From the edge of a roof you throw a snowball downward that strikes the ground with 100 J of kinetic energy.Then you throw a second snowball upward with the same initial speed,and this too falls to the ground.Neglecting air resistance,the second snowball hits the ground with a kinetic energy of

Question

Quizplus · Accepted Answer

The potential energy of the second snowball when it is thrown upwards is the same as the potential energy of the first snowball when it is just about to hit the ground. Therefore, the initial potential energy of the second snowball is equal to the final kinetic energy of the first snowball, which is 100 J. 
Due to conservation of energy, the initial potential energy of the second snowball is converted entirely into kinetic energy as it falls back to the ground. Therefore, the kinetic energy of the second snowball just before hitting the ground is also 100 J.

From the Edge of a Roof You Throw a Snowball