1. The problem statement, all variables and given/known data A mine car of mass 440kg rolls at a speed of 0.50m/s on a frictionless horizontal track. A chunk of coal of mass 150kg has a speed of 0.80m/s as it leaves a chute above the car. The angle of the chute is 25 degrees from the horizontal. After the coal has come to a rest in the car, what is the velocity of the car and coal system. 2. Relevant equations mv1 + mv1 = v(m1+m2) 3. The attempt at a solution Originally I thought all momentum was conserved but then that wouldn't make sense because if it did the cart would not continue rolling straight forward. I got the correct answer if only taking x component into account. I just don't understand why you do this and how come the y component is not included in calculating momentum. The solution says: Momentum is conserved in the horizontal direction during the "collision." Let the coal be object 1 and the car be object 2. So my question in other words is how come its only conserved in horizontal direction? Thanks.