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

At the highest point of the block's trajectory, it will momentarily stop before it starts to fall due to gravity. At this point, the ball can be thrown directly at the block, which will be stationary for a split second. If the ball is thrown accurately, it will hit the block at this point. Throwing the ball at the spring or the original position of the spring will not ensure that it hits the block, as they are both in motion and the ball's trajectory needs to be calculated precisely.

A Block Is Supported on a Compressed Spring, Which Projects