Centripetal force is the net force directed toward the center of an object's circular path, equal to mv²/r, that continuously changes the object's direction (but not its speed) in uniform circular motion. It is not a separate type of force; it's a role played by real forces like gravity, tension, or friction.
Centripetal force is the net force pointing toward the center of a circular path. For an object in uniform circular motion, that inward net force has magnitude F = mv²/r, and it produces centripetal acceleration v²/r. Here's the part that trips everyone up. Centripetal force is not a new kind of force. It's a job description, not a force itself. Gravity does the job for an orbiting spacecraft. Tension does it for a ball on a string. Friction does it for a car rounding a flat curve. Whatever real forces add up to point at the center, that sum is the centripetal force.
Because the force always points perpendicular to the velocity, it only changes the velocity's direction, never its magnitude. That's why an object in uniform circular motion moves at constant speed even though it's constantly accelerating. If the inward force suddenly disappeared (the string snaps), the object wouldn't fly outward. It would travel in a straight line tangent to the circle, exactly as Newton's first law predicts.
Centripetal force lives in Topic 3.6 (Centripetal Acceleration and Centripetal Force) and Topic 3.7 (Free-Body Diagrams for Objects in Uniform Circular Motion) in Unit 3. The skill the exam actually wants is translation. You have to look at a physical situation (a banked curve, an orbit, a ball on a string) and identify which real force or combination of forces is playing the centripetal role, then set that net inward force equal to mv²/r. It also connects to energy ideas in Unit 3, including translational kinetic energy K = ½mv² (LO 3.1.A). Since centripetal force is perpendicular to velocity, it does zero work, which is exactly why speed and kinetic energy stay constant in uniform circular motion.
Keep studying AP Physics 1 Unit 3
Centrifugal Force (Unit 3)
The 'outward push' you feel on a merry-go-round isn't a real force in an inertial frame. It's your inertia trying to carry you in a straight line while the centripetal force drags you into the circle. On the AP exam, never put centrifugal force on a free-body diagram.
Gravitational Force (Unit 3)
For anything in orbit, gravity IS the centripetal force. Setting GMm/R² equal to mv²/R is how you find orbital speed, and it's exactly the setup the 2018 FRQ used for a spacecraft of mass m circling Earth at radius R.
Tension (Unit 3)
A ball whirled on a string stays in its circle because tension supplies the inward force. At the bottom of a vertical loop, tension must exceed gravity, so T - mg = mv²/r. That extra tension is a classic FRQ calculation.
Friction Force (Unit 3)
A car turning on a flat road has nothing pulling it inward except static friction between the tires and pavement. When friction can't supply mv²/r (icy road, too much speed), the car slides tangent to the curve, not outward.
Multiple-choice questions love the conceptual traps. Which way does the net force point at the top of a loop? What happens the instant the string breaks? Why doesn't centripetal force do work? Free-response questions usually start with a free-body diagram of an object in circular motion, then ask you to apply Newton's second law along the radial direction. The 2018 FRQ gave a spacecraft of mass m in a circular orbit of radius R around Earth and expected you to recognize gravity as the centripetal force. The biggest scoring mistake is drawing a separate 'centripetal force' arrow on a free-body diagram. Only draw real forces (gravity, tension, normal force, friction), then show that their net inward component equals mv²/r.
Centripetal force is the real net force pulling an object toward the center of its circular path. Centrifugal force is the apparent outward push you feel inside a rotating frame, and it isn't a real force in the inertial frames AP Physics 1 uses. When a car turns left, you feel shoved right, but nothing is pushing you. The car door is pushing you left (centripetally) while your body tries to keep going straight. If a question asks what happens when the centripetal force vanishes, the answer is straight-line tangent motion, never 'flies radially outward.'
Centripetal force is the net inward force on an object in circular motion, with magnitude F = mv²/r, not a brand-new type of force.
Never draw 'centripetal force' as its own arrow on a free-body diagram; draw the real forces (tension, gravity, friction, normal force) and show their net inward sum equals mv²/r.
Because centripetal force is always perpendicular to velocity, it does zero work, so speed and kinetic energy (K = ½mv²) stay constant in uniform circular motion.
If the centripetal force disappears, the object moves in a straight line tangent to the circle, following Newton's first law.
For orbits, gravity supplies the centripetal force, so GMm/R² = mv²/R, the exact setup used in the 2018 FRQ about a spacecraft orbiting Earth.
It's the net force directed toward the center of an object's circular path, with magnitude mv²/r. It changes the object's direction continuously while leaving its speed alone, which is what keeps the object moving in a circle.
Sort of, but it's not a separate type of force. It's the name for whatever real force (or combination) points toward the center, like gravity for an orbit, tension for a ball on a string, or friction for a car on a curve. That's why you never label an arrow 'centripetal force' on a free-body diagram.
Centripetal force is the real inward net force keeping an object in its circle. Centrifugal force is the apparent outward push felt in a rotating frame, and in the inertial frames AP Physics 1 uses, it doesn't exist. The 'outward' feeling is just your inertia resisting the turn.
No. In uniform circular motion the force is always perpendicular to the velocity, so W = 0. That's exactly why the object's speed and kinetic energy (K = ½mv²) stay constant even though it's accelerating the whole time.
The object flies off in a straight line tangent to the circle at constant speed, per Newton's first law. It does not shoot radially outward. Answering 'outward' is one of the most common wrong answers on this style of multiple-choice question.