# max veloicty after collision

#### FistLength

7+ Year Member
The question involves a bob swung from height h and allowed to collide with bob m2 which is at rest.

EK physics 407
As the mass of m1 becomes much greater than m2, what is the maximum velocity that can be reached by m2? (assume completely elastic collision)

A: 0 m/s
B: v
C: 2v
D: The maximum velocity is infinite

Answer is C, 2v

Can someone please explain how?

5+ Year Member
OP
F

7+ Year Member

#### sps27

5+ Year Member
That was a good question. I had been thinking about it. The eqns are a bit tedious to go through but it does work out. I doubt EK would have taken the pains to go through all of that to explain.....

#### patrickmbyrne

They didn't really show the derivation of equations 9.21 and 9.22... the just kinda throw them out there

#### sps27

5+ Year Member
They didn't really show the derivation of equations 9.21 and 9.22... the just kinda throw them out there
V1(i) + V1(f) = V2(i) + V2(f) (shown in the derivation)
V1(f) = V2(i) + V2(f) - V1(i). Now apply this to 9:16 eqn and solve for V2(f). Likewise V2(f) = V1(i) + V1(f) - V2(i) and apply that to eqn 9:16. Solve for V1(f). You will get those 9:21 and 9:22 eqns.