Impluse from spinning a wheel while standing on a skateboard

In summary, the conversation discusses the concept of frictionless settings and the effects of impulse on a system with two skateboards and a waterwheel. It is noted that in a frictionless system, the time of impulse should not affect the movement of the skateboards. However, in a system with friction, time could make a significant difference. The conversation also addresses the misconception that neither skateboard will move when the wheel is punched, and explains that both will actually move due to Newton's laws. It is also mentioned that pushing directly on the skateboard requires less energy than pushing on the water wheel, as the latter also involves rotating the wheel.
  • #1
robhlee
52
0
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, the wheel will spin but neither skateboard will move.

What if you change the impulse of the punch from infinitesimal amount of time to say a few seconds?
 
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  • #2
robhlee said:
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, the wheel will spin but neither skateboard will move.
Why do you keep saying this? Did the person exert a (horizontal) force on the wheel/skateboard: Yes! So you can deduce, from Newton's laws, that:
(1) The wheel/skateboard exerted an equal and opposite force on the person/skateboard. (Newton's 3rd law)
(2) Since this is the only force exerted (no friction to stop the skateboards from rolling) on each skateboard system, each system has a net force acting on it and it will accelerate. (Newton's 2nd law)

And why did you start a new thread on the same topic?
 
  • #3
well, its not exactly the same...I just had a question on the effect of impulse in this situation.
 
  • #4
Yes but you're still making the mistake of stating that neither skateboard will move. They will both move.

This is more obvious if you draw a free-body diagram of the system and make sure you always draw paired forces because of Newton's 3rd Law.

In answer to your question about impulse, I think if the system is truly frictionless, time should make no difference. The reason this clashes with your intuition is that neither you nor I has ever seen anything remotely close to a frictionless system. In a system with friction, time could make a huge difference.

Also note that, even if pushing with the same force, you would have to exert more energy to push a skateboard by just pushing on the water wheel, compared with pushing directly on the skateboard. Why? Because work is force times distance, and since the wheel is moving as well as the skateboard, you have to push the fin a farther distance than the skateboard moves. The extra energy you expended then goes into the rotation of the water wheel.
 

Related to Impluse from spinning a wheel while standing on a skateboard

1. How does spinning a wheel while standing on a skateboard create an impulse?

Spinning a wheel while standing on a skateboard creates an impulse because of the conservation of angular momentum. When the wheel spins, it has a certain amount of angular momentum. When you suddenly stop the wheel's rotation, that angular momentum needs to go somewhere, so it is transferred to the skateboard and causes it to move forward.

2. How does the impulse from spinning a wheel affect the skateboard's motion?

The impulse from spinning a wheel while standing on a skateboard causes the skateboard to move forward. This is because the impulse changes the skateboard's momentum, which is the product of mass and velocity. As the wheel's angular momentum is transferred to the skateboard, it increases the skateboard's velocity, causing it to move forward.

3. Is the impulse from spinning a wheel while standing on a skateboard the same as pushing off with your foot?

No, the impulse from spinning a wheel while standing on a skateboard is not the same as pushing off with your foot. Pushing off with your foot creates a force that propels the skateboard forward, while the impulse from spinning a wheel is a transfer of angular momentum that causes the skateboard to move forward. Both actions result in forward motion, but they are caused by different physical mechanisms.

4. How does the radius of the spinning wheel affect the magnitude of the impulse?

The radius of the spinning wheel does not affect the magnitude of the impulse. This is because angular momentum is a conserved quantity, meaning it cannot be created or destroyed. The only way to change the magnitude of the impulse is by changing the initial angular momentum of the wheel or the duration of the wheel's rotation.

5. Can the impulse from spinning a wheel while standing on a skateboard be used to turn or steer the skateboard?

Yes, the impulse from spinning a wheel while standing on a skateboard can be used to turn or steer the skateboard. This is because the impulse affects the skateboard's momentum, which is a vector quantity. By changing the direction of the impulse, you can change the direction of the skateboard's momentum and thus steer it in a different direction.

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