A 0.25 kg ball is thrown straight up with a velocity of 25 m/s. Apply the law of conservation of energy to determine the ball's height when its velocity is zero.
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i am pretty sure it is c because when people are closer together, they cannot move as freely. picture being in a mostly full elevator, for instance. the people are right next to you, and you can barely move.

3. dark matter

empty space could also be a simple vacuum