# momentum/impulse car collision

• May 16th 2008, 03:10 PM
jason03
momentum/impulse car collision
Heres the problem Im working on

http://img91.imageshack.us/img91/1161/cartz0.jpg

Im trying to figure the setup to find which car was going faster and how fast....

Would I set up the equation before the collision and after...the collision is perfectly plastic so

$m_{a}v_{a} + m_{b}v_{b} = (m_{a} + m_{b})v\prime$

also since its perfectly plastic

$v\prime_{b} = v\prime_{a} = v\prime$
• May 16th 2008, 04:55 PM
mr fantastic
Quote:

Originally Posted by jason03
Heres the problem Im working on

http://img91.imageshack.us/img91/1161/cartz0.jpg

Im trying to figure the setup to find which car was going faster and how fast....

Would I set up the equation before the collision and after...the collision is perfectly plastic so

$m_{a}v_{a} + m_{b}v_{b} = (m_{a} + m_{b})v\prime$

also since its perfectly plastic

$v\prime_{b} = v\prime_{a} = v\prime$

1. I hope you understand the vector nature of momentum ......

2. The above point is actually moot since the application of Conservation of Momentum is NOT valid since there are external forces (how else are the skid marks produced?) acting on the system.

More information (such as the length of the skid marks) is necessary before the question can be correctly answered.

3. If you mean by perfectly plastic that the cars are stuck together after the collision, then of course it follows that $v\prime_{b} = v\prime_{a} = v\prime$. But you're told the cars are stuck together after the collision ...... why confuse the issue by talking about perfectly plastic collisions .....?

And you do realise that the collision is not elastic, right?
• May 18th 2008, 05:43 PM
jason03
What did you mean by the above point is moot?

I do understand what you are saying about the skid marks and such, but this textbook problem is simplyfying the problem of the two cars colliding...So I believe this problem as the others I;ve worked on External forces are neglected....

..and the problem says the cars are stuck together after the crash so it would be a perfectly plastic collision...meaning the coeficent of restituion is 0....