Two hydrogen atoms are in excited state with electrons residing in n = 2. First one is moving towards left and emits a photon of energy towards right. Second one is moving towards right with same speed and emits a photon of energy towards right. Taking recoil of nucleus into account during emission process
none
If only conservative forces act on a system, total mechnical energy remains constant -
-
In the first case K.E. of H-atom increases due to recoil whereas in the second case K.E. decreases due to recoil but