Let's Make Robots!

Need some help with physics question (non robot)

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



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I cringe when it comes to perpetual motion machines but a question comes to my mind while discussing getting energy from gravity. With Nasa and spacecraft speeds. And I know you are our resident rocketeer and space expert Marcus so maybe you could answer this for me. I believe they use the gravity of space bodies to slingshot space probes ever faster onwards to their objective.

Is this stealing energy from the planet to feed to the probe? Where does this gained energy go when the spacecraft slows down or stops at it's destination?

Perhaps Markus missed this question, but yes, you are correct. The net energies involved are equal. When the a spacecraft (for example, Voyager II) speeded up after executing a gravitationally assisted acceleration maneuver past a planet (ie. Jupiter) its velocity increased by a given amount. That energy was "robbed" from the planet, and just as you surmised, altered the planet's orbit. Because the spacecraft is so small with respect to the mass of the planet, this amount could be considered negligible.

Let us assume for a moment this spacecraft were interstellar and needed that speed-boost to reach solar escape velocity, and it is on it's way to another star system. When it reaches the far end of its journey, and detects a planet it wants to study, it will execute a gravity assisted braking maneuver past a large planet, or the star itself. It is the same maneuver, but whether you speed up or slow down depends on whether you pass in front of or behind the planet. The energy it loses at that end, will be given to the body it uses for braking.

Both NASA and ESA refer to what we call a 'slingshot' effect as gravitationally assisted trajectories (GAT). All spacecraft exploring the outer planets and beyond use GAT maneuvers to gain or lose speed. The spacecraft can pick up speed at Jupiter, then brake upon arrival at Saturn by going retrograde around it. After it is finished taking all the readings and photos they have it reverse course, perhaps taking a path around one of the larger moons and come back towards Saturn from the opposite direction, building speed again for a trip to Uranus or Neptune or beyond.

The planets are revolving around the sun and also rotating on axis, The direction you approach therefore determines whether you will increase or decrease your relative velocity.


I was too tired to think about this so I put my best man on it instead.
I'll let you know if he comes up with anything.

(In best Homer imitation):  "Now if we were to replace the water in the tank with beer... –and someone were to lie down under the leaky valve to catch the drippings... –someone like ME."

"Hmmm, I could even make the valve bigger, –uh, –to decrease that... what's it called? Oh yeah, the friction, then there would be more beer pouring out...  YES, this could work...! —and who cares if the stupid balls are moving, as long as the valve leaks?"

...however, the system won't work, and it's not because of the valve - the valve could easily be built with a ferrofluid seal.

In order to understand why the system won't work we need to imagine an equivalent system.

Let's first focus on the balls and chain and think about what happens as the chain moves through the water. If you try to imagine the volume of water displaced by the balls at any time you will notice that it is practically constant, it actually varies around an average that is constant by, at most, half the volume of one ball.

So, let's simplify our system and imagine that instead of a string with balls we have a rubber hose loop filled with air (with the ends of the hose connected togather). Can you see the reason why the system won't work now?

The buoyant force is given by the resultant of all the pressure vectors. In the simplified system with the hose you basically have pressure pressing only perpendicular to the axis of the hose, there is no force pushing the hose up or down.

A tube that goes through the bottom of a vessel filled with water will not float away even though it is empty (filled with air).

You can easily test this yourself with a quick experiment. Take a vessel with a hole in the bottom (this could be a flower pot or half a plastic bottle with a hole drilled in the bottom) and a piece of PVC pipe (or any other pipe that fits in the vessel), put the pipe over the hole and pour water in the pot (between the pot and the pipe, not into the pipe). You will notice that the pipe won't float away even though, the same pipe with its ends plugged would float.

I'll try to make a drawing :) Phear my mad mspaint skillz!!!

Getting away from the perpetual motion machine, I think you have just suggested a new use for ferofluids. Your idea would effectively be a form of liquid force field.

At the bottom of that page in the references was a link to a video by the company that produces them.

Watching it i dont think it would be able to effectively allow the balls to flow through very well as the seal has its own cylindrical center around which the water flows:




Still a pretty cool technology. And the point is moot regardless(there are more important reasons the machine wouldnt work).

Anyhow i thought the link was interesting. Cool idea.

Changing it to a rubber hose alters the experiment.  Let us say you are in the water and not a great swimmer. The normal method to keep you afloat is to attach a lightweight bladder filled with air which will hold you up. I would not want to try to use a hose attached to the bottom to hold me up, however. The hose as you pointed out has no lift. I'm sure you see the difference. 

The hose is a totally different thing than, say, an air-filled balloon, even if it were several balloons fastened together, extending down to the bottom of the pool. Said balloons will still exhibit an upward force, unlike a straight hose or pipe.

You sir, do indeed have mad MsPaint skills. Fantastic diagram.