# Centripetal Force and Novices

#### Jerry Walz

This is supposed to be: Centripetal Force, not Centrifugal.
This post is heavily edited.

This post actually originated from me thinking about designing a space game, and the first thing that I think about when it comes to games and movies, is how unrealistic they are. So it may be quite simple, to make a better space game one may only need to follow the actual rules of physics. Then that got me to think about this.
I read recently, but not in detail, that they are working on an idea of using centripetal force to recover energy from the energy spent, to say, make my game spaceship travel through space with very little energy. Now I realize that energy would have to be put into it before you could recover it, but at the end of the cycle, the initial energy would also be recycled. If the ship were going to turn, it would probably also try to slow down to make the turn faster, so there is the deceleration energy. My question then is this: Is it becoming accepted that, in Space, it is probable to recover energy spent through the means of equilibrium?
I don't know enough to talk about Ion energy creation, but I could see that there would be potential to use that method of recycling deceleration or centripetal energy by using a similar principal.

Jerry Ray Walz Jr.

Oh I thought that I would just add an article that may be more interesting than me. Ten to twelve times more efficient? So that would definitely include gasoline combustion engine, right?
https://www.nasa.gov/multimedia/imagegallery/image_feature_2416.html

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#### russ_watters

Mentor
Welcome to PF!
...how would a space ship actually slow down or turn in space. The answer seems obvious, it would move in a circular motion, but to do this would require a very nice Ion drive for rotation and to defy centrifugal force.
You're making that a bit more complicated than it is; there is no centrifugal force to "defy". Centrifugal force is the so-called "ficticious" reaction force that opposes a centripetal force:
https://en.wikipedia.org/wiki/Centripetal_force

Simply put, if you want to turn left, you point your engine to the right and it applies a force to the left. You, in the spaceship feel like you are being shoved into the right-hand wall, but that's just due to your inertia wanting to keep you moving straight. In other words, you think you are being pushed to the right by an unseen force but in reality you are being pushed to the left by the wall of the spacecraft.
I read recently, but not in detail, that they are working on an idea of using centrifugal force to recover energy from the energy spent, to say, make my game space ship turn.
Who? Where did you hear this? Altogether, that sentence doesn't appear to mean anything.
Do you believe that using mechanical energy, say using a container with liquid in it, having an object of mass inside of the liquid container, dividing the container into chambers, and then using the centrifugal force from the mass object onto the liquid. Lets say, create hydroelectric energy from that and feed the dispensed liquid back into the top of the emptying liquid container.
It's tough to tell because the description is kind of vague, but this sounds like a self-driving pump/turbine, which would be a perpetual motion machine. No, that wouldn't be possible.
Not much of a diagram...
Did you mean to post a diagram? I don't see one.
Is this the basic principal behind the concept of recycling centrifugal energy?
There is no such thing as "centrifugal energy", but perhaps you are referring to rotational kinetic energy?
Oh I thought that I would just add an article that may be more interesting than me. Ten to twelve times more efficient? So that would definitely include gasoline combustion engine, right?
https://www.nasa.gov/multimedia/imagegallery/image_feature_2416.html
They use a different measure of efficiency that does not apply to/compare to car engines.

#### Jerry Walz

Thanks for your post Russ. Ya I meant Centripetal force. I wanted to spell it centrifical, but that was't a word either. I guess I'm just getting a spelling lesson today. So that may make a difference in some of your analysis of what I wrote. Yes, the ship would have to burn at about 135 degrees, right-ish, then it would also have to rotate on the same plane, so it would need a "helicopter" or rotate type of burn at the front of the ship at 135 degrees. So center mass to slow, and top of the ship to rotate. That is what I was trying to get at, but ya, it would be deceleration force and not much centripetal force. I must have been making the comparison of when they fly by planets. If the ship were breaking near a planet, there would be centripetal force.

