Question

Ball B, with a mass of 30kg, is moving to the left at 10 m/s. With what velocity should Ball A, with a mass of 10kg, move to the right and collide with 2, so that ball A rebounds with a velocity of 30 m/s, and Ball B with velocity of 10 m/s, after the collision? Assume the collision to be perfectly elastic.

Answer 1

30(-10)+10(V)=10(+30)+30(-10)
it is unclear to me which direction B is after the collision(I assumed left above)
Left was negative, Right is positive

10V=300-300+300
V=30m/s if I got the directions correctly.

Answer 2

But it was elastic so B has to go right after
V = -30

before
A moving right at 30
after
A moving left at 30

before
B moving left at 10
after
B moving right at 10

speeds of both the same before and after so same Ke
momentum is zero before and after

Answer 3

Thank you SO MUCH!