Question

Using newtons 3rd law of motion prove that if no external force is acting linear momentum conserved

Answer 1

for every reaction there is an equal and opposite reaction

if no force acts on a object then the object doesn't cause any force

F = ma = 0

a = 0, ⇒ velosity is constant

then the linear momentum doesn't change

Answer 2

Newton's third law is For every action, there is an equal and opposite reaction.
one object A exerts a force FA on a second object B, then B simultaneously exerts a force FB on A, and the two forces are equal in magnitude and opposite in direction,
FA = −FB