If work is done on a body, the body will accelerate, change position or change shape. In the former case, the body gains kinetic energy.
If body A is moving, it can do work on body B through colliding.
Summary
The equation for Ek can be derived by considering the work done by a constant force. If the force is constant, the the acceleration is constant (since F=ma), so the suvat equations can be applied: