Question

Mark the correct statement(s).

• A. Total work done by internal forces of a system on the system is always zero.
• B. Total work done by internal forces of a system on the system is sometimes zero.
• C. Total work done by internal forces acting between the particles of a rigid body is always zero.
• D. Total work done by internal forces acting between the particles of a rigid body is sometimes zero.

Answer 1

Answer: (B), (C)

Consider a system of two point particles which are free to move on a smooth horizontal surface, then the work done by electrical forces acting on the two particles is non-zero. But it is not necessarily true always. For a rigid body, there is no relative displacement of particles, hence, internal forces do not do any work.