The Work-Energy Theorem states that the net work $W_{net}$ done by all forces (conservative and non-conservative) acting on a particle is equal to the change in its kinetic energy: $W_{net}=\Delta K=K_f-K_i=\frac{1}{2}m{v}_f^2-\frac{1}{2}m{v}_i^2$. If work is positive, the particle speeds up; if negative, it slows down. This theorem is a direct consequence of Newton's second law applied over a displacement.