A block is supported on a compressed spring, which projects the block straight up in the air at velocity . The spring and ledge it sits on then retract. You can win a prize by hitting the block with a ball. When should you throw the ball and in what direction to be sure the ball hits the block? (Assume the ball can reach the block before the block reaches the ground and that the ball is thrown from a height equal to the release position of the block.)
A) At the instant when the block leaves the spring, directed at the block.
B) At the instant when the block leaves the spring, directed at the spring.
C) At the instant when the block is at the highest point, directed at the block.
D) At the instant when the block is at the highest point, directed at the spring.
E) When the block is back at the spring's original position, directed at that position.

Question

A block is supported on a compressed spring, which projects the block straight up in the air at velocity  . The spring and ledge it sits on then retract. You can win a prize by hitting the block with a ball. When should you throw the ball and in what direction to be sure the ball hits the block? (Assume the ball can reach the block before the block reaches the ground and that the ball is thrown from a height equal to the release position of the block.)
A) At the instant when the block leaves the spring, directed at the block. 
B) At the instant when the block leaves the spring, directed at the spring. 
C) At the instant when the block is at the highest point, directed at the block. 
D) At the instant when the block is at the highest point, directed at the spring. 
E) When the block is back at the spring's original position, directed at that position.

Quizplus · Accepted Answer

The ball must be thrown when the block is at its highest point. This is because at maximum height, the block momentarily stops moving, and therefore there is no horizontal velocity to deflect the ball's trajectory. Additionally, throwing the ball at the block rather than the spring ensures that the ball hits the block itself rather than the ledge or the spring, which may not count towards winning the prize. Therefore, choice C is the best option.

A Block Is Supported on a Compressed Spring, Which Projects