Question

A 12kg cart rolling at 18m/s to the right collides with a 15kg cart that is at rest. After they collide, the 15kg cart is moving to the right with a velocity of 5m/s. What is the final velocity of the 12kg cart? 

Answer 1

$$mv=mv'+Mu\\(12)(18)=12v'+(15)(5)\\v'=11.75\frac{m}{s}$$

It is moving to the right .