Correct answer: A

The correct answer is Total momentum of the system.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: With negligible external impulse, vector total momentum before and after the collision is equal.
3. Check signs and units: The correct answer is Total momentum of the system.

Correct answer: B

The correct answer is A collision in which objects stick together afterward.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: Momentum is conserved in an isolated perfectly inelastic collision, but kinetic energy is not conserved.
3. Check signs and units: The correct answer is A collision in which objects stick together afterward.

Correct answer: C

The correct answer is A collision in which total momentum and total kinetic energy are conserved.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: In an ideal elastic collision, both total momentum and total kinetic energy remain constant.
3. Check signs and units: The correct answer is A collision in which total momentum and total kinetic energy are conserved.

Correct answer: D

The correct answer is The relative separation speed compared with the relative approach speed.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: The coefficient of restitution describes collision bounciness through relative speeds before and after impact.
3. Check signs and units: The correct answer is The relative separation speed compared with the relative approach speed.

Correct answer: A

The correct answer is $2\,\text{m/s}$ in the original direction.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: Conservation gives $2(4)+2(0)=(2+2)v_f$, so $v_f=2\,\text{m/s}$.
3. Check signs and units: The correct answer is $2\,\text{m/s}$ in the original direction.

Correct answer: B

The correct answer is It is transformed into thermal energy, sound, deformation, or other internal energy.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: Total energy is conserved, but kinetic energy can transfer to internal and non-mechanical forms.
3. Check signs and units: The correct answer is It is transformed into thermal energy, sound, deformation, or other internal energy.

Correct answer: C

The correct answer is The backward motion of an object when it gives forward momentum to another object.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: In an initially stationary isolated system, forward momentum of one object is balanced by backward momentum of the other.
3. Check signs and units: The correct answer is The backward motion of an object when it gives forward momentum to another object.

Correct answer: D

The correct answer is It moves backward to conserve total momentum.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: The throw gives the ball forward momentum, so the person and skateboard gain equal opposite momentum.
3. Check signs and units: The correct answer is It moves backward to conserve total momentum.

Correct answer: A

The correct answer is It remains constant when net external force is zero.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: Internal forces cannot change total momentum, so the center of mass moves at constant velocity without external net force.
3. Check signs and units: The correct answer is It remains constant when net external force is zero.

Correct answer: B

The correct answer is Their masses can be different.

1. Identify the momentum system: Determine the objects, directions, initial and final velocities, and whether external impulse is negligible.
2. Apply the momentum relationship: Since $p=mv$, a heavier object can have smaller speed while matching the magnitude of a lighter object's momentum.
3. Check signs and units: The correct answer is Their masses can be different.