JEE Advance - Physics (2009 - Paper 1 Offline - No. 12)
If the resultant of all the external forces acting on a system of particles is zero, then from an inertial frame, one can surely say that
Linear momentum of the system does not change in time.
Kinetic energy of the system does not change in time.
Angular momentum of the system does not change in time.
Potential energy of the system does not change in time.
Erläuterung
According to Newton's second law, we have
$$\sum {{{\overrightarrow f }_{ext}} = {{d\overrightarrow p } \over {dt}}} $$
If $$\sum {{{\overrightarrow f }_{ext}} = 0} $$, we find that the linear momentum $$\overrightarrow p $$ as constant.