 13m/s, what
is the height of the roller coaster at point B? [6.8 m]
15. A 3.50kg object is dropped from a height of 0.50m onto a vertical spring with a
spring constant of 2.0x103 N/m. What is the maximum compression of the
spring? [0.13 m]
16. A compressed spring, with a spring constant of 145N/m, is used to fire a dart
from a dart gun. If the spring in the dart gun is compressed 0.10m, what is the
maximum horizontal speed of the 10.0g dart? [12 m/s]
17. A 12.0kg box, initially at rest, slides 50.0m along a 40.0 o
ramp as shown in the diagram to the right. The speed of
the box at the bottom of the ramp is 18.4m/s.
a. Is the system isolated or not?
b. Explain the energy transformations that take place.
18. A 60.0kg student runs at a constant speed up the incline as shown in the
diagram below. If it takes the student 4.5s to climb to the top of the incline,
calculate the power done by the student. [7.8x102 W]
19. A 1.00x103 kg object is raised vertically at a constant velocity of 4.00m/s by a
crane. What is the power output of the crane is the object was raised 8.0m from
the ground? [3.9x104 W]
20. A hydraulic hoist is used to lift a 105kg pallet from the ground to a shelf that is
1.50m off the ground. If the hydraulic hoist uses 95W, what is the time it takes
the hoist to move the pallet from the ground to the shelf? [16 s]
21. A 7.50kg cat uses energy at a rate of 1.29J/s for a total of 3.00s. If the energy
used by the cat is to accelerate the cat from rest, what maximum speed could the
cat reach in 3.00s? [1.02 m/s]
22. A 15.0kg dog is startled awake from a nap. He starts from rest and accelerates
across the floor to 5.00m/s in a distance of 2.25m. Determine the power output
of the dog during this period of acceleration. [208 W]
Answers
1. B
17.
a. System is not isolated because the mechanical energy at the top of the ramp
is 3783 J and the mechanical energy at the bottom is 2031J.
b. The box initially has only gravitational potential energy at the top of the ramp.
As the box begins to slide down, the gravitational potential energy is being
converted into kinetic energy. At the very bottom, the box only has kinetic
energy. Since the system is not isolated, energy is being lost due to friction
(ie. work is being done by friction).