# Thread: Friction when pool balls collide

1. Hi all

simple question really, when two pool balls collide, does the amount of friction at the point of contact between them increase or decrease relative to their speed?
I would think that the more energy there was the more friction there would be, is that right?

Many thanks

2. Maybe not as simple as I originally thought. Anyone?

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3. No it's not simple. There are quite a few factors but the quick answer is yes. I think the amount of energy transferred would be a combination of the force perpendicular to the contact point and the angle of the glancing blow from the other ball and the share would depend on the friction between the balls. At the moment of impact the balls compress ever so slightly and that is the opportunity for some energy to be transferred in the form of spin. It would be very little compared to the force that is sending the object ball in the required direction though and not very much spin would be imparted.

So the amount of energy transferred in spin would *very* roughly depend on the force, the coefficient of friction, and the time period of the contact (surface area isn't a factor in friction calcs). The greater the force, the more compression and so the longer the time they are in contact and able to transfer energy as spin.

4. Thanks very much Rarry. If you don't mind my taking it a little further... Assuming a a full on contact, do you think there would be a drop off, a flattening of the curve at some point, i.e. speed reducing the amount of time the balls are in contact and so the friction?

5. I'd be really guessing. There are too many factors with a lot of uncertainty but I would think it would drop off, yes. With extra speed, and so extra spin the surfaces might not interact so well and simply ricochet without imparting so much spin, and as you say, with extra speed the contact time would lessen as there is only so much compressibility to balls and you'd expect the extra speed would shorten the bounce time. Mark up some balls with a felt tip and try a high speed phone camera? There's more art than science to a question like this and it would be easy to overlook or misjudge some factor that would change things a lot so I wouldn't have a high degree of confidence in predicting the outcome.

6. Yes, there are a lot of variables involved aren't there. I've already done a little high speed videoing and nearly gone blind watching such like on youtube.

Thank you once again for your input, it's much appreciated.

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