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Circular Motion (Vertical)

Hello everyone,
I have a (hopefully) quick question regarding vertical circular motion.



Say you have a bucket connected to a string and you are swinging it over your head, I understand that the tension in the string at the top is much less than that at the bottom as at the top, gravity is actually helping the string maintain the circular motion. This means you have:

Top: F = mg + T
Bottom: F= T - mg



My question is, and I have only ever seen this come up in an exam once, what actually keeps the water in the bucket?

Almost my entire class got lost as our teacher attempted to explain this the other day but I could not get my head around it. If you draw a free-body force diagram, at no point is there an upward force on water when it is at the top of the circle. I understand what keeps the bucket moving in a circle but not what keeps the water in the bucket.

The way I see it is similar to how I explain a satellite orbiting Earth. It is constantly falling towards Earth under the force of gravity but its horizontal velocity (perpendicular to the force of gravity) effectively means it constantly "misses" the planet, but I can't exactly write that in my exam now can I? :colondollar:


Thanks for your help,
Conor
(edited 9 years ago)
Original post by InfernoxCJC
Hello everyone,
I have a (hopefully) quick question regarding vertical circular motion.



Say you have a bucket connected to a string and you are swinging it over your head, I understand that the tension in the string at the top is much less than that at the bottom as at the top, gravity is actually helping the string maintain the circular motion. This means you have:

Top: F = mg + T
Bottom: F= T - mg



My question is, and I have only ever seen this come up in an exam once, what actually keeps the water in the bucket?

Almost my entire class got lost as our teacher attempted to explain this the other day but I could not get my head around it. If you draw a free-body force diagram, at no point is there an upward force on water when it is at the top of the circle. I understand what keeps the bucket moving in a circle but not what keeps the water in the bucket.

The way I see it is similar to how I explain a satellite orbiting Earth. It is constantly falling towards Earth under the force of gravity but its horizontal motions effectively means it constantly "misses" the planet, but I can't exactly write that in my exam now can I? :colondollar:


Thanks for your help,
Conor



Ask yourself why the water falls out of the bucket when it isn't being swung around. What is present (force, acceleration, direction?) that causes this.

Now swing the bucket around and ask yourself if these same conditions also apply to the water staying in the bucket. If so, you have your answer. Both situations require the same set of conditions.

Your satellite idea is on the right track.
Reply 2
Original post by Stonebridge
Ask yourself why the water falls out of the bucket when it isn't being swung around. What is present (force, acceleration, direction?) that causes this.

Now swing the bucket around and ask yourself if these same conditions also apply to the water staying in the bucket. If so, you have your answer. Both situations require the same set of conditions.

Your satellite idea is on the right track.


Thanks for your response! (Love your Display Picture by the way #EasilyDistracted :biggrin:)

Is it to do with the fact that at the top of the circle the water wants to accelerate in the direction perpendicular to the centre of the circle? (just like the bucket would if you let go of the sting)
Or, going by the satellite scenario, is it that the water is indeed falling out of the bucket but that the circular motion of the bucket effectively "catches" the water similarly to how the satellite "misses" the Earth?

Conor
Original post by InfernoxCJC
Thanks for your response! (Love your Display Picture by the way #EasilyDistracted :biggrin:)

Is it to do with the fact that at the top of the circle the water wants to accelerate in the direction perpendicular to the centre of the circle? (just like the bucket would if you let go of the sting)
Or, going by the satellite scenario, is it that the water is indeed falling out of the bucket but that the circular motion of the bucket effectively "catches" the water similarly to how the satellite "misses" the Earth?

Conor


Yes to all.
You could say the water is falling out of the bucket (it has a downwards acceleration) but the motion of the bucket prevents this.
At the top when swinging round the water is accelerating downwards (centripetal) and has a net downwards force (centripetal) on it.
When the bucket is not swinging, at the top it accelerates downwards (falls) as a result of a net downwards force (gravity).
In both cases you have the same set of conditions.

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