## Thursday, July 31, 2008

### More Bad Usage of "Centripetal" Force

I mentioned earlier of the erroneous usage of centripetal force when the author actually was thinking of "centrifugal" force, fictitious or not. This is another similar situation.

Some of the ideas are entertaining methods to demonstrate inertia and other physics principles to your dining companion. For example, you can hold the bottle by its end and swing it around in a circle as fast as you can, letting the centripetal force push the contents toward the cap.

Of course, any first year undergraduate physics students can already tell you that a "centripetal force" is a central force that acts INWARDS, towards the center of the revolution (in a circular motion). It is the force that causes the ketchup bottle and its content to move in the circular path. A ketchup in the bottle will have a friction (and viscosity) with the wall of the bottle that will transfer this centripetal force from the source of the force to the wall of the bottle to keep that volume of ketchup in motion. However, at some point, the centripetal force will be greater than the friction between the ketchup and the wall of the bottle so that the ketchup will slid outwards.

Of course, one could also use the "centrifugal force" to explain this from the reference frame of the ketchup. This is the outward force that will "push the contents towards the cap". So again, as in the previous bad usage of "centripetal force", the author had "centrifugal" force in mind but didn't use it.

Zz.