B Questions about momentum and force as well as normal force/friction

[Lnewqban]

in the article they mention that the model they use for friction does not work the same for objects at high velocities, do you have any source for me to read about high velocity affect on friction and what is considered high enough velocities? it seems like it might be the answer I am looking for since the wheel's RPM seems to determine projectile velocity aka the force exerted on it.

I am unable to find the mention you refer to in the article.
Could you guide me to it, please?

 

[cardboard_box]

I am unable to find the mention you refer to in the article.
Could you guide me to it, please?

"The equations given for static and kinetic friction are empirical laws that describe the behavior of the forces of friction. While these formulas are very useful for practical purposes, they do not have the status of mathematical statements that represent general principles (e.g., Newton’s second law). In fact, there are cases for which these equations are not even good approximations. For instance, neither formula is accurate for lubricated surfaces or for two surfaces siding across each other at high speeds. Unless specified, we will not be concerned with these exceptions."

seems to suggest kinetic friction (no idea if static friction too) acts differently then the formulas at high velocities. I don't know what is considered high enough velocity for it to apply and it is probably not very useful for my mechanism, but I am intrigued about this.

 

[Lnewqban]

"... For instance, neither formula is accurate for lubricated surfaces or for two surfaces siding across each other at high speeds..."

seems to suggest kinetic friction (no idea if static friction too) acts differently then the formulas at high velocities. I don't know what is considered high enough velocity for it to apply and it is probably not very useful for my mechanism, but I am intrigued about this.

Thank you for pointing that out.

I have found this article that describes the range of velocities that can cause changes in the molecular structure of the surfaces of metals, I believe:
https://www.sciencedaily.com/releases/2022/09/220906114220.htm

I also believe that your machine does not reach speeds of that magnitude.

 

[A.T.]

yes that makes sense, but that also means that the contact area of the wheel with the projectile just doesn't really matter

That is a weird reply to a post that had nothing to do with contact area, but was about velocity. You seem to be all over the place.

In any case, the simple model of friction, that is independent of velocity and contact area is just an approximation. It doesn't necessarily hold over arbitrarily large ranges of these variables, and for arbitrarily short interactions.

 

[cardboard_box]

That is a weird reply to a post that had nothing to do with contact area, but was about velocity. You seem to be all over the place.

yes, let me give a hopefully better example, if we are using a linear shooter (2 counter-rotating wheels and the projectile in the middle) then we add another set of wheels (fully identical to the first set with same RPM and everything). it really shouldn't matter at all to the final velocity of the projectile, that seems a little counter intuitive since there are more wheels pushing on the projectile. similarly it means you don't really need both wheels to "push" since it can't exceed maximum surface velocity anyways.

In any case, the simple model of friction, that is independent of velocity and contact area is just an approximation. It doesn't necessarily hold over arbitrarily large ranges of these variables, and for arbitrarily short interactions.

I doubt any mechanism in my range of building get to large enough velocities or contact areas for it to matter.

 

[A.T.]

if we are using a linear shooter (2 counter-rotating wheels and the projectile in the middle) then we add another set of wheels (fully identical to the first set with same RPM and everything). it really shouldn't matter at all to the final velocity of the projectile,

A second wheel pair at the same speed can help, if the first wheel pair fails to achieve the desired speed, for example because of insufficient traction.

similarly it means you don't really need both wheels to "push" since it can't exceed maximum surface velocity anyways.

You do need both wheels to push equally, if you want the ball's center of mass to match the wheels' circumferential speed. A single wheel can only accelerate one side of the ball to its own circumferential speed, while the ball center of mass moves slower as the ball rotates.