1. The problem statement, all variables and given/known data :block slides on smooth triangular wedge kept on smooth floor.Find velocity of wedge when block reaches bottom. (image is attached) 2. Relevant equations:Let the velocity of the triangular wedge be V.And v be the velocity of block m. applying conservation of linear momentum in the horizontal direction we get MV +m(v cos theta -V)=0 And because of conservation of energy 1/2 MV^2+1/2 m[(v cos theta - V)^2 +v sin theta]^2 The answer comes out to be 3. The attempt at a solution:I understood all these except conservation of linear momentum part. MV +m(v cos theta -V)=0 here MV +m(v cos theta -V) is final momentum and 0 is taken as initial momentum. why their initial momentum and velocity is taken as zero?I have learnt in constrains, force responsible for motion of wedge is Nsin theta which is always there as long as block and wedge are in contact.And similarly motion of block is because of mg sin theta which should be always there if block is on the wedge.Then which initial condition are you referring to when these forces are not there so that they have zero velocity?I know we are taking blocks and wedge as our system now so normal forces become internal and cancels out.This problem must be a part of kinematics,Kinematics is the branch of classical mechanics which describes the motion of points, bodies (objects) and systems of bodies (groups of objects) without consideration of the causes of motion. But I am confused why initial momentum is zero?