Suppose the gas is held in a cube container,one molecule moves ceaselessly The increase of molecule's momentum -MVis -MlVi =-2MVa The impulse on the wall of the container x dA The increase of molecule‘s momentum i x i x i x   v   v  2 v The impulse on the wall of the container ix 2 v Suppose the gas is held in a cube container, one molecule moves ceaselessly