Guys I'm having a little problem in undersatnding a few things: We have a wall and a ball of mass 'm'. The ball is thrown towards the wall with speed u and rebounds with speed 'v'. The coefficient of restitution is, e is 1/3. The question is to find the value of 'v'. Thats how I think it should be solved: mu=mv (since the final velocity of the wall is "CONSIDERED" zero) => u=v Now if I apply newton's law of restitution and simplify: v=-eu (putting this into u=v gives us) u=-eu cancelling u gives e=-1 but e=1/3. Where did I go wrong? I just cant understand it. Any help will be greatly appreciated.