Work Power Energy

Work/Power/Energy Concept Review Questions

1. Define work and energy.

 Work is the displacement of an object in a parallel direction to an

applied force.

 Work is the transfer of energy.

 Energy is the ability to do work.

2. Define power.

 Power is the rate of doing work.

 Power is the rate of energy transfer.

3. What is the relationship between work and energy?

 Work is the transfer of energy.

 Energy is the ability to do work.

4. When is work accomplished on a mass?

When a force is parallel to the displacement of an object.

5. When is work accomplished on a mass positive? When is it negative?

 Work is positive when force and displacement are in the same

direction.

 Work is negative when force and displacement are in opposite

directions.

6. How much work is accomplished by gravity on a box that rests on a shelf?

Zero because there is no displacement.

7. What happens, in general, when work is accomplished on a mass?

Energy is transferred.

8. What happens when the net work is zero in lifting a mass at a non-zero constant

velocity?

The potential energy increases.

9. What happens when there is positive work accomplished by pushing a mass

horizontally?

The kinetic energy of the mass increases.

10. How much work is done on a 7.0 kg bowling ball by gravity on a bowling ball

traveling along a 19 m level alleyway?

Zero because the displacement of the bowling ball is perpendicular to the

gravitational force (weight) on the ball.

11. How much work is accomplished while carrying a 15 kg a distance of 8.0 m along a

level surface?

Zero because the displacement is perpendicular to the gravitational force and

the force supporting the mass.

12. Two people carry, A and B, each carry an identical mass box from the first

floor to a room located farther down the hall on the third floor. Person A takes the

nearest staircase and travels down the hall to the room. Person B walks along a first

floor hallway, takes an elevator to the third floor and then walks to the room.

Person A arrives at the room first.

a) On which person is more work accomplished? Why?

Same amount of work because the displacements are the same.

b) On which person is more power generated? Why?

Person A is more powerful because the person was able to accomplish the

same task in less time.

13. By what factor does kinetic energy increase, if the speed of an object is tripled?

KE is proportional to the velocity squared. So, if the velocity is tripled, then

the kinetic energy is increased nine times the original kinetic energy.

14. How much work is accomplished by gravity on a satellite that is in circular motion

around a planet?

None. There are two ways to explain this. First, the gravitational force does

not cause displacement of the satellite in a parallel direction to the force.

Second, if the satellite is in circular orbit around a planet, then its speed is

constant which means that the net work and force are zero; no change in

speed.

15. Define kinetic energy.

Kinetic energy is a mass’ ability to do work based on its motion.

16. State the work-energy theorem, explain its meaning, and give an example of its

application.

The work-energy theorem states that the work accomplished on a mass

changes its kinetic energy by the same amount.

17. How much work is accomplished on a 65 kg mass that has a change of kinetic energy

of 320 J?

W = ΔKE  W = 320 J

18. Define potential energy.

Potential energy is the amount of capable work a mass can accomplished

based on its height above a reference.

19. Define mechanical energy.

Mechanical energy is the sum of potential and kinetic energies.

20. What is the conservation of mechanical energy? Express in text and in equation

form.

Conservation of mechanical energy states that mechanical energy remains

constant in the absence of non-conservative forces (forces that permanently

remove energy from a system).

ME 1 =ME 2

PE 1 +KE 1 =PE 2 +KE 2

mgh 1 + ½ mv 1 2 =mgh 2 + ½ mv 2 2