Physics question!?

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Physics question!?

Postby Crescent Pulsar R » Sat Jul 30, 2011 1:12 am

Something's been bugging me, and I have nothing in the way of education to know what the answer could be.

I was wondering if the speed of an air bubble would change if the water it was in was moving in the opposite direction (i.e. the air bubble is going up and the water is going down). The speed would be based on the air bubble's actual movement, rather than the distance it has to cover to reach the water's surface.

Would differing movement speeds of the water make a difference? And, ultimately, would the air bubble speed up, maintain a consistent speed, slow down, stop, or even reverse depending on or regardless of how fast the water is moving against/around it?
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Re: Physics question!?

Postby talonhunter » Sat Jul 30, 2011 8:19 am

hmmm...well conceder that the air bubble is suppose to be lighter then the denser water and therefore would rise to the surface very quickly in your air up water down analogy. But the best way to think about it is friction -vs-gravity. By utilizing gravity as the directional propulsion...(ie magnetics)...your speed would be dependent on the frictional forces against it...
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Re: Physics question!?

Postby Ellen Kuhfeld » Sat Jul 30, 2011 9:07 am

This is one of those problems so beloved by physics teachers. Everything depends on the conditions. The easy answer comes from having the water in an elevator, moving at constant speed. In that case the bubble will rise at the same rate as usual, relative to the elevator.

If the elevator is accelerating, you have (in effect) modified the force of gravity acting on the water and bubble. In that case, the bubble will rise (relative to the interior of the elevator) more rapidly if you have increased the gravity, less rapidly if you have decreased it. You are starting to get into fluid mechanics.

If the water is pouring out of a faucet, or going down a drain, forget it. You've just invoked turbulent flow. Nobody can calculate it exactly, and if you measured it, it wouldn't be the same twice in a row.
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