When a box rests on a round sheet of wood on the ground, it exerts an average pressure p on the wood. if the wood is replaced by a sheet that has half the diameter of the original piece, what is the new average pressure? i think the answer is 4p but i don't know why.

Pressure is (weight of the box) / (area under the bottom of the box). Neither of those quantities depends on the area of whatever is under the box. You could move the box from the deck of a battleship and put it on the kitchen table. The pressure it exerts doesn't change.

moving ball hits still ball. mover stops, hit ball moves. momentum transfer.

the answer would be c