centripetal force

Meaning: An object moving in a circle changes direction continuously, so it accelerates toward the center even at constant speed. By Newton's second law, that acceleration requires a net inward force. "Centripetal" describes this role (center-seeking), not a separate force type. Some real force or combination of forces must supply it: tension in a string, gravity for an orbit, friction for a car on a flat curve, or the normal force on a banked track.

Equation: Fc = mv²/r, where Fc is centripetal force (N), m is mass (kg), v is speed (m/s), and r is the radius of the circle (m). Equivalent form: Fc = mω²r, with ω the angular speed (rad/s).

On the MCAT: Passages describe a car on a curve, a satellite, a centrifuge, or a mass on a string. You identify which force supplies the centripetal force, then predict how it changes. Doubling v quadruples the needed force; doubling r halves it. Because the force is perpendicular to velocity, it does no work and does not change speed. If the force vanishes (string cut), the object moves off in a straight line tangent to the circle.

Common confusion: Students draw "centripetal force" as an extra arrow on a free-body diagram and add it to the real forces. Instead, draw only the real forces; their net component toward the center equals mv²/r. Also, "centrifugal force" is not a real force. It is the apparent outward push felt in the rotating frame, caused by inertia.