Work done by a constant force is $W = \vec{F}\cdot\vec{d} = Fd\cos\theta$. Work transfers energy to or from a system, depending on the direction of the force relative to motion.

Kinetic energy is $K = \tfrac{1}{2}mv^2$. Near Earth, gravitational potential energy is $U_g = mgh$ relative to a chosen reference level. A spring stores potential energy $U_s = \tfrac{1}{2}kx^2$.

The net work on an object equals the change in kinetic energy: $W_{\text{net}} = \Delta K$. This theorem is especially useful when forces vary with position or when you only care about speed changes.

If only conservative forces do work, then $K_i + U_i = K_f + U_f$. Nonconservative forces such as friction change mechanical energy by converting it into thermal energy or other forms.