Module 09: Conservation of Energy

The potential energy in the spring increases because the spring does negative work on the block.

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the potential energy stored in the spring during this motion?

The spring does negative work on the block because the spring force is in the opposite direction of the block's displacement.

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the work done by the spring on the block during this motion?

The elastic potential energy in the spring decreases while the kinetic energy of the block increases.

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction to compress the spring and then release the block, what happens to the energy in the system immediately after the block is released?

The spring has zero potential energy when the block is at x = 0, where the spring is neither stretched nor compressed.

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. The block can be pushed in the negative x-direction to compress the spring or pulled in the positive x-direction to stretch the spring. Where along the x-axis does the block have to be for the spring to have zero potential energy?

The frictional force does negative work on the block and decreases its kinetic energy.

A block slides along a rough surface and comes to a stop. What can you conclude about the frictional force exerted on the block?

When the total work done on the object is positive, the object's speed will increase.

A variety of forces are applied to an object such that the net force does positive work on that object. What can you conclude about the speed of the object?

0 joules

A weightlifter brings a 400-N barbell upward from his shoulders to a point 50 cm higher at a steady speed. During this process, what is the total work done on the barbell?

0 watts

A weightlifter exerts an upward force on a 1000-N barbell and holds it at a height of 1 meter for 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time?

500 watts

A weightlifter exerts an upward force on a 1000-N barbell and lifts the barbell 1 meter upward in 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time?

quantity X for that system does not change in any way during that process

If Quantity X is conserved for a certain system and during a certain process, that means__________.

Both particles will have the same kinetic energy at the bottom.

In both figures, a particle of mass m is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest down a frictionless ramp. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom?

The particle in figure (a) will have more kinetic energy than the particle in figure (b) at the bottom.

In both figures, a particle of mass m, is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest and slides down a ramp with a rough surface. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom?

The particle in figure (c) will have more kinetic energy than the particle in figure (b) at the bottom.

In figure (b), a particle of mass m is released from rest at the top of a frictionless ramp of height h and in figure (c), a particle of mass 2m is released from rest at the top of the same frictionless ramp. Which particle, the one in figure (b) or (c), will have more kinetic energy at the bottom?

Two balls of putty (the system) collide in midair and stick together.

In which of these processes is the total energy of the system conserved?

The work done by the force is equal to the area under the force versus position function.

The graph shows the x-component of a force applied to an object versus the position of that object in the x-direction. How is the work done by this force determined from the data in this graph?

non-conservative forces do zero total work on the system during that process

The mechanical energy of a system is conserved during a certain process only if __________.

The work done by a conservative force is always path independent.

What is true about the work done by a conservative force?

The work done by a non-conservative force will always change the total mechanical energy of a system.

What is true about the work done by a non-conservative force?

The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does negative work on the system.

When helping a friend move into a new home, you push a chair across the room. What do you know about the force of friction applied to the chair by the floor?

The force of gravity applied to the chair does not change the energy of the system (the chair) and therefore does no work on the system.

When helping a friend move into a new home, you push a chair across the room. What do you know about the force of gravity applied to the chair?

The force you exert on the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does positive work on the system.

When helping a friend move into a new home, you push a chair across the room. What do you know about the force that you exert on the chair

The work done by the force of gravity is negative and proportional to the upward displacement of the book.

When you lift a book upward off of a table, what is true about the work done on the book by the force of gravity?

Gravity

Which of the following forces is considered a conservative force?

The force due to a spring

Which of the following forces is considered a conservative force?

They have twice the kinetic energy of either ball by itself.

You observe two identical balls of putty heading directly toward each other at equal speeds. What can you say about their total kinetic energy?