QUESTION Ball A is moving with velocity of v, it collides elastically with five stationary balls. All six balls are of equal mass. What happens next? SOLUTION I want to find out how many balls move off from those that were stationary and with what velocity they move away. I apply the elastic collision equation ①, conservation of momentum ② and conservation of kinetic energy ③. After substituting into equation ② from ① and deriving an equation for the outgoing mass (mB) I then think about what possible outcomes there could be. This leads me to the conclusion that mB = m is the only valid solution. Then I feed that solution back into equation ② to give me vB = v. Please see attached PDF. I don't actually use equation ③ directly so it is superfluous, though equation ① is derived from it. My main concern is the use of equation ① in the absence of knowing exactly how much mass moves out of the problem - is my use of it correct? If my solution is not valid is there a way of using cons. of momentum and KE to solve this problem?* Any insight gratefully received. * I did try thinking about the problem with mA margining into the masses and having an unknown mC and mD moving away either left and right or right or stationary and right and using mA + mB = mC + mD, but even with that I couldn't derive enough equations to eliminate unknowns.