Question

F1=-f2 why negative sign?

Answer 1

Newtons third law states that every action has an equal and opposite reaction. F1 is basically the force that you are exerting, but F2 is the same amount of force exerted in the opposite direction, thats why its -F2.

Answer 2

Answer:

The negative sign indicates that the direction of forces is opposite to each other.

Explanation:

F₁ = - F₂ is the mathematical expression of Newton's third law of motion. The law states that to every action there is an equal and opposite reaction. That is if a force is applied to object A by another object B, then object A will, in turn, apply a force on B. These two forces will be in equal magnitude, but their direction will be opposite to each other. It is to indicate this opposite direction that we use a negative sign in the mathematical expression of the 3rd law of motion.