Physics
posted by Becky .
A ball is moving at 82 m/s when it hits a ball at rest which has a mass fours times as big as its own. The formally stationary ball leaves the collision with a speed of 7.5 m/s what is the final velocity of the first ball?

The law of conservation of linear momentum for elastic collision
m1•v1 = m1•u1 + m2•v2.
The velocities after collision are
u1 =(m1m2) •v1/(m1+m2)
u2 =2•m1•v1/(m1+m2)
Since m2=4m1,
u1 =(m 4m) •82/(m+4m) =  49.2 m/s
u2 =2•m •82/(m +4m) = 32.8 m/s