# Potential Energy for objects on the groun

1. Dec 20, 2005

### paprika

Since energy is conserved, when an object falls from a given height, the energy which it had as potential energy is converted into kenetic during the fall, then it is turned back into potential once the object hits the ground (which it must in order to maintain same total energy).

How does kenetic energy become potential energy for an object which has fallen to the ground if the amount of potential energy (in any given situation) is based on an object's position (usually height), but the position in the case of being on the ground is height = 0?

I think my first mistake is believing that the object must maintain constant total energy, where am I going wrong here? I'm not sure what to believe.

2. Dec 20, 2005

### neutrino

It is true only when it has to maintain the same mechanical energy(K+U).

When it hits the ground, you may here a sound, and some heat might be generated. These are just other forms of energy, and thus total energy should account for all of this.

3. Dec 20, 2005

### Staff: Mentor

You assume that mechanical energy is always conserved: Not so! When the only force acting on the falling object is gravity, mechanical energy (K + U) is conserved. But as soon as the object hits the ground, other non-conservative forces act on the object. As neutrino stated, the object's kinetic energy is transformed into various other (non-mechanical) forms of energy: thermal energy, sound, deformation.

Total energy is conserved, but mechanical energy is not.

4. Dec 20, 2005

### Staff: Mentor

Neither said it, but also, potential energy is energy due to height, so kinetic energy doesn't turn back into potential when the object hits the ground.