1. The problem statement, all variables and given/known data Two cars 1 and 2 moving against each other collides.There are no external forces acting on them. 2. Relevant equations conservation of linear momentum: m1*v1+m2*v2 =(m1+m2)v12 3. 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 dont understand this part can someone clarify it please.