Kaplan Momenta Q

[Hexon]

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Two baseballs undergo a head on collision. Ball 1 is twice as heavy as ball 2. Ball 1 was travelling at an initial speed of v1, while ball 2 had an initial speed of v2. The type of collision is elastic.
If ball 1 travels at a speed of 7/5 v1 after impact, what is the speed of ball 2?

Answer:
the speed of ball 2 after impact = 1.6 v1 = 2 v2

i got mixed up with this one, how does one go about doing this?

 

[mynamehere]

if it's elastic, momentum and KE is conserved so:

Ball 1 - mass 1 = 2 mass 2
Ball 2 - mass 2
I made ball 1 go to right (+) so ball 2 goes in (-) direction
after collision, they go in opposite direction:
ball 1 goes opposite direction (-), same with ball 2 (+2)

m1v1 + m2(-v2) = m1(-7/5 v1) + m2 (vf2)
m1=2m2

2m2v1 - m2v2 + 2m2 (7/5 v1) = m2(vf2)
m2 (2v1 - v2 + 14/5 v1) = m2(vf2)
cancel m2

2v1 - v2 + 14/5 v1 = vf2
24/5 v1 - v2 = vf2


?

 

[Hexon]

if it's elastic, momentum and KE is conserved so:

Ball 1 - mass 1 = 2 mass 2
Ball 2 - mass 2
I made ball 1 go to right (+) so ball 2 goes in (-) direction
after collision, they go in opposite direction:
ball 1 goes opposite direction (-), same with ball 2 (+2)

m1v1 + m2(-v2) = m1(-7/5 v1) + m2 (vf2)
m1=2m2

2m2v1 - m2v2 + 2m2 (7/5 v1) = m2(vf2)
m2 (2v1 - v2 + 14/5 v1) = m2(vf2)
cancel m2

2v1 - v2 + 14/5 v1 = vf2
24/5 v1 - v2 = vf2


?

that's exactly where i got stuck as well,
where do i go from there?

 

[mynamehere]

since it's elastic, KE is also conserved.. i think if you use this fact and substitute , you might be able to get the answer?

sorry i started doin this and saw the heavy algebra which will most definitely not be necessary on mcats... .. RIGHT?! D: