For a surface molecule a) the net force on it is zero b) there is a net downward force c) the potential energy is less than that of a molecule inside d) the potential energy is more than that of a molecule inside
Correct options are B) and D) A molecule viewed from the top to surface of the fluid. Force exerted on a molecule by all nearby molecule are equal and opposite, so net horizontal component of force is zero. As a molecule is viewed horizontally to the surface of fluid a vertically downward force acts on molecule due to which it does not escape from surface. Hence the right option is (b). There is a net attractive force on molecules at surface acting downwards. So the P.E of molecules at the surface is negative to lower ones but in magnitude PE (mgh) is more than that of the lower or inside molecules. Thus this verifies option (d).
