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AP® Physics 1 & 2

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Moderate

Inertial References Frames: Comparison of Forces

APPH12-K81RKR

A physicist on a train moving at a constant rate $v$ measures the average acceleration of an object under a applied force and determines the force has average magnitude $F$. While this is happening a rival physicist speeds by in a train moving at a constant rate of $2v$ on a parallel track and observes the same experiment.

Which of the answer choices below describes the magnitude of the force which the second physicist should calculate and gives the correct reasoning?

A

The second physicist should calculate a smaller force because she is moving faster so she will observe a smaller acceleration and since $F=ma$ she will calculate a smaller force

B

The second physicist should calculate the same force because both physicists are in inertial reference frames which mean they will measure the same acceleration and calculate the same net force.

C

The second physicist should calculate a greater force than the first physicist because she will measure the same acceleration but it will take more force to achieve that acceleration in a reference frame which is moving faster.

D

It is impossible to know what force the second physicist will calculate because the problem does not state whether the second physicist is moving in the same direction as the first physicist or in the opposite direction.