NEWTON’S THIRD LAW OF MOTION

Physical Science Ch. 6

Forces are part of a mutual action called INTERACTION.

WHERE IS THE INTERACTION IN THIS PICTURE?

NEWTON’S THIRD LAW STATES: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Action VS. Reaction

ACTION OBJECT A EXERTS A

FORCE ON OBJECT B.

Example: hammer hits nail

REACTION OBJECT B EXERTS

FORCE ON OBJECT A Example: nail hits

hammer back

Boulder vs. Earth Example:Earth pulled up by boulder, boulder pulled to ground by Earth. We can see the boulder move, but why don’t we notice the Earth being pulled? MASS DIFFERENCES!!F/m=a Boulder, f/M=a EarthMore mass, less acceleration

How do planes and birds fly?

THRUST

LIFT

DRAG

WEIGHT

ACTION-REACTION FORCE PAIRS!!!!

Do action-reaction forces cancel?

Only internal forces cancel! See below… hand pushes box so box pushes hand. BUT, they are external… so the box accelerates

the hand and the hand accelerates the box!!!

Have you ever noticed that you can’t really

push yourself?

Forces exerted by one part of an object on another part of the same

object are INTERNAL FORCES!

Internal forces NEVER influence the acceleration of an object!

Only external forces are responsible for acceleration!

Pg. 80-81 Horse-Cart Problem

Moral of the story according to Newton’s Third Law of Motion:If you push hard on the world, the world will push hard on you… if you push gently on the world, it will push gently

on you!

THE WAY YOU TOUCH OTHERS

IS THE WAY OTHERS WILL TOUCH YOU!