Superman is asked to calculate the momentum of two objects in motion, a speeding bullet and a locomotive. He finds that the momentum of the two objects is equal. What is the best explanation for this?

A. The bullet and the locomotive must have the same mass.
Incorrect. If the masses are equal it does not mean the momentum is equal. Mass and velocity are both components of momentum.

B. The bullet and the locomotive must have the same velocity.
Incorrect. If the velocities are equal it does not mean the momentum is equal. Mass and velocity are both components of momentum.

C. The locomotive is moving at a much higher velocity than the bullet.
Incorrect. The locomotive is already more massive than the bullet, if it also has a greater velocity than the bullet, then it will have a much larger momentum than the bullet.

D. The bullet is moving at a much higher velocity than the locomotive.
Correct! Momentum is equal to mass × velocity, so the only way to have the two moving objects have the same momentum is for the bullet to have a much higher velocity than the locomotive.

Determine the momentum of a 50-kg halfback moving westward toward the goal line at 10 m/s.

A. 5 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity. This result comes from dividing m by v.

B. 500 kg m/s
Correct! The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity.

C. 60 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity. This result comes from adding m and v.

D. 40 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity. This result comes from subtracting m – v .

An automobile has a momentum of 20,000 kg m/s. If the velocity is increased so that it is doubled, what is the new momentum of the vehicle?

A. 10,000 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity.

B. 20,000 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity.

C. 40,000 kg m/s
Correct! The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity. The new momentum if you double the velocity is p=m (2v) which is the same as p=2(mv). If p=mv is 20,000 kg m/s, then 2 times 20,000 kg m/s equals 40,000 kg m/s.

D. 80,000 kg m/s
Incorrect. The equation describing momentum is p=mv, where p is momentum, m equals mass, and v is velocity.

If a box has a momentum of 36 kg m/s and a velocity of 12 kg, what is its velocity?

A. 3 m/s
Correct! The velocity can be found by dividing momentum by mass.

B. 24 m/s
Incorrect. You subtracted, you need to divide.

C. 48 m/s
Incorrect. You added, you need to divide.

D. 432 m/s
Incorrect. You multiplied, you need to divide.

A student in a lab has three carts that she will be using for an experiment. At one point during the experiment, the masses and velocities of the carts are:
Cart A – 2kg, 3m/s right;
Cart B – 4kg, not moving; and
Cart C – 1kg, 2m/s left.
What is the magnitude of the total momentum of this system?

A. 4 kg m/s
Correct! Since carts A and C are moving in opposite directions, you need to subtract the one momentum from the other.

B. 8 kg m/s
Incorrect. Since carts A and C are moving in opposite directions, you need to subtract the one momentum from the other.

C. 35 kg m/s
Incorrect. You multiplied the total mass by the total velocity, you need to calculate the momentum of each cart, and then combine them.

D. 6 kg m/s
Incorrect. This is the momentum of Cart A by itself, you need to find the total momentum.