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Name:  Giselle R.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
What is centrifugal force? Is it a real force? Could you give us a simple explanation and some examples of centrifugal force so that my elementary students will understand this concept? Thank you,

Miss R.

Additionally:

Thank you very much for responding to our question about centripetal force. We will be trying the experiment. We have one more related question. What then is centrifugal force? and What is the difference between centripetal and centrifugal forces?


Replies:
Giselle,

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 relevant).

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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