Why do two colliding cars have the same speed after collision?

[jahlin]

Homework Statement

Two cars 1 and 2 moving against each other collides.There are no external forces acting on them.

Homework Equations

conservation of linear momentum:
m1*v1+m2*v2 =(m1+m2)v12

The Attempt at a Solution

I have solved the problem but my question is how come they have a common speed after the collision?I don't understand this part can someone clarify it please.

 

[mikelepore]

The equation you wrote is the one used when the two objects stick together when they collide, so after the collision there is only one object with a mass of m1+m2. This is called a perfectly inelastic collision. You can't use that equation if the objects bounce off each other.

 

[jahlin]

thanks a bunch mikelepore!