Mechanical energy in a body moving upwards

Question

Why is it that mechanical energy is always conserved, I mean when an object is thrown in air, why does the kinetic energy convert to potential energy and not any other form of energy?

Answer 1

Energy forms are associated with forces. Gravitational forces cause the presence of gravitational potential energy. Therefor we might expect this energy form to be involved.

Had other types of forces, such as electrical forces, elastic forces etc., been involved then we would have expected electric potential energy, elastic potential energy etc. to be involved.

In fact in your scenario there might be other forces involved. Such as air drag. Then energy would have been lost to air resistance (heat lost to the air). This is typically a tiny amount so we often ignore it for low speeds.

Finally, note that the statement "mechanical energy is always conserved" is incorrect in general. Mechanical energy is only conserved when only mechanical forces are involved.

Answer 2

Mechanical energy is not always conserved. It is only conserved if the system is not subject to a net external force.

In your example the object and the earth are the system. If you throw an object in air mechanical energy of the system will not be conserved because the air exerts an external force due to air resistance dissipating part of the kinetic energy of the system as heat due to air friction. This results in a gain of gravitational potential energy less than that if there was no air.

Hope this helps.

Answer 3

Mechanical energy is not always conserved. If you throw the object up hard enough to hit the ceiling then suddenly its remaining kinetic energy is converted into heat, sound waves and deformation energy, and it is left with just potential energy. When it falls back down and hits you on the head, same thing happens again.