physics

[rubah]

Our lab today was just to work two problems, and I really failed at them both. One of them was because of algebra and I'm hoping the other was also, but who knows?!


You and your friend are playing soccer. Your friend kicks a soccer ball at 8 m/s toward you. Instead of kicking it back like normal, you decide to kick a slightly larger ball at it to make it reverse directions. They hit head on with a coefficient of restitution of .9

a) If the soccer ball has an inertia of 600g, how fast would the larger 750g ball have to be travelling to exactly reverse the soccer ball's velocity?

For this one, I wrote a couple of equations based off the conservation of momentum law and the definition of coefficient of restitution.


1) [600g (m1) * 8 (v1)] - [750g (m2) * v (v2)] = [600g (m1) * -8 (reversed v1)] + [750g (m2) * v` (v4)]

2) v` = -.9v - .8
and when I solve those two, I get v = 136 m/s, which is kinda way unrealistic. Leaving the masses in grams doesn't affect it at all, does it?

Anyways, equations for those:
1) P(sys)i = P(sys)f
2) e = (vel2f - vel1f)/(vel2i - vel1i)

[qwertysaur]

yes, you should definatly convert all mass units to Kilograms. Remember, a Newton of force is a kilogram meter per second squared.

[blackmage_nuke]

for the first formula, using kg i got 12.8 = v2 + v4

Then with coefficient of restitution using this formula i got from wikipedia
http://upload.wikimedia.org/math/4/4/2/442cb57fc9daffed79a534632d0a5aad.png
which is slightly different to your formula (particularly the bottom)
I got 10(v4)+80 = 72 +9(v2)

then using simultaneous equations with
12.8 -v2 = v4 and
10(v4)+80 = 72 + 9(v2)
(subbing)
i got v2 = 7.15789...
or v2 = 136/19

but i could be wrong, this coefficient of restitution stuff is new to me. I might have gotten some of my negatives in the wrong place too.

I dont think it has anything to do with newtons

edit: you have to be pretty damn good at soccer to kick a ball and hit someone elses ball in mid air

[rubah]

yeah, that worked, blackmage_nuke :) Stu showed me how to do the rest of it, so~