Let's Make Robots!

Need some help with physics question (non robot)

The ten year old really got me stumped on this one...



Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Some comments say that it would not work, but I disagree. It would only NOT work IF it were off-planet...  away from Earth's gravity.

Given certain conditions it would work.

Here is what I see... Gravity is what powers it.  It is not exactly the free energy system some are considering.  It is getting its energy from the Earth's gravitational field.

one side of the "chain" is in the water and being forced upwards because the water which is more dense than the air in each 'ball' is being pulled down with more force displacing the ball moving it upwards. Most of the rest of the chain in out of the water and being pulled down by gravity of the Earth which is making it displace the air that surrounds it and move downwards.

The generator would indeed rob some of the energy of motion due to its friction, but I am thinking more along the lines of a tiny motor similar to those we use on robots, and having good low-friction bearings. Given larger balls in the chain, you will have more force to turn the motor. A large belt (aka chain) will have friction at each joint or resistance to flexing, but that is one of the details taht could be worked out. 

Another concern was the "magic valve" as it was called, but again, that depends on how it is built.

Rather than a valve as they are normally thought of, why not a an iris similar to that of a camera combined with a pair of tiny sensors watching for the end ot one cylander moving past and when the next one comes up to the iris from below. The sensors could be light/dark sensing, similar to those used in small line-follower robots. Rather than round balls filled with air, I picture them being cylanders with flat bottoms and tops. They go through an opening through which they fit perfectly, perhaps with a very thin neopreme "wiper" to minimize the leakage. When the "ball"(cylander) is just clearing the  area of the seal, an iris device such as used for camera shutters snaps closed. I see a tiny amount of water possibly leaking out very slowly, but the water can be replaced at intervals. The blades of the iris could themselves be made from or coated with neopreme rubber to seal better.

The iris would take a tiny amount of power to move it in and out and operate the sensors, but think "camera Iris" and what a tiny amount of power that takes to operate. Power from the generator might be small, but should be more than enough to run the electronics package moving the iris.

The limits would be defined by scale. Several places around the world there are quite large generators being powered by rivers or water falls such as at the Aswan Dam in Egypt, or the generators at Niagara Falls between the US and Canada. The energy is harvested by water being moved by the power of gravity pulling the water downhill. In our case we are also harvesting the power of gravity forcing the displacement of the air-filled cylanders by water being pulled down by gravity.


SHORT ANSWER: Yes, I see it as feasible, if built properly.

Take your son out to a burger-style dinner, (or fried chicken or whatever he prefers) as a reward for imaginative thinking.


Good post. You make a lot of good points. But the one I have to say I most agree on is Your last one. 'the kid' deserves a pat on the back. Practicality aside it is certaintly a cool concept and a 'good' idea. :)

I tried to edit that to make it clearer, but once there was an answer, I could not.  Hopefully it was clear enough the first time around.


according to description it's a perpetual motion machine of the first kind. It violates the first law of thermodynamics, hence it works not.

This kind of 'water perpetual motion machine' can be simplified as following:











Will this move forever? No, because you have always frictional losses between the spheres and the water, the mechanical system (pivot points), etc.



Obviously that will seek equilibrium. I mentioned that the system is NOT a "perpetual motion" scheme. It is powered by one of the four main forces of nature, –gravity. Just as the Earth will balance the force of gravity against centrifugal force, it reaches equilibrium and will neither fall into the Sun, nor fly off into outer space. As to spinning around the Sun, how can it keep circling the Sun forever? Since there is no such thing as perpetual motion it must run out of energy...  perhaps by noon tomorrow?

Now if we put the other containers below, will the balance stop when V2 is halfway submerged?


Now since the original experiment did not have an upward force on the balls out of the water, the major force acting on that side is the downward force from gravity.

As you drew it, this system would come to a balance with v1 and v2 equally submerged, but you have changed the experiment by introducing a new upward force on v2 once it enters the water. In the original thought experiment above, there is no upward force on the balls/containers outside the tank. (If there is, please point it out to me. Perhaps I missed it?)

...the weight of the water holding the ball down before it gets to a point where it can float --just as it enters. I think that just may be it. 

I was quite impressed with the boy's thinking on this one. I was even more impressed when I could not figure out why it would not work. To be honest, I knew in the back of my head that it wouldn't --but gosh darn it, I could not see why.

Oh, I love fun little recreational thinking like this.

According to Archimedes' principle (http://en.wikipedia.org/wiki/Archimedes#Archimedes.27_principle) will the lift of each floating ball be equal to the mass of the amount of water they displace.

The downward pressure on the ball trying to enter the tank from below will face the pressure of the full water column above it.

Even the volume of multiple balls pulling upwards can never exceed the volume of the unbroken column above the one going in through the valve.

I agree with the first sentence, ("A body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces. ")

However, I disagree with the latter assertion. All the balls/containers have the weight of the water above them, but that does not excuse them from floating, since the upward force is greater than the downward force from the water above them. Likewise it would not stop the one at the bottom, for it has the combined upward force of all the ones above it.

Let us say you have a buoy or even an air-filled balloon in a pond. You bury it on all sides under the top-soil, so it is like the container entering the tank from below. Are you maintaining that the air-filled balloon will stay on the bottom because of the weight of the water above it?

Only if you pack it under enough dirt will you be able to make it stay down,

–and we are considering a minimum friction at the valve just as there would not be enough friction on the sides of the balloon to hold it down, even if only the top of it were open to the weight of the water above it.  This could be easily tried. I am betting there is no way within the parameters of the experiment that you can hold that balloon down.   :-)


If you were to take a balloon into the ocean and took it down 1000 meters, would it stay down because of the weight of the water above it?

I say no matter how deep it is, how much water weight is above it, it will still rise to the top. Ask someone you know who scuba dives if they have a marker buoy to release, if there is any depth to which they could take it that the weight of the water above it would keep it from rising.

My answer to that is no.

This just clicked for me i think. so here goes:

The thing is that the ball in this case isn't at the bottom of the ocean. See the thing is we (or at least i do) tend to think of it as the ball's bouyancy 'fighting gravity' which isnt true. the ball's lower density causes it to move through the water to get to an area of less pressure. Because the ball on the bottom isnt in water on all sides it doesnt move up; it is being pushed down upon by the entire column of water above it. Which the force of the other balls cannot ever equal.

Make sense?

It makes sense (as in understandable), but I disagree.

As per my example to mogul just below, pulling the plug in a sink full of water is a good example. You feel a resistance to being pulled upwards, but when you apply enough pressure, it suddenly breaks free. As was mentioned about Archimede's principle, the upward force the balls apply will relate to their weight versus the mass of the volume of water being displaced, not its upper surface area. That is another reason I used a taller cylander, not a ball. Actually inasmuch as the water displacement pushes the cylander upwards, gravity is also pulling down on it, so the bouyancy is "fighting" (opposing) gravity after all. The upward force each cylander applies is multiplied by the number of balls or cylanders in the water at a time.