This is the specific question I refer to (exam practice):
Particle P has mass 3kg and particle Q has mass 2kg. The particles are moving in opposite directions on a smooth horizontal plane when they collide directly. Immediately before the collision, P has speed 3 ms^–1 and Q has speed 2 ms^–1. Immediately after the collision, both particles move in the same direction and the difference in their speeds is 1 ms^–1.
I did the following to (correctly) calculate the speed of each particle:
3kg * 3ms^-1 + 2kg * -2ms^-1 = 3kg * v + 2kg * (v + 1)
v = velocity of particle P = 0.6ms^-1
v + 1 = velocity of particle Q = 1.6ms^-1
My question is this: how do I know that the greater speed (v + 1) is for particle Q? Is it because it had the greater momentum before the collision, so it's supposed to have the greater velocity after the collision? If I assume that particle P has the greater velocity after the collision, the answer is different (and incorrect).