3. Centripetal Force
• is a force that makes a body follow a
4. Constant Velocity
The velocity vector
tangential to the
5. Centripetal Force
• Newton’s 2nd Law
– Whenever there is acceleration there is a force to
– the force acts towards the center of the circular
path, the acceleration must also be towards the
• F = ma
• ac = v2 / r
• Fc = mac
• Centripetal Force is not a new, separate
force created by nature!
• Some other force creates centripetal force
– Swinging something from a string tension
– Satellite in orbit gravity
– Car going around curve friction
• How Centripetal Acceleration Keeps an
Object Moving in a Constant Circle?
8. • Velocity constantly changes direction,
but not magnitude, when an object is in
9. Conceptual Problem
• What affects Fc more: a change in mass, a
change in radius, or a change in speed?
• A change in speed since it is squared and
the others aren’t.
10. Conceptual Example
• During a circus trapeze act, a man
hangs from a bar and swings his
partner from their arms. Which is
harder to do: hold the partner still under
the bar, or to swing the partner under
the bar? Why?
11. • Swing because not only is there gravity,
but there is also centripetal force.
• What provides the Fc?
12. •Tension their arms.
13. Centripetal Force
“Objects in motion tend to remain in motion, at the same rate,
And in the same direction, unless acted on by an outside force”
• Newton’s First Law tells us that only a force can
cause a body to move out of a straight line path.
In circular motion the direction of the body is
continually changing at every instant. Therefore a
force must be acting. That force is called
centripetal (central) force since it acts toward the
circle of the circular path.
ALL CIRCULAR MOTION REQUIRES A
CENTRIPETAL FORCE, OTHERWISE THE
BODY CONTINUES IN A STRAIGHT LINE PATH.
14. Centripetal Force
• All circular motion requires a centripetal force.
Newton’s Second Law of Motion tells us that
force equal mass times acceleration. Therefore,
centripetal force must produce an acceleration
(centripetal acceleration). Since the force acts
towards the center of the circular path, the
acceleration must also be towards the center !
ALL CIRCULAR MOTION IS ACCELERATED
MOTION. THE ACCELERATION IS ALWAYS
TOWARDS THE CENTER OF THE CIRCULAR PATH.