The work-energy principle states that the change in kinetic energy of an object is equal to net work done on it (or by it).

In mathematical terms,

`W_n = KE_f - KE_i = 1/2 mv_f^2 - 1/2 mv_i^2`

In simpler terms, if the kinetic energy or the velocity of a body increases, it is because of net positive work done on it. If the kinetic energy or the velocity of the body decreases, it means that either negative work has been done on it or the body itself did some positive work.

The work-energy principle is also applicable for rotating bodies.

Please note that work-energy principle is only applicable for rigid bodies. If the body deforms because of the work done on it, part of the work done causes deformation and rest converts to the kinetic energy of the body and the principle is thus not applicable.

hope this helps.