# Gravity and Centrifugal Force

Q:
Picture a shaft with a small pivot point in the center and a small spring on the other end of the pivot point. just above the the pivot point on the spring side is a small weighted object. if the shaft is spinning at a high rate of speed, what would be the effect of the weighted object? in which direction would it want to move? kinda stumped on this. Thanks
- Dan (age 16)
Maine
A:

I don't have your exact picture clearly in mid, but I think the key part of your question is about how the little weight pulls on the shaft as it swings around. It's easiest to think of this in the frame rotating along with the shaft. The weight has ordinary gravity (mg) pulling it down plus a centrifugal pseudo-force pulling it out away from the rotation axis. The magnitude of that pseudo-force is mrω2, where m is the mass, r is the distance to the axis, and ω is the angular velocity, 2π/T where T is  rotational period. So you have to add those two forces as vectors to see which way the result points. It can either pull that end of the shaft down (at low speed) or up (at high speed).

Mike W.

(published on 09/04/2016)

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