AP Physics C: Mechanics Practice Quiz: Conservation of Energy

Correct Answer: B

The content explicitly states in point 3 that 'Mechanical energy is the sum of a system's kinetic and potential energies.'

Correct Answer: C

Point 5 states a fundamental principle: 'Energy is conserved in all interactions.' This is a broader concept than the conservation of mechanical energy, which has specific conditions.

Correct Answer: C

Point 6 directly provides the two conditions for the conservation of mechanical energy: 'If the work done on a selected system is zero and there are no nonconservative interactions within the system, the total mechanical energy of the system is constant.'

Correct Answer: A

This scenario fits the conditions for conservation of mechanical energy (point 6). As the ball falls, its height (potential energy) decreases while its speed (kinetic energy) increases. According to point 2, we can describe this behavior using conservation of mechanical energy principles, where one form of mechanical energy is converted into another.

Correct Answer: C

Point 6 states that mechanical energy is constant IF work done on the system is zero AND there are no nonconservative interactions. If mechanical energy is not constant (in this case, it decreased), then at least one of these conditions must have been violated. This could be due to negative work done on the system or energy being transformed by nonconservative forces (like friction) as described in point 4.

Correct Answer: C

Point 3 explicitly identifies the components of mechanical energy: 'Mechanical energy is the sum of a system's kinetic and potential energies.'

Correct Answer: B

If mechanical energy (KE + PE) is constant, any change in one type must be balanced by an equal and opposite change in the other (point 4). If KE increases by 50 J, PE must decrease by 50 J to keep their sum constant.

Correct Answer: B

Friction is a nonconservative interaction. According to point 6, this means mechanical energy is NOT constant. However, point 5 states that energy is conserved in ALL interactions. Therefore, the decrease in mechanical energy must be balanced by an equivalent increase in other energy types (like thermal energy) within the system, as described by point 4.

Correct Answer: C

This question directly paraphrases point 4: 'Any change to a type of energy within a system must be balanced by an equivalent change of other types of energies within the system or by a transfer of energy between the system and its surroundings.'

Correct Answer: B

Point 6 gives two conditions for constant mechanical energy. While there are no nonconservative interactions (frictionless surface), the first condition is violated: work is done on the system by the external force. This work increases the system's kinetic energy, and therefore its total mechanical energy.

Correct Answer: D

In the absence of nonconservative forces (air resistance) and external work, the total mechanical energy of the pendulum-Earth system is constant (point 6). Therefore, the mechanical energy at the highest point (maximum potential, zero kinetic) is equal to the mechanical energy at the lowest point (zero potential, maximum kinetic).

Correct Answer: D

Point 6 states that mechanical energy is constant when there are 'no nonconservative interactions within the system' and no external work. Therefore, the principle is most applicable when these nonconservative interactions (like friction or air resistance) can be ignored.

Correct Answer: C

This question is the reverse of point 6. The observation is that total mechanical energy is constant. The inference, based on the provided principles, is that the conditions for this to happen must be met: zero work done on the system and no nonconservative interactions within it.

Correct Answer: B

In an ideal orbit, the only significant force is gravity, which is a conservative force. There are no nonconservative forces like air resistance and no external work being done on the satellite-planet system. Therefore, according to point 6, the total mechanical energy of the system is constant.

Correct Answer: C

This is a subtle but critical point. The box's mechanical energy (KE+PE) increases because its potential energy increases. According to point 6, mechanical energy is constant only if the work done on the system is zero. Here, the person does positive work. Even if the net work is zero (as per the work-energy theorem, since ΔKE=0), the condition in point 6 for *mechanical energy conservation* refers to the work done by external forces other than the conservative forces already accounted for in potential energy. The person's force is an external force doing work, thus changing the system's total mechanical energy.