Define conservation of linear momentum?

Dear student

Conservation of linear momentum statement: If the external force applied on a system is zero then the linear momentum of the system will be conserve.
By Newton's second law of motion
$$\begin{gathered} F=\frac{dP}{dt} \\ ifexternalforce=0 \\ \frac{dP}{dt}=0 \\ P=cons\tan t \\ momentum=cons\tan t \\ {P}_{initial}=P_{final} \end{gathered}$$
so linear momentum will be conserve if applied external force =0
Regards,