Two children are throwing a ball back and forth to each other aboard a rapidly moving cruise ship. the observer who is in the best frame of reference to accurately decide which child is throwing the ball harder is
a) standing on the shore.
b) in another boat driving at the speed of the ball.
c) standing on board a ship that is moving in the opposite direction.
d) standing on board a ship that is moving alongside the children’s ship.

D) standing on board a ship that is moving alongside the children’s ship.

Explanation:

If the observer is in other frame of refference, he will see that the velocity of the ship affects how the ball moves (because the ball and the kids are moving in the same frame of reference than the ship), independent of the fact that the ball is in the air for a brief lapse.

Then he must be in the same frame than the children. If he sees that in this frame one of the children, let's call him "children a" throws the ball with more speed than the other, then he can know that the "children a" is throwing the ball harder.

Then the correct option is D

measure their densities same density, same sub