DaveC426913
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Doc Al said:And the answer to that, which I thought I gave a while back, is that we are assuming that the balls are smooth--no friction. Thus any force they exert can only be perpendicular to their surface.
Yep. I think that's the crux. But I'm not sure it's entirely obvious why that's so.
I've been trying to come up with a macro-scale example of poking an object such that you can't impart any directional momentum on it. Like pushing a floating boat with your finger, or tapping a balloon.