Homework Help: Mechanics : Collision

1. Aug 19, 2009

sranjanr

Three perfectly elastic balls A, B, and C of masses 2 Kg, 4 Kg, and 8 Kg, respectively move along the same straight line and in the same reaction with velocities of 4 m/s,
1 m/s, and 0.75 m/s. The ball A collides with ball B and subsequently the ball B impinges with ball C. Make calculations to show that the balls B and C will be brought to rest after the impacts

As I solved :

Putting the equation

v1=u1(m1-m2)+2*m2*u2/m1+m2
v2=u2(m2-m1)+2*m1*u1/m1+m2

Now m1=2Kg , m2=4Kg , u1=4m/s , u2=1m/s

v1 =4(2-4)+2*4*1/2+4=0
v2 =1(4-2)+2*2*4/2+4=3

now ball A get rest and ball B is in 3m/s

now as same before putting equation for B and C

m1=4kg , m2=8 kg ,v1 =3 m/s ,v2 ,0.75 m/s

v1=3(4-8)+2*8*0.75/4+8=0
v2=0.75(8-4)+2*4*3/4+8= 27/12

but as per question V2 should be 0

Am i doing any wrong step ?

2. Aug 19, 2009

kuruman

Your starting equation is incorrect. The first term has dimensions of momentum (as it should not) and the second term dimensions of velocity (as it should). Please check your starting formula again.

3. Aug 19, 2009

sranjanr

I could not get .could you tell me which equestion should i apply ?

4. Aug 19, 2009

kuruman

$$v_{1} = \frac{m_{1}-m_{2}}{m_{1}+m_{2}}u_{1}+\frac{2m_{2}}{m_{1}+m_{2}}u_{2}$$

To get v2, just swap subscripts 1 and 2.