work-energy principle

Meaning: The work done by all the forces on an object, added together, equals the change in its kinetic energy. Positive net work speeds the object up, because the net force has a component along the displacement. Negative net work slows it down. Zero net work leaves its speed unchanged. The principle follows from Newton's second law applied over a displacement, and it holds for conservative and nonconservative forces.

Equation: Wnet = ΔKE = ½mvf² − ½mvi², where Wnet is net work (J), m is mass (kg), and vf and vi are final and initial speeds (m/s). For a constant force, W = Fd cosθ, with F in N, d in m, and θ the angle between force and displacement.

On the MCAT: Typical setups are a braking car, a block on a rough ramp, or a falling object, asking for final speed or stopping distance. With a constant braking force, d is proportional to v², so doubling the speed quadruples the stopping distance. Force-versus-displacement graphs also appear: the area under the curve is the work.

Common confusion: Students use the work of one force and forget friction or gravity. Add every force's work, with its sign. Constant speed means net work is zero, not that no force does work. If gravity is counted as potential energy, do not also count its work.