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Law of Restitution & Momentum 
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#1
Apr1812, 01:09 PM

P: 64

Hello everyone , here we go :
I understand that momentum is always conserved as long as no external forces act on a given system . I also understand that the law of restitution applies to the axis perpendicular to the line of impact between two objects .However i have a problem . For example . A ball hits the ground at an angle of 45 degrees and bounces up at an angle of 30 degrees. So from the law of restitution : e=  ( Vball*sin30  Vground)/(Uball*sin45  U ground) , the ground does not move therefore Uground = Vground=0 so e =  Vball*sin30/(Uball*sin45). My question is , why can't we apply momentum conservation on the y axis ? Is it because the reaction force when hitting the ground is considered an external force ? I am very confused please explain :S Thank you ! 


#2
Apr1812, 01:28 PM

Mentor
P: 41,315

If you expand your 'system' to be 'ball + ground/earth', then momentum will be conserved again. (The force between ground and ball would then be an internal force.) 


#3
Apr1812, 01:32 PM

P: 64

So my restitution formula is wrong and the only thing i have in my "system " is the ball , no ground etc . So it should be e=  V*sin30/(u*sin45) but e can't be negative ....argh !!! :S



#4
Apr1912, 05:58 PM

P: 64

Law of Restitution & Momentum
Let's assume that two balls(m1=m2=m ) collide with one another .
The first ball is traveling at a speed u while the second ball is stationary.The first ball strikes the second one at an angle θ to the line of impact.If the coefficient of restitution is e , find the angle at which the second ball travels after the impact. What i don't understand in this case is why isn't momentum conserved both along the line of impact and on the axis perpendicular to it as well ? I mean , both balls are part of our system so there are no external forces ?! Could you please solve this exercise and explain each step thoroughly so i can finally understand ? Thank you very much !!! 


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