1. The problem statement, all variables and given/known data a) The number of molecules hitting unit area of a surface per second is: 0.5vnf(v)dvsin(s)cos(s)ds Show that the average value of cos(s) for these molecules is 2/3 b) Using results above, show that for gas obeying Maxwellian distribution, the average energy of all th molecules is (3/2)kT, but the average energy of the molecules which hit the surface is 2kT. 2. Relevant equations 3. The attempt at a solution For part a, I just integrated sin(s)((cos(s))^2) from 0 to pi. This gives the correct answer, but I'm not sure if its the correct method. I know how to find the average energy of all the molecules I'm not sure of how to find the average energy of the molecules that hit the surface.