Elastic Collision: Rotational Momentum and Linear Momentum

[luckis11]

Since I smell intention of deceit and covering up the problem, I now tell you that I PROOVED that the average velocity of the circles in the box will keep increasing as the number of oblique collisions increases, and now you have to proove me wrong.

 

[Doc Al]

Since I smell intention of deceit and covering up the problem, I now tell you that I PROOVED that the average velocity of the circles in the box will keep increasing as the number of oblique collisions increases, and now you have to proove me wrong.

I must have missed your "proof". Which post contained that proof (which would violate known laws, by the way).

DaleSpam did prove you wrong, quite elegantly. Take a specific case where you have the sum of the speeds increasing. State the initial parameters and final parameters (angle, speeds, etc). Then just take that "final" state and reverse the velocities. That will be an example of a collision where the sum of the speeds decreases.

 

[Dale]

Since I smell intention of deceit and covering up the problem,

This is very rude and completely unwaranted.

In post 6 I gave you a reputable link about your original question as requested. In post 14 I gave several physical examples of conservation of momentum and an explanation from first principles about conservation of angular momentum. In post 19 I clearly answered your follow-up question and carefully explained that your sum can increase, decrease, or stay the same depending on the details, and therefore that it is not conserved in general nor can you make general statements about it. In post 24 I corrected your incorrect assumption and pointed out that your own thought experiment should have lead to your rejecting your incorrect assumption. In post 26 I carefully detailed a computation that you could do to find cases where the sum decreases. In post 28 I explained a much more elegant solution to the problem.

In all of these I have not been the least bit decietful. On the contrary I have provided a wealth of good and pertinent information and answered every question you posed or provided a detailed method for you to answer it yourself.

I do not deserve that at all.

 

[luckis11]

Do you mean reverse the motions that took place, like playing backwards a movie?

Dalespam with the "deceit" I was referring to the responses for the last problem. And "smell" means suspect.

 

[Doc Al]

Do you mean reverse the motions that took place, like playing backwards a movie?

Yes.

 

[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.