# free fall scenario

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Say you have a fitted but freely moving piston falling in a large cylinder. The piston weighs 100kg but only weighs 1 kg under water. The cylinder is filled with water, but there is plumbing going from the bottom of the cylinder to the top. The plumbing is also filled with water. As the piston moves down in the cylinder the water is free to move from beneath it, through the plumbing, and back to the top of the cylinder in a continuous loop.Lets assume the plumbing is of the appropriate shape and design to allow the water to travel as freely as possible. What would the velocity of the piston be after a 20m freefall?

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Ugh, I've always been bad at these things. I can't tell you the answer, but I'll do my best to help. Firstly, when you say "as freely as possible", does that mean no resistance? Otherwise, we need more specs. I think that you then have to dicover the boyancy of the object, subtract that from the accell. due to gravity, and do something with the area under a graph...?

Ok, so that was totally unhelpful. Sorry about that.

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ok the equations you need to do this are,

1. F=ma

2. s=ut+0.5at^2

3. v=u+at

F=force(N) m=mass(kg) a=acceleration(ms^-2) s=displacement(m) u=initial velocity(ms^-1) t=time(s) v=final velocity(ms^-1)

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Oh you forum whore! If you can't get your forum needs met at home, you just go elsewhere? You could have said something. We could have sought counselling.

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Oh you forum whore! If you can't get your forum needs met at home, you just go elsewhere? You could have said something. We could have sought counselling.

umm... wtf?

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I think we both knew we were beyond counciling a long time ago. I forgive you, when will you forgive yourself?

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no, really. wtf is going on here?

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(Sorry, beckerman posted this elsewhere. I don't want to hijack this thread. Please continue.)

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nothing in the rules says he can't post elsewhere. its a free net.

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Lets say no resistance for the piston. Insane looks like he knows something. I am too ignorant to apply his equations he gave me properly. Maybe a graph would help, I dont know, Please help.

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ok, what level of education are you at? i'm figuring if you get a question like this then you should already know those equatins and how to apply them.

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Art degree holder, working in metal fabrication.

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I will understand what youve done if I see the work written out. I just do not trust myself to do it and get a correct answer.

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Then do it and present the answer here to have someone have a look at it

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Lets say no resistance for the piston. Insane looks like he knows something. I am too ignorant to apply his equations he gave me properly. Maybe a graph would help, I dont know, Please help.

Do you expect that the piston will accelerate at a different rate than the other components?

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Well the only other moving component is the water. The water in the cylinder has to move at the same speed as the piston. Since the "external" plumbing can be larger or smaller in volume, the water here could be moving at a different speed. I am thinking that if the components were sized appropriately the piston could descend quite quickly, despite its nearly neutral bouyancy. My reasoning is the 1kg of weight we have only weighs 1kg when the water is stationary. Once the water is flowing it will have a diminishing effect on the 100kg mass. Likewise the increase in weight the mass will be experiencing will apply a progressively increasing amount of force to the water beneath it. Therefore the final velocity would be much quicker than the final velocity would be if the object were thrown into a pool for instance.

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Anyone have anything interesting to say?

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Even if the water is moving it will still be under influence of gravity. When the piston is submerged it will be like a seesaw with the 100 kg piston on one side and 99 kg water in the plumbing on the other.

The water in the cylinder and in the plumbing will have to accelerate at the same rate as the piston, otherwise a pressure that will counteract the descend will build up in the cylinder under the piston.

The minimum weight on the parts to be accelerated is 199 kg, with larger plumbing more water is added and the weight raises.

Now, if there was a race between to identical motorbikes, the first carrying a weight of 100 kg and the other 199 kg, which one would be able to accelerate fastest ?

The only difference between the plumbing and the pool scenario would be friction inside the cylinder and the plumbing, if the friction is as low as in a pool, (which problably requires plumbing of the size of a pool), then the piston would descend with the same rate as if dropped in a pool.

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