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Hello Ute, When you lift a ball to a certain height above the ground work is done against the weight of the ball.The amount of work needed to lift a ball to a certain height is stored as gravitational potential energy. When the ball falls from this height the force of gravity (weight) on the ball will accelerate the ball.This means its velocity will increase...it gets faster (Newton's Second Law). Since the kinetic energy is the energy of motion, the kinetic energy will also increase. Just at the point of impact all the potential energy has been converted into kinetic energy. When it strikes the ground the ball has an upward force acting on it by the ground.It also give the same force to the ground,in the opposite direction (Newton's Third Law).The upward force on the ball makes the bottom of the ball compress ,and energy is stored in the ball as elastic potential energy.It only takes a short time for the ball to come to rest (velocity = 0). Then the elastic energy of the ball is released and the ball gives a force on the ground. There is an equal and opposite force on the ball,in the upward direction (Third Law again). The elastic energy is now released as kinetic energy. The kinetic energy is then converted into gravitational energy as the ball moves upwards. If the ball is perfectly elastic then no other energy changes will happen. In practice,some of the elastic energy will be converted into heat, by internal friction, as the molecules move against each other in the ball. A small amount of kinetic energy will also be converted into sound. These other energy changes are the reasons why the ball does not return to the same height. A cement ball is unable to store much of the kinetic energy as elastic potential energy ,so the original energy is converted into heat and also the work needed in deforming the ball,and sound energy. For simplicity I have assumed that (i) the effect of air friction can be neglected and (ii) the surface of contact is not deformed/compressed. In practice both of these effects will result in extra energy changes ,reducing the the kinetic energy of the ball. I hope this has answered your question(s) !

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