we know that the acceleration of an object in circular motion points toward the center of the circle. So we
can conclude that the centripetal force acting on an object also points toward the center of the circle.
The formula for centripetal acceleration is
In this equation, v is the object’s velocity, and r is the radius of the circle in which the object is
traveling.
Centrifugal acceleration: As far as you’re concerned, nonsense. Acceleration in circular motion is
always toward , not away from, the center.Centripetal acceleration is real; centrifugal acceleration is nonsense, unless you’re willing to read a
multipage discussion of “non-inertial reference frames” and “fictitious forces.” So for our purposes, there
is no such thing as a centrifugal (center-fleeing) acceleration. When an object moves in a circle, the
acceleration (and also the net force) must point to the center of the circle .
The main thing to remember when tackling circular motion problems is that a centripetal force is
simply whatever force is directed toward the center of the circle in which the object is traveling . So,
first label the forces on your free-body diagram, and then find the net force directed toward the center of
the circle. That net force is the centripetal force. But NEVER label a free-body diagram with “Fc .”
Exam tip from an AP Physics veteran:
On a free-response question, do not label a force as “centripetal force,” even if that force does act
toward the center of a circle; you will not earn credit. Rather, label with the actual source of the force;
i.e., tension, friction, weight, electric force, etc.
—Mike, high school juniorMass on a String
A block of mass M = 2 kg is swung on a rope in a vertical circle of radius r constantspeed v . When the
block is the circle, the tension in the rope is measued to be 10 N. What is the tension in the rope when
the block is at the bottom of the circle?