Question

If the resultant of all the external forces acting on a system of particles is 0, then from an inertial frame, one can surely say that ___________

(a) Linear momentum of the system does not change in time

(b) Kinetic energy of the system does not change in time

(c) Angular momentum of the system does not change in time

(d) Potential energy of the system does not change in time

The question was posed to me in class test.

Asked question is from Moment of Inertia and its Significance in chapter System of Particles and Rotational Motion of Engineering Physics I

Answer 1

Right answer is (a) Linear momentum of the system does not change in time

To explain I would say: According to the law of conservation of linear momentum if external force is 0 then the linear momentum of the system does not change in time. There may be external forces acting due to which kinetic energy or potential energy or both may change. Also, net force is 0 does not mean net torque is 0. So angular momentum may change. Hence only linear momentum of the system does not change in time.