Question

A box contains 10^5 molecules. The average kinetic energy of each molecule is 0.5*10^-6J. With respect to an observer at rest the box, having a mass of 1kg, is moving with a velocity of 2m/s. What is the value of kinetic energy that will contribute to the internal energy?

(a) 0.05J

(b) 2.05J

(c) 0

(d) 2J

The question was asked in semester exam.

This interesting question is from Thermodynamics in section Thermodynamics of Physics – Class 11

Answer 1

The correct option is (a) 0.05J

To elaborate: When calculating internal energy of molecules, the system is to be studied from a frame in which the centre of mass is at rest.                                                                                                                                  So the kinetic energy due to movement of the entire box will not be considered.                                                                                                                                                                             Only the individual kinetic energies of molecules will be considered, and their value is:                                     10^5*0.5*10^-6

= 0.05J.