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Dr. Christopher S. Baird

Why is the centrifugal force talked about so much if it's not real?

Category: Physics      Published: December 15, 2012

washing machine
A washing machine depends on the centrifugal force being real within the rotating frame of reference. Public Domain Image, source: Christopher S. Baird.

The centrifugal force is very real if you are in a rotating reference frame. It causes objects in a rotating frame of reference to accelerate away from the center of rotation. Washing machines, uranium enrichment centrifuges, and biology lab centrifuges all depend on the reality of the centrifugal force . However, the centrifugal force is an inertial force, meaning that it is caused by the motion of the frame of reference itself and not by any external force. If I stand on the ground and watch children spinning on a playground toy then in my stationary frame of reference their outward acceleration is caused simply by their inertia. In my frame, which is external to the rotating frame, there is no centrifugal force at work. But in the rotating frame of reference of the children, there is a centrifugal force.

This oddity arises from the fact that forces only take on their expected meaning in Newton's laws when we are in non-rotating (inertial) reference frames. In rotating reference frames, Newton's laws take on a more complicated, non-intuitive form. But Newton's laws in the rotating frame can be made to look like the regular Newton's laws if we treat the extra pieces in the equations as inertial forces. In other words, the intuitive nature of pushes and pulls in everyday life can be extended to rotating reference frames if we call the effects of the rotation inertial forces. The centrifugal force is one of these inertial forces. The Coriolis force is another one.

geosynchronous satellite
Geosynchronous satellites depend on their centrifugal force canceling gravity, so that they hover motionless over the same spot on earth. Public Domain Image, source: NASA.

As an analogy, consider friction. The force of friction is fundamentally due to the electromagnetic forces between molecules. Even though friction itself is not fundamental, that does not make it any less real. A block of wood sliding on the ground feels an opposing force that is real. We call it friction. In a similar way, the centrifugal force has very real effects on objects in a rotating reference frame and is therefore real. But the centrifugal force is not fundamental. Rather it is caused by the rotation of the reference frame. The centrifugal force is not some psychological oddity humans experience. It affects everything in a rotating reference frame, not just the humans. The earth bulges out at the equator because of the centrifugal force. Geosynchronous satellites (those that hover constantly over the same spot on earth) depend on the centrifugal force exactly canceling gravity so that the satellite remains stationary in the earth's reference frame. The eye of a hurricane (the calm spot in the middle) is caused because the centrifugal force cancels the air pressure gradient force at that point. When the hurricane air that is spiraling inwards due to the pressure difference hits the point where it gains enough centrifugal force, it stops.

Topics: centrifugal, centrifugal force, force, inertia, inertial force, reference frame, rotational motion