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Old Jul 6th 2008, 09:13 PM   #1
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Water bucket

Suppose I make a bucket that is full of water into circular motion, if the speed is large enough the water would not fall towards the centre.

However, if suddenly there is a hole at the bottom of the water bucket, would the water escaple?

Anything that is not attached would be thrown out if the bottom is not closed?
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Old Jul 7th 2008, 05:36 AM   #2
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Originally Posted by werehk View Post
Suppose I make a bucket that is full of water into circular motion, if the speed is large enough the water would not fall towards the centre.

However, if suddenly there is a hole at the bottom of the water bucket, would the water escaple?

Anything that is not attached would be thrown out if the bottom is not closed?
Consider a slightly simpler problem: A rock on a swing that is moving in a vertical circle such that the rock does not leave the swing. Why doesn't the rock leave the swing? Because there is a normal force acting on the rock at all times. (Though its magnitude varies.) This is what keeps the rock on the swing and thus it is the same kind of situation that keeps the water in the bucket.

So if there is a hole in the bucket (dear Liza, dear Liza!) the water will pour out of it because there is no normal force from the bottom of the bucket to keep it in.

-Dan
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Old Jul 7th 2008, 08:48 PM   #3
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For the normal force which allow the rock in circular motion, must there be friction between the surfaces or friction is unimportant?
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Old Jul 8th 2008, 05:33 AM   #4
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Originally Posted by werehk View Post
For the normal force which allow the rock in circular motion, must there be friction between the surfaces or friction is unimportant?
Yes, there would have to be some static friction present, unless you could manage to rig the motion such that the rock has a constant angular speed (not the typical case). In the latter case all we would need is friction to get the motion started.

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