This discussion focuses on the mechanics of momentum, normal force, and friction, particularly in the context of collisions and interactions between objects. The participants analyze scenarios involving two cubes in a vacuum and a spinning wheel interacting with a projectile. Key concepts include the coefficient of restitution, conservation of momentum, and the role of normal force and friction, which are not dependent on surface area. The conversation highlights misconceptions about how friction operates and the implications of surface area on force interactions.
PREREQUISITESPhysics students, mechanical engineers, and anyone interested in understanding the principles of momentum, force interactions, and friction in mechanical systems.
I am unable to find the mention you refer to in the article.cardboard_box said: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.
"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."Lnewqban said:I am unable to find the mention you refer to in the article.
Could you guide me to it, please?
Thank you for pointing that out.cardboard_box said:"... 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.
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.cardboard_box said:yes that makes sense, but that also means that the contact area of the wheel with the projectile just doesn't really matter
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.A.T. said: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.
I doubt any mechanism in my range of building get to large enough velocities or contact areas for it to matter.A.T. said: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.
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.cardboard_box said: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,
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.cardboard_box said:similarly it means you don't really need both wheels to "push" since it can't exceed maximum surface velocity anyways.