An experimenter walks briskly forwardwith a ball in one hand, and attempts to release the ball so that it will fall into a bucket, as shown in the photo below:

 

The question is where one must release the ball in order for it to fall into the bucket. In the photograph the ball has been released and is falling, but whether it will fall into the bucket is not certain. As you walk along, must you release the ball before or after the position shown here? Must you wait until your hand is directly above the bucket, or slightly beyond that point, due to the effect of conservation of momentum between your hand and the ball at the time when it is released?

In order for the ball to fall into the bucket, you must release it:

• (a) somewhere to the left of where the scientist is in the picture.
• (b) somewhere to the right of where the scientist is in the picture, but not as far as the bucket.
• (c) when your hand is directly above the bucket.
• (d) when your hand is just past the center of the bucket.

After May 1, click Read More for the answer.

 The answer is (a): the ball must be dropped at a location to the left of her position at the time of the photo above. Exactly where you must drop the ball depends on how fast you are walking. See this video for an example. 

Notice that dropping the ball without changing its velocity in the horizontal direction has the result that the ball will move along a with the walker and remain under the hand from which it was released until it goes into the bucket. The horizontal speeds of the ball and the walker are the same.