Question

If Newton’s 3rd law of motion is written in the form F21 = F12; what is
the mistake?

Answer 1

The Mistake is You have to put a minus sign on the left hand side Just like this F12 = -F12

Answer 2

☆Answer☆

This shows that force on a body A by B is equal an opposite to the force on body by A. Hence, the forces of action and reaction are equal and opposite, which proves Newton's 3rd law of motion.

The third law states that all forces between two objects exist in equal magnitude and opposite direction: if 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..