Free Body Force Diagramsand Newton’s Third Law

Types of Forces

• 1. Gravitational or Weight• 2. Electrostatic• 3. Electromagnetic• 4. Nuclear – strong and weak • 5. Push or pull - Contact• 6. Elastic or Strain • 7. Normal or Reaction• 8. Frictional, Drag, Air Resistance – (Contact)

Free-Body Force Diagrams• A diagram of a single body,

showing the forces on that body only.

• Example: A person standing on Earth

• The man’s free-body diagram will be with him on his own, without the Earth.

• The diagram is only about what is happening to him.

Describing Forces

• The arrow indicates the size and direction.

• What should the description consist of ?

• 1) what is exerting the force• 2) on what the force is

exerted• 3) the type of force• The magnitude can also be

labelled near the arrow.Gravitational force of the Earth on the man

Normal Forceof the Earth on the man

750N

750N

Gravitational force of the man on the Earth

Normal Forceof the man on the Earth

How about the Earth?

Identifying Newton 3rd pairs of forces

• What is Newton’s Third Law?

• “For every action there is an equal and opposite reaction”.

• Can you identify the Newton’s Third Law pairs in the previous example?

• What do you notice about them?

Gravitational force of the man on the Earth

Normal Forceof the man on the Earth

What do notice?

750N750N

Gravitational force of the Earth on the man

Normal Forceof the Earth on the man

750N

750N• Same line of action• Act on different bodies• They are the same type• They are equal and opposite• Always concerns two forces

(pair)

Try another

Identify the Newton 3rd Law Pairs

Gravitational force of the Earth on the cat

Gravitational force of the Earth on the table

Gravitational force of the tableon the Earth

Gravitational force of the caton the Earth

Normal Forceof the table on the cat

Normal Forceof the cat on the table

Normal Forceof the Earth on the table

Normal Forceof the tableon the Earth

Tangential Forces

• What causes tangential forces?

• Friction

Gravitational force of the Earth on the cat

Normal Forceof the table on the cat

Frictional Forceof the table on the cat

Demo: Driven wheels vs. Trailing wheels

Which pencilwill fall?

Horse pulling a cart

The horse

Gravitational force of the Earth on the horse

Normal Forceof the Earth on the horse

Applied Forceof the cart on the horse

Frictional Forceof the Earth’s surface on the horse

Normal Forceof the Earth on the cart

Applied Forceof the horseon the cart

Frictional Forceof the Earth’s surface on the cart

Gravitational force of the Earth on the cart

The Earth

Frictional Forceof the horse on the Earth’s surface

Gravitational force of the horseon the Earth

Normal Forceof the horseon the Earth

Gravitational force of the carton the Earth

Frictional Forceof the cart on the Earth’s surface

Normal Forceof the carton the Earth

What do younotice about the sum of the forces?

Force on a car

Gravitational force of the Earth on the car

Frictional Forceof the Earth’s surface on the car

Frictional Forceof the Earth’s surface on the car

Normal Forceof the Earth on the car

Air resistanceof the airon the car

Are the horizontalforces balanced?