A block is supported on a compressed spring,which projects the block straight up in the air at velocity $\overrightarrow { \mathbf { v } } = v _ { 0 y } \hat { \mathbf { j } }$ .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

Quizplus · Accepted Answer

When the block is at the highest point, it has zero velocity, which means it will be stationary for a brief moment before it starts falling back down. At this point, throwing the ball towards the block with the same upward velocity as the block had initially will ensure that the ball hits the block. If the ball is thrown at any other time, it will miss the block either because the block has already fallen back down or because it is moving too quickly for the ball to catch up to it.

A Block Is Supported on a Compressed Spring,which Projects the Block