By Valentina Tobos | Laurentiu Tobos

Updated on Sep 26, 2011

Review these concepts at: Mechanical Energy Study Guide

**Practice Questions**

- Find the work done by a force of 300.0 N exerted on an object to lift it vertically to a distance of 240.0 cm above the ground.
- Find the work done by the force of friction of 100 N on an object moving on a horizontal surface along a distance of 2 km.
- An object of mass 450 g moves along a horizontal surface with constant velocity due to a force that pulls the object (traction force). Find the work done by the net force and the force of friction over a distance of 2 m if the coefficient of kinetic friction is 0.6.
- In practice problem 3, find the work done by the weight, and the work done by the force of friction.

- At a construction site, a plank of mass 425 kg is lifted to a height of 11.2 m above the ground by a constant force in 1.00 minute. The plank is lifted at a steady pace. Find the power needed to produce this displacement.
- The same power is applied to two spinning wheels: One has a radius of 25 em and the other has a radius of 10 em. On which wheel is the applied force greater if they both describe a complete rotation in 30 s? The force on each wheel is always perpendicular to the radius as shown in Figure 7.3. This is similar to the construction of a bicycle with multiple speeds! Explain.
- Consider two engines working the same amount of time. Can you conclude that one engine is doing twice the work if it is generating twice the power?
- A regular light bulb has a power of 60 watts. If you are to give the number in horsepower, how much would you end up with?

- If the initial kinetic energy is larger than the final kinetic energy, what significance does the minus sign have on the work? Explain briefly.
- An object is slowing down to a final speed three times smaller than the initial speed. What is the ratio of the final and initial kinetic energies?
- Is it possible for a small object to have a large kinetic energy? Explain briefly.
- An object is moving with constant speed, overcoming friction. What is the work performed by the net force on the object?
- An object is moving up on an inclined plane until it stops. Find the speed at the bottom of the plane if the work done on the object is W = –(50 · m) J, where
*m*is the mass of the object. - An object is kicked and starts moving with a speed of 3.0 m/s on a surface with a coefficient of kinetic friction of 0.80. Find the distance over which the object will stop.

- An object has a potential energy relative to Earth of 1,200 J. If the potential energy reduces to 800.0 Jby dropping for 20.0 m, find the mass and the weight of the object.
- An object has been raised on an inclined plane free of friction to a distance
*d*with respect to the bottom of the plane where the potential energy is 1,200 J. If the component of the weight along the plane is 600 N, find the distance*d*.

- Consider a process in which the mechanical energy is conserved. What happens to the kinetic energy when the potential energy is increasing?
- Does an increase of the kinetic energy of an object necessarily mean an increase of mechanical energy for that specific object?
- An object is slowing down due to friction with the surface on which it is moving. In this case, is the total mechanical energy conserved? Explain briefly.
- An object is placed on a kitchen table. What do you have to do in order to reduce the potential energy of the object? Explain briefly.
- The first swing of a pendulum raises the ball at the end of a string 30.0 em long at an angle of 60.0° with the vertical. Find the kinetic energy when the ball returns to the bottom of the swing if the mass of the ball is 0.25 kg and the ball is released at the maximum angle.
- An object is thrown perfectly vertically from the surface of Earth, with an initial speed of 15.0 m/s. Assuming no air resistance and using the principle of conservation of mechanical energy, find the maximum height reached by the object.
- A volleyball of mass of 450 g at 1.8 m above ground is served so that it has an initial velocity of 15 m/s directed at an angle of 60° above the horizontal. Find the kinetic, potential, and mechanical energies at the beginning of the trajectory.
- In practice problem 23, assume there is no friction with air, and find the maximum height reached by the ball.
- A 0.750 kg ball is dropped from rest at a point 0.80 m above the floor. There is no air resistance, and the floor is at zero potential energy. Find the potential and the mechanical energy at the highest point.
- For practice problem 25, find the velocity of the ball when it reaches the floor.

**Answers**

- 7.2 · 10
^{2}J - –2 · 10
^{5}J - 0 J and 5.4 J
- 0 J and –5.4 J
- 7.93 · 10
^{2}watts - On the small wheel (
*F*·*v*= constant) - Yes, since time is the same.
- 8 · 10
^{–2}hp - The object is slowing down; work is done by the object on the surroundings.
- 1/9
- Yes, large speed
- As much as the work lost by friction
- 10 m/s
- 5.6 · 10
^{–1}m - 1.5 kg and 15 N
- 2 m
*KE*is decreasing.- No, if the process is conservative, that just means the
*PE*is decreasing. - No, friction is not a conservative force.
- Lower it on the ground.
- 37.5 J
- 11.3 m
- 51 J, 8.1 J, 59 J
- 10.2 m
- 6 J, 6 J
- 4 m/s

From Physics Success in 20 Minutes A Day. Copyright © 2006 by LearningExpress, LLC. All Rights Reserved.

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