# Elastic collision?

You are standing on a sheet of ice that covers the football stadium parking lot in Buffalo; there is negligible friction between your feet and the ice. A friend throws you a ball of mass 0.400 kg that is traveling horizontally at 10.8 m/s. Your mass is 70.4 kg.

If you catch the ball, with what speed do you and the ball move afterwards?
6.1 cm/sCorrect

Part B
If the ball hits you and bounces off your chest, so afterwards it is moving horizontally at 7.90 m/s in the opposite direction, what is your speed after the collision?
cm/s

with the second part, i calculated the first momentum before the collision then the one for after. i subtracted the after from before and devided my the mass of the boy and i got 1.64 cm/s. is this wrong? the velocity of the boy before the collison is zero becasue he's at rest.

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#### OlderDan

Homework Helper
The final momentum of the ball is negative compared to the inital momentum. Your calculation has treated it as positive.

you mean 1.65 should be a negative if so, i still ended up wrong with it.

#### OlderDan

Homework Helper
you mean 1.65 should be a negative if so, i still ended up wrong with it.
No.

You calculated the initial momentum of the ball and got _________

You calculated the final momentum of the ball and got _________

You subtracted the second number from the first treating both as postive, but in fact your second number is negative. If you treat it correctly, your final velocity will be greater in part B than it was in part A.

thnaks dude. i solved it was 10.6cm/s

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