A man named Harold is standing in a train car that has no windows. Since there aren't any windows, he doesn't know how fast or in what direction the train is moving or whether it is accelerating or decelerating. Harold decides that he might be able to figure something out by dropping a tennis ball and watching where it lands. He does know which way the front of the train is, so when he does his ball-drop experiment he is facing forward. To make his experiment easier, Harold draws a red X on the floor of the train car. He then holds the tennis ball directly over the X, and releases the ball from a height of about five feet.

Question

A man named Harold is standing in a train car that has no windows. Since there aren't any windows, he doesn't know how fast or in what direction the train is moving or whether it is accelerating or decelerating. Harold decides that he might be able to figure something out by dropping a tennis ball and watching where it lands. He does know which way the front of the train is, so when he does his ball-drop experiment he is facing forward. To make his experiment easier, Harold draws a red X on the floor of the train car. He then holds the tennis ball directly over the X, and releases the ball from a height of about five feet.

The first time he drops the ball, it lands directly below his release point, right on the red X.

What can Harold conclude about the train's velocity?
A. The train is moving forward.
B. The train is moving backward.
C. The train isn't moving.
D. He hasn't learned anything about the train's velocity.

What can Harold conclude about the train's acceleration?
A. The train is undergoing forward acceleration.
B. The train is undergoing backward acceleration.
C. The acceleration is zero.
D. He hasn't learned anything about the train's acceleration.

Answer 1

Harold can make some conclusions about the train's velocity and acceleration based on the outcome of his ball-drop experiment.

Regarding the train's velocity, since the ball lands directly below the release point, on the red X, Harold can conclude that the train is not moving horizontally. This means that options A and B, which indicate that the train is moving forward or backward, can be eliminated.

Now let's consider the train's acceleration. The fact that the ball lands directly below the release point indicates that there is no horizontal acceleration acting on it. If there was a forward or backward acceleration, the ball would have landed in a different position relative to the release point. Therefore, option D, which states that Harold hasn't learned anything about the train's acceleration, can be eliminated.

Based on these conclusions, Harold can determine that the train isn't moving (option C) and that its acceleration is zero (option C).