Centripetal vs. Centrifugal Forces

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Centripetal vs. Centrifugal Forces

Post  Admin on Mon Dec 07, 2009 4:03 am




Centrifugal force is a virtual force. It is not really a force. There are
some situations you can be in that have you accelerating without speeding
up. One of these is a carnival ride that spins around in a circle at a
constant speed. I know one where you feel pressed against the wall very
tightly, and then the floor drops out. Most people would believe they were
moving steadily, with there bodies being pressed tightly against the wall
(outward, in a cetrifugal direction). This is centripetal force. This is
not really what happens.

When moving fast, a great deal of force is required to make you change
direction. Your body "wants" to continue in a straight line. The curved
wall gets in the way. The wall pushes in against your body. The "outward
force" is just your body trying to move in a straight line. It is not a
force at all. It is inertia, your body resisting the effects of the forces
it feels.

Virtual forces exist when your body is accelerating. Objects moving in a
steady direction at a steady speed appear to accelerate, as you see them.
Place a ball on a car seat while moving at a steady speed. Have the driver
slam on the brakes. Observe the ball appear to be pushed forward and off
the seat. The ball just continued to move forward. It is the car that felt
the backward force of the brakes. Hang a heavy ball from a spring in an
elevator. As the elevator begins to rise, the ball begins to move, as if
someone pushed down on it. It is the ball just "trying" to stay still as
the elevator accelerates upward. While rising, you can stop the motion. It
will start again when the elevator stops. The faster the acceleration (the
more you can "feel" it in your body), the stronger the virtual forces appear
to be.

Dr. Ken Mellendorf
Illinois Central College

When the trajectory of an object travels on a closed path about a point --
either circular or elliptical -- it does so because there is a force pulling
the object in the direction of that point. That force is defined as the
CENTRIPETAL force. It has not been more simply, or directly stated than by
one Isaac Newton in his famous "Principia" (definition 5):
"A centripetal force is that by which bodies are drawn or impelled, or any
way tend, towards a point as to a center."

This force can be demonstrated by twirling a ball on a string, and either
actually or conceptually cutting the string. The ball's trajectory is then a
straight line tangential to the closed trajectory at the instant the string
is cut. This is also illustrated by what happens to the ball in the "hammer
throw" of track and field. The athlete spins the heavy ball around several
times then lets it fly. It takes off in a straight line (not quite, because
the hammer is actually not spun parallel to the ground, but that is not

That is really all that is necessary. The term CENRTIFUGAL force appears to
have come about because of a mistaken perception that there is a force that
operates in the opposite direction as the CENTRIPETAL force. But that is a
misconception. The "pull" that is felt by the ball on a string or by the
hammer thrower is the force that has to be applied toward the center, to
keep the ball from flying off tangentially, not radially.

Unfortunately, the terms are often used interchangably, or incorrectly.
Newton's term, which I think should take the prize is CENTRIPETAL.

Vince Calder

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