NEWTON’S THIRD LAW

HOW DID THE APPLE FIT INTO THIS• As the story goes, Newton was sitting in a

garden and watched an apple pulled from a tree by the force of gravity.

• As the apple fell, Newton realized that the earth was pulling the apple down with the force of gravity.

• However, Newton also recognized that at the same time as the earth was pulling on the apple, the apple must be pulling on earth with the exact same force.

STATEMENT OF NEWTON’S THIRD LAW• For every force acting on one object in

one direction, there is another force that is the exact same magnitude acting on another object in the exact opposite direction.

OVER-SIMPLIFICATION OF NEWTON’S THIRD LAW• Newton’s Third Law

is often oversimplified with the statement “For every action there is an equal but opposite reaction.”

• This statement winds up being applied to situations that do not involve forces.

WHEN NEWTON’S THIRD LAW WORKS• All forces are created in pairs.

• However, some pairs of forces act on the same object.

• For example, if you push on the back of your head, you do not go anywhere.

• Newton’s Third Law is really useful when the action and reaction forces act on different objects.

ROCKETS AND NEWTON’S THIRD LAW• Rockets, jets,

planes, and motor boats all use Newton’s Third Law.

• Many small particles are pushed back and fired away from the engine. The particles have push against the engine.

GUN RECOIL

• A gun pushes a small bullet to accelerate the bullet down the gun barrel.

• Meanwhile, the bullet pushes back on the gun.

• The size of the force is the same on both objects, but the gun has more mass.

• Therefore, it’s acceleration is smaller.

• http://www.break.com/index/gun-recoil-knockout.html

MOVING ON A SKATEBOARD• A skateboard

reduces the amount of friction between a person and the ground.