My question is... when a ball falls vertically on an inclined plane with a velocity [v][/0] and let it collide elastically with the incline plane...let the angle of inclination be 'β' ... Now, here we conserve the momentum of the ball in the direction of commomn normal of the two objects.... So we write an equation initial momentum of the ball (along common normal) = final momentum of the ball (along common normal)... Assuming that the inclined plane is fixed... THE REAL QUESTION (CONFUSION).. i think that the net force on the system (ball + plane) is acting along the common normal ..as gravitational force is acting ..so how are we conserving the momentum along common normal and even along the line tangent to the inclined plane.... Even along that line Gravity is acting.... Sorry for the length of the question...I was too descriptive..:D..