Store the velocity of one item, when it hits the other sprite, set the velocity to 0 and set the velocity of the other sprite to the original velocity of the original sprite.
That's the only thing I can think of
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First of all, here's a very basic simulation of two balls colliding accurately (and transferring momentum from one to the other): http://scratch.mit.edu/projects/amcerbu/2182230
If that doesn't answer your question, give me a bit of time, I'll have to prepare a longer response.
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