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Q & A: rotation from gravity

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Most recent answer: 12/29/2015
Q:
If objects fall at the same rate regardless of weight, why does the heavier end or side of an object rotate downward while falling? Eg: A weighted ball attached to the inside of a hollow ball.
- Tim (age 45)
Iowa City,Iowa
A:

Great question. Pure uniform gravity wouldn't make anything rotate. So this rotation must come from the other force on these falling objects, air friction.

Your example of a weight inside a hollow sphere is very nice, because it doesn't let us get away with treating the object as two separate parts falling at different rates due to friction. Instead lets thing of this as a sphere whose center of gravity is off toward one side. Therefore,as measured around that center of gravity, there's more force on one side than the other.  So the air friction causes a torque around the center of gravity, and that makes the sphere rotate.

Mike W.


(published on 12/29/2015)

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