# hitting a pool ball at exactly 90 degrees

So a friend and I had an argument and I thought I might get some unbiased proof here. Is it possible to hit a pool ball with another pool ball at exactly 90 degrees. Ie if I had a white ball could I hit another ball to go exactly in a perpendicular direction to the white ball?

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Yes and no. You'd have to hit exactly tangentially and the resulting speed would be exactly zero. (I assume you mean perpendicular to the original direction o fth ewhite ball) –  Hagen von Eitzen Dec 29 '13 at 21:57
But would it impact. Imo it would not impact. Ie if it were to hit it was by definition not tangential ( –  Murdock Dec 29 '13 at 22:00
And yes. Thats what I meant –  Murdock Dec 29 '13 at 22:00
Its more a theoretical question. I would argue that 89.99999... degrees is possible but not 90 degrees –  Murdock Dec 29 '13 at 22:08
Just use a masse shot to curve the ball, that will work –  Asimov Sep 6 '14 at 15:41

Unless the ball you strike is able (or allowed) to change direction before it hits the second ball, (i.e. by bouncing off a cushion) the answer is no. This is due to conservation of momentum and energy. Consider the white ball of mass $m_w$ travelling along a horizontal plane with velocity $v$. The second ball, with mass $m_s$ is initially stationary, so has velocity 0. If the second ball were travelling at right angles to the white ball after impact with velocity $w$, by conservation of momentum, the momentum of the white ball is now $m_w*v-m_s*w$, and for the second ball it is $m_s*w$. But here you have created energy from nowhere - the total kinetic energy of the balls is now greater after the impact than it was before the impact, a contradiction.