有無人幫到我，urgent!!!物理...

1. 水彈

1.1 Kinetic energy perspective

there is no gravitation acting on the water balloon. when the bullet which carries certain KE hits the surface of the balloon, the bullet loses a little bit energy to cause deformation of the surface of the balloon. however, the deformation reaches its limit and then it collapses. at this moment, the balloon surface is broken, however the water inside isn't affected. then the bullet reaches the water.

although there is no gravitation, there is friction in between the water molecules and the bullet, contributing to the water resistance in our macroscopic world. So intuitively, we can see the bullet needs to give up some of its KE against the water resistance. In other words, water receives energy from bullet. the water splashes out because every water molecule receives different amount of energy and has diffferent tendency or different directions to move

So, possibly some water molecules will stay on the surface of the bullet due to friction. and most of the water molecules will spash out in different directions and move in different velocities

1.2 momentum perspective
although the water molecules move in different directions and in different velocities, the overall momentum is still conserved

2. apple

2.1 energy perspective
the bullet hits the skin of the apple and passes through the apple. the bullet all the way loses its kinetic energy to apple. apple will break down into some tiny pieces and a large piece (with a hole), here i assume the kinetic energy of the bullet is large enough and the bullet can go against the resistance between the apple particles and the bullet.

also, note that the bullet is spinning itself after being fired. when the bullet decelerates inside the apple, the bullet spins at a larger radius and produces a larger hole at the other side of the apple.

2.2 momentum perspective

Part of the apple pieces receive some momentums from the bullet and we know that the momentum is conserved unless there is external forces acting on the system (friction between the bullet and apple is internal force).

Hence, apple and its pieces will move in different directions and in different velocities. however, note that the overall momentum must conserve

So, apple and its pieces DO NOT NECESSARILY move in the same direction as the bullet

蘋果是一個固體，雖然被子彈擊穿，但子彈必有部份能量轉變移至蘋果上，蘋果必會隨子彈飛行方向移動。
至於水彈，因為子彈進入水彈時，會先擊破外皮，於是水變成獨立個體，當子彈穿過水，其部份能量會形成水波分散整個水，於是只有少量水點隨子彈飛行，大部份水點，因能量分散，會向四方八面而去。