Hello everyone , here we go : I understand that momentum is always conserved as long as no external forces act on a given system . I also understand that the law of restitution applies to the axis perpendicular to the line of impact between two objects .However i have a problem . For example . A ball hits the ground at an angle of 45 degrees and bounces up at an angle of 30 degrees. So from the law of restitution : e= - ( Vball*sin30 - Vground)/(Uball*sin45 - U ground) , the ground does not move therefore Uground = Vground=0 so e = - Vball*sin30/(Uball*sin45). My question is , why can't we apply momentum conservation on the y axis ? Is it because the reaction force when hitting the ground is considered an external force ? I am very confused please explain :S Thank you !