## nasNs's Profile User Rating:

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1. #### In Topic: How to get out of collision when using posteriori (discrete) method

Posted 24 Sep 2012

macosxnerd101, on 24 September 2012 - 09:50 AM, said:

This shouldn't happen. If the step size is n, then there is an overlap between the two circles of diameter n. On the next step, they should move n distance away from each other. Since you are dealing with one dimension, there is no issue.

This does happen. I was only giving a simple example just for illustration. In a general case, with many particles moving in all directions, this suitation can develope. Here is a simple example. Imagine 4 balls in the x axis like this, where ball A and D are moving as shown, and ball B and C are not moving. Now ball D will knock ball C to the left and ball A will knock ball B to the right:

-> <-
AB CD

In the next time step, ball B and C will move towards each others and they can enter into each others may be becuase the time step is too large

-><-
B C

Now in the next time step, B and C will have speeds in the other directions,

<- ->
B C

but the new position will be such that they are still a little bit inside each others. So they are still in collision. So in next step after that they will move back into each other

-><-
B C

and so on.

In other words, the 2 balls remain in collision for more than one time step. Once the 2 balls remain in collision for more than one step, it is hard for them to separate unless a 3rd ball comes in to break them apart.

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