Ball 1, with a mass of 10kg, is moving to the right at 20m/s. Ball 2 is moving to the left at 20 m/s. Upon collision, ball 2 comes to a standstill, and ball 1 moves to the left at twice its original speed. What is the mass of Ball 2?

Short answer, 10kg, it must be the same mass as Ball 1Long calculation to prove the answer:M1 = Mass of Ball 1M2 = Mass of Ball 2U1 = Initial speed of Ball 1U2 = Initial speed of Ball 2V1 = Final speed of Ball 1V2 = Final speed of Ball 2M1U1 + M2U2 = M1V1 + M2V210 x 20 + M2 x 20 = 10 x 40 + M2 x 0200 + 20M2          = 40020M2                   = 200M2                      = 200 / 20M2                      = 10 kgAlternatively you could say that because the velocity of ball 1 increased by the exact velocity of ball 2, their masses must be equal, so both ball 1 and 2 are 10 kg.