Fill in the blanks:
Newton's ___ law is te real law of motion.

Explanation

According to Newton.s 2nd law of motion, F = ma. This law contains both 1st and 3rd law of motion.
Explanation of 1st law of motion: F=ma, if `F = 0 implies a=0`.
It means that if no external force acts on body then it remains in state of rest or keeps moving with uniform velocity, which is Newton.s 1st law of motion.
Explanation of 3rd law of motion
Consider a system of bodies 1 and 2. Let `vecF_(12)` and `vecF_(21)` be the forces applied on 1 due to 2 and on 2 due to respectively.
`vecF_(12)= (dvecp_(1))/(dt)` ....(i)
`vecF_(21)= (dvecp_(2))/(dt)` .....(ii)
Adding (i) and (ii), we have `vecF_(12)+ vecF_(21)= (dvecp_(1))/(dt)+ (dvecp_(2))/(dt)= (d)/(dt)(vecp_(1)+vecp_(2))`
Since there is no external force acts on the system `(d)/(dt)(vecp_(1)+vecp_(2))=0`
or `vecF_(12)+ vecF_(21)=0 implies vecF_(21)= -vecF_(12)` which is Newton.s `3^(rd)` law of motion. Therefore Newton.s `1^(st)` and `3^(rd)` law of motion can be explained from `2^(nd)` law. Therefore Newton.s `2^(nd)` law is real law of motion.