Elastic Collision: Rotational Momentum and Linear Momentum

[Dale]

Dalespam with the "deceit" I was referring to the responses for the last problem.

My responses for the last problem were accurate and complete. I gave you a detailed approach for working out the problem (which is sufficient even in the homework forums) and then followed up with a far more elegant symmetry approach. I gave you two complete methods for solving the problem!

Your accusation of intentional deciet is completely unwarranted, especially regarding my responses for the last problem.

 

[luckis11]

Take the case that one of the two balls (call it B) was still before the collision, regarding the duration of the collision as zero (in the case of that simulation with the circles). The velocity of the moving ball (call it A) before the collision (and before reversing the movie) is u and is on the axis of x.

So when reversing the movie, after the collision the sum of the absolute values of the component velocities at the axis of x remains the same and the two opposite component velocities at the axis of y will cancel each other out, right?

But in the normal direction of the movie, two opposite rotational momentums are created. I can't see how will they cancel each other out at the reversal of the movie. Or no rotation can be created if the duration of the collision is zero according to the simulation? What about the case of reality?

Thanks, you did help me think of this reversal of the movie (although I had to lie that I prooved something whereas I was doubting about it, in order to force you to give me an understandable epxlanation). Also thanks that you let me know that not only momentums are created out of zero, but also angular momentums out of zero.

However, leaving the theoretical simulation, in reality, the two opposite momentums that were created out of zero in the normal direction of the movie, I suspect that they have come out of the rest momentum of the balls, i.e. the balls are actually springs. If this is so, then the absolute values of the two opposite momentums that were canceled out at the reversal of the movie collision, is transferred somewhere else, e.g. rest momentum and frictions.

The problem also troubles me regarding what should be reasonably expected in the collisions of particles «with no mass» ignoring the theory of relativity, e.g. at the simulation with the circles. Then the cancelling out of the two opposite velocities seems reasonable on one hand and unreasonable on the other hand. And in this case, transfer of momentums into and out of rest momentums and frictions, is out of question. By "rest momentum" I mean the momentum of the molecules, electrons etc of a still ball.

 

[Dale]

I had to lie that I prooved something whereas I was doubting about it

That would certainly explain the smell of deceit. I always find it interesting when one person brazenly accuses another of being guilty of the very thing that they themselves are doing in that very moment.

Frankly, it is not worth the aggravation to correct your mistaken notions. I suggest that you take an introductory physics course from a local community college or university. Hopefully your professor will be willing to spend the time required to help you work through your confusion. However, I would recommend that you treat him or her with more courtesy than you have shown here, or they will likely be as disinclined to help as I am.