Question

Consider a system to be two train cars traveling toward each other. What is the total momentum of the system before the train cars collide? kg • What must the total momentum of the system be after the train cars collide? kg •

Answer 1

What is the total momentum of the system before the train cars collide?

 (1600) kg • ms

What must the total momentum of the system be after the train cars collide?

 (1600) kg •  ms

Answer 2

Let say the two train cars are of masses $m_1$ and $m_2$

now if the speed of two cars are $v_1$ and $v_2$

then we can say that the momentum of two cars before they collide is given by

$P = m_1v_1 - m_2v_2$

here two cars are moving in opposite direction so we can say that the net momentum is subtraction of two cars momentum.

Now since in these two car motion there is no external force on them while they collide

So the momentum of two cars are always conserved.

hence we can say that the final momentum of two cars will be same after collision as it is before collision

$P = m_1v_1 - m_2v_2$