Action = Reaction
“To every action there is always opposed an equal reaction”
Isaac Newton
F1= -F2
If one object (A) exerts a force on a second object (B), then object B simultaneously exerts a force on A, and the two forces are equal and opposite. The force of the first object is called “Action” and the force of the other is the “Reaction”. When one object changes the motion of the other, it will undergo an equal change in its own motion due to the reaction force. The state of the object is the result of the interaction forces. The three forces - normal, frictional, and tension forces - are the result of contact interactions.
The normal force on an object is generally associated with the force that the surface of the object exerts perpendicularly to the surface of the other, like the force of our weight and the reaction force of the ground on us.
Friction, on the other hand, is the force resisting the relative motion between two surfaces sliding against each other. When surfaces in contact move relative to each other, friction converts kinetic into thermal energy. Friction can have dramatic consequences; for instance, rubbing two pieces of wood together can start a fire.
Tension is also a contact force. Tensile strength, or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. Stress expresses the internal forces that neighboring particles of a material exert on each other; strain is the measure of the deformation of that material. Flexible materials capable of undergoing larger deformations are also able to absorb more stresses without failure, as opposed to brittle materials such as glass which will break easier. If the reaction force is not too great, the object may resist the applied force and allow a new equilibrium state, returning to its original state when the load is removed. A larger applied force, however, may lead to a permanent deformation of the object or even to its structural failure. The larger the action, the greater the reaction force.