Newton’s Third Law of Motion

Chapter 6

Action and Reaction

Question:

• Is it possible for me to touch the table but not have the table touching me?

6-1Forces and Interactions

•A force is a push or pull.

• Forces do not act alone. Forces are mutual actions called INTERACTIONS between two things.

For example…

• A hammer hits a nail…

• The nail interacts with the hammer. The hammer hits the nail and the nail hits the hammer.

6.2 Newton’s Third Law

“Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first”

In other words…

“For every action there is an EQUAL and OPPOSITE reaction”

6.3 Action vs. Reaction

For example:One Force (an action):Pushing on a wall

Second Force (a reaction):The wall pushing back

What happens to you when you jump off a boat onto the shore?

What happens to the boat?

6.3 Action and Reaction

“ACTION” happens first:Driving in a car, the tires push on the

road.Dropping a book, the Earth pulls on

the book.“REACTION” happens second:The road pushes back on the tires.The book pulls back on the Earth.

6.4 Action and Reaction on Different Masses

FORCES

are equal even if

MASSES

are different.

6.4 Action and Reaction on Different Masses

The force exerted on a cannon is equal to the force exerted on the cannonball

So, why does the cannonball move so much faster and farther than the cannon?

6-5 Why don’t they cancel each other out?

Page 78-79Figure 6.11-6.12Do the action and reaction act

on the same object? If so, they can cancel each

other out.

Action VS Reaction

Equal in magnitude and opposite in direction

Forces do not cancel each other out.

6.6 The Horse-Cart Problem

Observe the pairs of forces acting on the horse and cart

6.7

ACTION = REACTION

Example: Pushing on a wall

vs

Punching a wall

Chapter 6

Newton’s 3rd Law of Motion

Vocabulary:

1. action force 3. Newton’s 3rd Law

2. interaction 4. reaction force