A beam of gas molecules is incident normally on a plate AB. Each gas molecule has mass m and velocity V. The incident beam falls on an area A on the plate and all the molecule strike the plate elastically. Number of molecules in unit volume of the beam is n. When the plate is moved to right (see fig.) a force `F_(1)` is needed to keep it moving with constant velocity u `(gt V)` . When the plate is moved to left with constant velocity u, an external force `F_(2)` is needed. Find `F_(2) – F_(1)`.