force

Meaning: A force is a push or pull one object exerts on another. It is a vector, so it has magnitude and direction. The net force is the vector sum of every force acting on one object, and it sets that object's acceleration. Common forces are gravity (weight, mg), the normal force, tension, friction, the spring force, the buoyant force, and the electric force. By Newton's third law, forces come in equal and opposite pairs, but each member of a pair acts on a different object.

Equation: Fnet = ma, where Fnet is the net force (N, equal to kg·m/s²), m is mass (kg), and a is acceleration (m/s²).

On the MCAT: Expect free-body diagrams for inclines, pulleys, elevators, and circular motion. Resolve forces into components along convenient axes, set the sum along each axis equal to ma, and solve. In uniform circular motion the net force points toward the center (centripetal force). Questions often ask whether an object speeds up, slows down, or moves at constant velocity.

Common confusion: Students think motion requires a net force. A net force is needed to change velocity, not to maintain it, so an object at constant velocity has zero net force. A related slip is cancelling third-law pairs: they do not cancel, because they act on different objects, and only forces on the same object are added to find Fnet.