Newton’s 3rd Law

Newton’s 3rd Law Forces always exist in pairs

Examples of Newton’s 3rd Law When you walk across the floor: you push against the floor and the floor pushes against you

For Every Action There is an Equal and Opposite Reaction When two objects interact with one another, the forces they exert on each other are called action-reaction pairs

Action-Reaction Pairs Whenever a force is exerted, there is another force that is equal in size and opposite in direction This is true even if there is no motion

Person Pushing Against a Wall

Force pairs do not always act on the same object Even though the forces are equal and opposite, they do not cancel each other out because they act on different objects

Swimming You push the water backward and the water pushes you forward

Hammer & Nail Nail exerts a force on the hammer equal and opposite the force the hammer exerts on the nail Net force acting on the nail, drives it into the wood

Rocket at Lift Off Action: rocket pushes on gas Reaction: gas pushes on rocket

Equal Forces don’t always have equal effects Because of Newton’s second law (F=ma) , an object’s mass will have an impact on the acceleration from the force

For Example When a gun is fired, the force exerted on the bullet is as great as the reaction force exerted on the gun. Why is the recoil not at the same speed as the bullet?

These forces depend on friction A person or car on ice may not be able to exert the action force to produce the needed reaction force