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Perfectly elastic collision 
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#1
Nov308, 12:08 PM

P: 21

1. The problem statement, all variables and given/known data
A 15 g ball is fired horizontally with speed v0 toward a 117 g ball hanging motionless from a 1.53mlong string. The balls undergo a headon, perfectly elastic collision, after which the 117 g ball swings out to a maximum angle θmax=53°. What was v0? 2. Relevant equations h=L(Lcos(theta) V=Square root of (m*g*h/(.5*M) little m is ball moving and M is the ball not moving (v2x)f=2m1/m1+m2 (V1x)i 3. The attempt at a solution Im not sure if im using the right equations and whether the angle given is the one we use in the equation, or if we have to subtract it from 90 first 


#2
Nov308, 12:41 PM

HW Helper
P: 5,341

Initial KE is 1/2*m*v^{2} The second is that the angle because it is hanging is with the vertical. Hence the height should be given by the Cosθ times the string length subtracted from the length. (h = length  projection of the string to the vertical.) So ... 1/2*m1*v^{2} = m2*g*(L  L*cos53) v^{2} = 2*(m2/m1)*g*(LL*Cos53) 


#3
Nov308, 01:07 PM

P: 21

is this the equation i use to get the answer or do i have to plug it into another one 


#4
Nov308, 02:09 PM

HW Helper
P: 5,341

Perfectly elastic collision
With that final velocity for the stationary ball after impact and the usual equations for conservation of energy and momentum, you should now have 2 equations and 2 unknowns, one of which is the Vo that they ask for. 


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