Newton's third law of motion
[tex]F_{12}=-F_{21}[/tex]Here F_12 is the force due to the first body on the second body.
F_21 is the force due to the second body on the first body.
The negative sign indicates that the direction of force is opposite.
Example: Hitting the wall with your hand.
You are applying force on the wall with your hand and the wall will exert an opposite force on the hand of equal magnitude.
