Question

prove that 2nd law of motion is the real law of motion ?

Answer 1

Newton second law states that
F=mxa
Force=mass x accelaration
if you don't give a acceleration then there will no force acting on it and which led to the contradiction that the Newton second law is a real law of motion

Answer 2

f12=dp1/dt

f21=dp2/dt

f12+f21=(dp1/dt+dp2/dt)

=d(p1+p2)/dt

according to law of conservation of momentum

d(p1+p2)/dt=0

therefore

f12+f21=0

f12=-f21

hence,it is real law of motion