# Newtonian Mechanics Question

1. Apr 22, 2007

### robhlee

Hello,
Say you have a frictionless setting. In this setting are two skateboards. One has a waterwheel (or any wheel with fins) propped up on beams so that it is on the skateboard and can freely turn. On the other skateboard is a person standing on it. If the skateboards are one behind the other (like train carts) and the person punches the wheel on a fin so that the wheel spins, what will happen?

In the instant of physical interaction (when the person punches the wheel) the fist/person is exerting a force on the fin, and, according to Newton's Third Law, the fin is exerting a force on the fist/person.

Correct?

If so, since the fin is part of a wheel, the force exerted by the person turns into centripetal acceleration and there is no net force on the waterwheel-skateboard apparatus. Meanwhile, there is a net force being exerted toward the person.

Correct?

If so, the person will be moved and the waterwheel-skateboard will remain stationary relative to its environment, but the water wheel will spin.

Please post any flaws or fallacies in theory you see in this situation.
:rofl: :rofl: :surprised :tongue2: :tongue:

2. Apr 22, 2007

### Staff: Mentor

No. You are missing the force that the skateboard exerts on the person to keep the person from moving. The skateboard does not move.

3. Apr 22, 2007

### robhlee

The only force the skateboard could exert would be friction. This is frictionless. Unless you meant inertia, but thats not force. Force overcomes inertia.

Last edited: Apr 22, 2007
4. Apr 22, 2007

### robhlee

so, russ_watters, your saying if someone flatout pushes you, you wont move because the skateboard is exerting a force back? is that what youre saying?

As far as the situation above is concerned, the person and the skateboard are one thing. If you dont agree, then say a mutant human with wheels on their feet replaces the person on the skateboard.

5. Apr 22, 2007

### Staff: Mentor

By "frictionless" I assume you mean no friction between wheels and floor.

Right! They both accelerate in opposite directions.

Huh? You just pushed on the waterwheel-skateboard--of course there's a net force on it.
The same net force is exerted on both.

Nope.

The tricky thing is that it will be difficult to exert a force on that spinning wheel, but if you do, it will accelerate.

6. Apr 22, 2007

### Crosson

I guess that:

is the crux of your discussion. It sounds similar to various (fallacious) "get rid of the force" ideas I used to have before getting formal training in physics.

I think the misunderstanding occurs in the difference between force and energy; energy is conserved and force is not. If somehow all of the energy of the punch went into the wheel, there would be no kinetic energy to move the person, but the punch is a net force and so it has to move the COM of the person attached to the flywheel.

7. Apr 22, 2007

### robhlee

Crosson, what do you mean by "COM"?

8. Apr 22, 2007

### robhlee

Hey guys,

What I meant by "no net force on the waterwheel-skateboard apparatus" was that there is no net force relative to environment. There is a force, but it is converted to centripetal force, since the fin is part of a wheel, I think.

Doc Al, I know there is a net force exerted on the fin and the fist, HOWEVER, the fin is part of a wheel and thus centripetal acceleration occurs when force is applied to the fin, so looking at the waterwheel/skateboard AS A WHOLE, there is no net DIRECTIONAL force.

Last edited: Apr 22, 2007
9. Apr 22, 2007

### robhlee

Doc Al, the punch is nearly instantaneous, just one punch. (In response to your last comment on your last post)

10. Apr 22, 2007

### robhlee

Wait, wait: "PERSON ATTACHED TO FLYWHEEL"? What do you mean Crosson?

11. Apr 22, 2007

### robhlee

Oh and Crosson, there is no force disappearing. Opposite and equal reactions on fin and person. The main question is, is the reaction onto the fin "converted" into a non-directional centripetal force?

12. Apr 22, 2007

### robhlee

Doc Al, by "frictionless", I mean "frictionless".

13. Apr 22, 2007

### robhlee

By "accelerate", do you mean the wheel's spinning or the movement of the waterwheel-skateboard?

14. Apr 22, 2007

### robhlee

sorry for the numerous posts, guys. Also I appreciate all your help.

15. Apr 22, 2007

### Staff: Mentor

Sure there's a net horizontal force on both. And forces don't "convert". True, if the wheel spins there'll be a centripetal force acting on all parts of it--but that's not the force that you hit it with.

Nope. Did you exert a horizontal force on it? Yes. Then there's a net force on it AS A WHOLE. Regardless of whether the wheel spins or not.

So?

16. Apr 22, 2007

### Staff: Mentor

I mean that the center of mass (COM) of the system will accelerate--the whole waterwheel-skateboard system.

17. Apr 22, 2007

### robhlee

Doc Al,

I know forces dont "convert", I felt iffy typing that word :), but the centripetal force is the result of the force on the fin.

Say the wheel was locked, it didnt spin. Comparing locked wheel punching and unlocked wheel punching, would you get the same result?

Oh, I said the punch was instantaneous just for your clarification. It seemed my first description was not clear enough.

18. Apr 22, 2007

### Staff: Mentor

The spinning is the result of the impulse you delivered with your punch.

Sure, if you delivered the same impulse--the same force for the same time. But you'd most likely be able to exert more force on the locked wheel.

Regardless: However you did it, if you delivered a horizontal impulse to the waterwheel-skateboard system, its total momentum will change--it's center of mass will move. No way around it.

19. Apr 22, 2007

### robhlee

Hey thanks. I dont wanna waste your time anymore, so could you tell me where I can get more information related to this on center of mass?

20. Apr 22, 2007

### robhlee

Wait one second...if you say the horizontal force of a locked and unlocked wheel would result in the same effect, in the unlocked wheel at any instant of movement there is an opposite side with equal momentum acting the opposite direction..so is there a net momentum?

21. Apr 24, 2007

### Staff: Mentor

Sorry, I skimmed quickly and misread your scenario - I thought the skateboards were connected. This sounds like an attempt at reactionless propulsion.

22. Apr 24, 2007

### Staff: Mentor

(I missed this last post.)

Sorry, I don't understand the phrase I highlighted. Considering the two skateboards as a whole, there's no net momentum. But considered separately, each skateboard system has equal and opposite momentum after the force is exerted.

Ah, wait. I see you are talking about the blades of the wheel itself. Realize that the wheel/skateboard system will have a net momentum. The entire wheel moves (think of the center of the wheel) horizontally; it doesn't just spin.

23. Apr 25, 2007

### robhlee

well, not exactly reactionless propulsion, I was just wondering if someone could give me an explanation that "makes sense to intuition".