# Is it possible to transfer kinetic energy from rigid body 1 to body 2 and conserve total kinetic energy? [duplicate]

This question comes from working with rigid rolling disks and wheels. For example, when a rotating disk comes in contact with a non-rotating disk, the non-rotating disk will start to rotate due to friction creating a torque on the body.

So if we just look at two rigid bodies, and forget about air resistance and other forces acting on the system of two bodies, is it in the theoretical case possible to transfer some or all of the kinetic energy of body 1 to body 2 without losing energy, in the form of friction, sound and heat, in the process? Does it make a difference in the result, if the kinetic energy of body 1 is pure rotational energy or translational energy or a combination of both?

To be clear, body 2 should have gained some or all of body 1's kinetic energy while the total kinetic energy of (body 1 + body 2) is the same.

• Possible duplicate of Are there perfect elastic collisions in Nature? – sammy gerbil Jul 21 '18 at 13:29
• Possibly the question you are trying to ask is whether rotational energy can be transferred from one body to another without the loss of energy which occurs in your last question. If so, you should be careful to identify the situation you are thinking about. Very general questions can be interpreted in ways which you did not intend. I think that has happened here. – sammy gerbil Jul 21 '18 at 17:31

At the macroscopic level some kinetic energy will be converted to some other form (heat, sound, etc.) making such collisions inelastic. At the microscopic level some collisions (e.g., those involving ideal gases molecules) approach being perfectly elastic. I believe there are some other examples at the microscopic level.

The conservation of kinetic energy occurs only in a perfectly elastic collision where coefficient of restitution is equal to one. Hence, yes, in an ideal situation, where no energy is lost through heat and sound upon collision and contact, it is possible. However, in reality, a perfectly elastic collision is never possible, so conservation of kinetic energy is not possible.

• The collisions between the molecules in the air are elastic. – Pieter Jul 21 '18 at 13:49

Yes. For example Newton's cradle. Same weight of the spheres, all kinetic energy is transferred to the sphere at rest.

• Nevertheless, the Newtonâ€™s cradle doesnâ€™t Ensure 100% Transfer of KE. Some is lost through the sound of collision. While this is negligible, one cannot say that the cradle is a perfect system – QuIcKmAtHs Jul 21 '18 at 13:44
• OP specified that air resistance etc would be disregarded. The physics is the same as for colliding nuclei and stuff like that. – Pieter Jul 21 '18 at 13:46