Consider a case- Force excerted on a stationary flat plate held normal to the jet. The Jet strikes on plate (see fig.) Which is smooth hence the exit velococity of jet = entering velocity of jet. According to theory. The fore is applied on plate due to change in momentum of jet. But my doubt is that since the jet have constant kinetic energy due to same velocity at entering and exit i.e. their is no loss of energy of the jet then how it can apply force on plate. It seems like energy is generated which voilates first law. I mean in order to apply force the jet should dissipate its energy but it is coming out with same velocity which means it have constant energy. Please clear my doubt.