Alright, forget the illustration lets just ask the question more simply. Is it possible to recycle energy spent. They have designed cars that recover energy from breaking to slow down. From my perspective that in space there can be an almost perfect equilibrium between energy used and energy recovered. I guess I should research this more. It seems that I have achieved the desired affect by consideration of the question, and again that is all I find on the internet and why I am in this forum, because too many people are talking about perpetual motion machines. That is not the effect that I'm trying to achieve here. I'll do some more research and get back with you all, but I may need to revise my initial post since it is causing too much confusion.

Jerry Ray Walz Jr.

#### jbriggs444

Science Advisor
Homework Helper
Is it possible to recycle energy spent.
In this context, "no". It is impossible to perform an energy-conserving 90 degree turn in empty space. You will expend a huge amount of energy into the exhaust stream. Conservation of momentum demands that the exhaust stream be flung outward at high speed and forbids that it be recovered. That's where the expended energy goes.

The best you can do is make the reaction mass huge and the payload small. But that is inefficient too.

Those X-wings and TIE fighters in the Star Wars universe are the stuff of fiction.

#### Jerry Walz

Ya I was just watching a video on the conservation of energy yesterday of some guy in space throwing two tennis balls at each other with velcro on them. So the Conservation of Energy are the three laws of mass, velocity, and time.

Dictionary.com definition
1. the principle that in a system that does not undergo any force from outside the system, the amount of energy is constant, irrespective of its changes in form.
Ok, so from this definition, the environment remains constant, because it is space, a friction-less environment. So according to my High School physics 102 skills go. The environment in space is basically the lack of environment when it comes to this law minus the probability of getting hit by an asteroid, right. Fuel spent is transferred into another form of energy. So were not trying to recycle the Fuel, were trying to recycle the energy. The Fuel is only the means by which we get the object of Conservation of Energy.

ConservationOfEnergy energy = new ConservationOfEnergy(mass, velocity, time);

Jerry Ray Walz Jr.

#### jbriggs444

Science Advisor
Homework Helper
Fuel spent is transferred into another form of energy. So were not trying to recycle the Fuel, were trying to recycle the energy.
Yes. The chemical potential energy in the unused fuel becomes kinetic energy in the exhaust gasses flowing out the rocket nozzle.

But if were are in the middle of empty space, that kinetic energy is not recoverable. We cannot reel it back in and use it again. If we tried, conservation of momentum would demand that we end up right where we started with no net change in velocity.

#### Jerry Walz

Never mind what I just said. I was supposed to by default recognize that we are actually talking about Vectors here. An object in motion tends to stay in that motion (Vector). So I see what you mean by the continuous need to burn fuel to make a turn which would require giving up the kineticEnergy that I put in my example. Hmm, well at least I don't believe in perpetual motion machines anymore, that's a start, but to my defense I think that we are leaving out the conversation, the use of planets to regain lost momentum by another change in Vector. This is probably what they were referring to for deep space travel, using outer gravitational fields for the duel purpose of changing a Vector. I'm just beginning to program with Unity Graphics, and I have not worked with vectors in a very long time, but this is a very useful real example for me as to how that matters.

#### jbriggs444

Science Advisor
Homework Helper
If instead of empty space, one is working within a solar system, for instance, then it is possible to use gravitational assist "slingshot" maneuvers to gain more energy than your thrusters alone can ordinarily provide.

That's less about recycling energy and more about drawing from the kinetic energy present in the massive planets already in motion.

#### Jerry Walz

Ya so using a small amount of energy to change a vector by doing some pretty darn good pre-planning, and combing that by drawing energy from the environment. They made it sound like way too easy. They made sound like a ship could just go anywhere it wanted to. Fake news!

#### CWatters

Science Advisor
Homework Helper
Gold Member
As others have said, the main problem you have is with momentum which is always conserved. To slow down you have to get rid of your momentum somehow. Both options mentioned (burning fuel or gravity assist) work by giving it to something else (exhaust gas or a convenient planet).

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