How do we know the direction of the impulse(which is the change in momentum)? I thought lets say if an object travels at -ve x direction at a certain speed and someone hits it back at another speed (towards the +ve x direction), how do we tell the direction of the impulse? Isnt the direction always in the +ve direction in this case if we have defined our +ve and -ve directions as stated???So its always true that the impulse is in the final direction? Cos final momentum - initial momentum = change in momentum =impulse. So it will always be in the direction of the final velocity right?