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# Physics - Momentum Question watch

1. "Two balls are moving in opposite directions with velocities 5ms−1 and 10ms−1. They collide, and move off in opposite directions with new velocities of 7.5ms−1 each. If the mass of the first ball was 1.25 kg, what is the mass of the second ball?"

My current attempt:
P = mv | Momentum Before Event = Momentum After Event

BEFORE EVENT
I knew the momentum of the:
First ball: ([5 ms^-1]*[1.25kg])=6.25 kg ms^-1
Second ball: 10m kg ms^-1

AFTER EVENT
First Ball: ([7.5 ms^-1]*[1.25kg])=9.375 kg ms^-1
Second Ball: 7.5m
-----------------------------------------------------------------------------------
So I put this into an equation:
10m + 6.25 = 7.5m + 9.375
2.5m + 6.25 = 9.375 (-7.5m)
m = 3.125/2.5 (/2.5)
m = 1.25 kg

So I got the same mass as the first ball for the second ball. I feel this is incorrect, can someone check please?
2. (Original post by Y10_GCSE)
"Two balls are moving in opposite directions with velocities 5ms−1 and 10ms−1. They collide, and move off in opposite directions with new velocities of 7.5ms−1 each. If the mass of the first ball was 1.25 kg, what is the mass of the second ball?"

My current attempt:
P = mv | Momentum Before Event = Momentum After Event

BEFORE EVENT
I knew the momentum of the:
First ball: ([5 ms^-1]*[1.25kg])=6.25 kg ms^-1
Second ball: 10m kg ms^-1

AFTER EVENT
First Ball: ([7.5 ms^-1]*[1.25kg])=9.375 kg ms^-1
Second Ball: 7.5m
-----------------------------------------------------------------------------------
So I put this into an equation:
10m + 6.25 = 7.5m + 9.375
2.5m + 6.25 = 9.375 (-7.5m)
m = 3.125/2.5 (/2.5)
m = 1.25 kg

So I got the same mass as the first ball for the second ball. I feel this is incorrect, can someone check please?
should be correct theoretically as they both move off at same speed

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