# What is the problem here?

## Main Question or Discussion Point

Hey everyone, this is my first post so I appreciate the replies.

Ok, like any good engineer shouldn't, I have no intention of violating the conservation of energy.
That being said, I need some help identifying where the violation occurs in the following mechanism I've been thinking about lately. (or does it?....)

Imagine a hollow metal sphere, say 10 kg, which has sufficient volume to make it positively buoyant.
It starts at the top of a vertical pipe and free-falls for say 10 m. At this point the mass has 1000 J of kinetic energy.
At this point, you harvest this energy as best you can (likely a mechanical wheel, or perhaps the sphere is magnetic and it passes an induction coil). If you can do this at even 50% efficiency, you just gained 500 J of energy.
After this, the sphere passes through a one-way valve via its own momentum into a vertical column of water, the water level being just slightly lower than the top of the air pipe. The vertical pipe and the column of water are connected at the top by a small ramp, and an actuator is used to push the sphere the final distance, say 1m, into the air pipe where the cycle starts again. Moving the 10 kg mass 1 m takes 100 J of energy, so thats subtracted from the 500 J gained to yield a positive 400 J.

So....isn't this performance over-unity? Of course, any work done on an object by gravity must be equal to the work done to get it up to that height in the first place, but in this case, isn't that being primarily taken care of by buoyancy?

Whats the issue here?
-Max

Related General Discussion News on Phys.org
WarPhalange
Flaws: assumption that you will get 50% efficiency and that there is some magical column of water that gets the sphere up to the top again. So how would that work?

I don't know what is magic about the column of water. It is simply there, next to the column of air, and the mass floats to the top because it is buoyant. Also, the principle I am curious about is still true if I assume any efficiency greater than 10%, which seems reasonable to me.

WarPhalange
I don't know, I don't get how you have a column of water just standing up instead of falling down under gravity.

russ_watters
Mentor
The flaw is the assumption about how much energy is required to push the ball into the water at the bottom of the device. It's wrong. In the real-world, a lot of that is surface tension and viscosity, but even if there existed a zero viscosity, zero surface tension fluid, you'd still need all of the kinetic energy of the ball (minus what it took to lift the ball out of the water at the top) to get it into the tube at the bottom.

If you can't visualize why this is true, take a step back and think about what happens to the column of water when you put the ball in at the bottom: it rises. The entire mass of the 10m column of water rises. That's a lot of energy.

http://www.lhup.edu/~dsimanek/museum/buoy4.htm