Centripetal force is the net force directed toward the center of a circular path that causes centripetal acceleration, with magnitude Fnet = mv²/r. It isn't a new kind of force; it's the role played by real forces like gravity, tension, friction, or a normal force when an object moves in a circle.
Centripetal force is the net inward force required to keep an object moving along a curved or circular path. By Newton's second law applied to circular motion, that net force has magnitude Fnet = mv²/r (or mω²r), and it always points toward the center of the circle, perpendicular to the object's tangential velocity. Because it's perpendicular to the motion, it changes the direction of the velocity without changing its speed.
Here's the part that trips people up. "Centripetal force" is not a separate force you add to a free-body diagram. It's a job description, not a force. Gravity does the job for an orbiting satellite. Tension does it for a ball on a string. Friction does it for a car rounding a flat curve. The normal force does it for a block at the bottom of a loop. On AP Physics C, your move is always the same. Draw the real forces, sum the components pointing toward the center, and set that sum equal to mv²/r.
Centripetal force shows up in two big places in AP Physics C: Mechanics. First, it's the engine of all circular motion problems in dynamics (Unit 2), from banked curves to conical pendulums to vertical loops. Second, it's the bridge to gravitation in Topic 7.2, Orbits of Planets and Satellites (Unit 7). For a circular orbit, the gravitational force GMm/r² is the centripetal force, so setting GMm/r² = mv²/r gives you orbital speed, orbital period, and ultimately Kepler's Third Law. If you can confidently identify which real force is acting centripetally and write Fnet = mv²/r, you've unlocked a huge slice of both units. It's also a classic place where the exam tests whether you actually understand Newton's second law as a vector statement rather than a plug-and-chug formula.
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Circular Motion (Unit 2)
Uniform circular motion is the kinematic picture (constant speed, changing direction); centripetal force is the dynamic cause. Centripetal acceleration v²/r exists because some real net force points at the center. Every circular motion FRQ starts with you identifying that force.
Circular Orbit and Kepler's Third Law (Unit 7)
In Topic 7.2, gravity IS the centripetal force. Setting GMm/r² = mv²/r and substituting v = 2πr/T gives T² ∝ r³, which is Kepler's Third Law. The entire orbits topic is centripetal force with gravity cast in the starring role.
Tangential Velocity (Unit 2)
The centripetal force is always perpendicular to the tangential velocity, which is why it does zero work in uniform circular motion. Cut the string and the inward force vanishes, so the object flies off along the tangent, not outward.
Mechanical Energy (Unit 3)
Loop-the-loop problems combine both tools. Energy conservation gets you the speed at a point on the loop, then Fnet = mv²/r at that point tells you the normal force or the minimum height needed. The 2021 FRQ Q2 (block, loop, and spring) is built exactly on this pairing.
Centripetal force is a workhorse on both sections. Multiple-choice stems give you a car on a banked curve, a mass on a string, or a satellite, and ask for the force, the speed, or what happens when r or v changes (watch the v² dependence). On FRQs, the standard sequence is: draw a free-body diagram with only real forces, write Newton's second law toward the center, and solve symbolically. The 2021 FRQ Q2 is the template, where a block goes around a vertical loop of radius R and you need the condition at the top of the loop, where gravity (and possibly the normal force) supplies the centripetal force, so the minimum-speed condition is mg = mv²/R. Two graded details matter: never label an arrow "centripetal force" or "mv²/r" on a free-body diagram (that loses points), and remember the centripetal direction counts as positive when you sum forces toward the center.
Centripetal force is the real net inward force measured in an inertial frame. Centrifugal force is a fictitious outward force that only appears if you analyze the problem from the rotating (non-inertial) frame, like the "push" you feel against a car door in a turn. In AP Physics C you work in inertial frames, so the outward force does not belong on your free-body diagram. What you feel in the car is your inertia carrying you along the tangent while the door pushes you inward.
Centripetal force is not a new force; it's the net inward component of real forces (gravity, tension, friction, normal force) when an object moves in a circle.
Apply Newton's second law toward the center: sum the real force components pointing at the center and set them equal to mv²/r (or mω²r).
Because centripetal force is perpendicular to the tangential velocity, it changes direction but not speed, and it does zero work in uniform circular motion.
For circular orbits in Topic 7.2, gravity is the centripetal force, so GMm/r² = mv²/r gives orbital speed and leads directly to Kepler's Third Law.
At the top of a vertical loop, the minimum-speed condition is mg = mv²/R, because at that instant gravity alone provides the centripetal force.
Never draw a separate "centripetal force" arrow on a free-body diagram; graders deduct points for it because it isn't a real, identifiable force.
It's the net force directed toward the center of a circular path, with magnitude Fnet = mv²/r. It's always supplied by real forces such as gravity, tension, friction, or a normal force, and it causes the centripetal acceleration that bends an object's path into a circle.
Yes and no. The inward net force is real, but "centripetal" is a description of direction, not a new type of force. On a free-body diagram you draw the actual forces (like tension or gravity) and then show that their inward components add up to mv²/r.
Centripetal force points inward and is real in an inertial frame. Centrifugal force points outward and is fictitious, appearing only when you analyze motion from a rotating frame. AP Physics C problems use inertial frames, so centrifugal force should never appear on your free-body diagram.
Only at minimum speed. At the top of a vertical loop, gravity and the normal force both point toward the center. At the slowest speed that maintains contact, the normal force drops to zero, leaving mg = mv²/R. The 2021 FRQ Q2 loop problem hinges on exactly this condition.
Not in uniform circular motion. The force is perpendicular to the velocity at every instant, so W = F·d gives zero. That's why a satellite in a circular orbit keeps constant speed and constant mechanical energy even though gravity acts on it continuously.