How did Newton deduce the law of conservation of linear momentum? Can it be derived only by Newton's laws, or does it follow from practical experiments?
If the law of conservation of linear momentum could be deduced by Newton's laws, why did Einstein try to redefine the linear momentum of a particle, assuming this law is true?
"If the law of conservation of linear momentum could be deduced by Newton's laws, why did Einstein try to redefine the linear momentum of a particle, assuming this law is true?"
Ans: No, Newton's law of conservation of momentum is not true. Einstein discovered that it is not $mv$ that is conserved, but $\gamma mv$.
From Newtons second Law which tells that force equals rate of change of momentum. If external force is zero then rate of change of momentum is zero which means momentum is conserved.
