1. Free Body Force Diagrams and Newton’s Third Law

2. 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)

3. 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.

4. Describing Forces 750N 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 Force
750N

5. How about the Earth? Normal Force of the man on the Earth
Gravitational force
of the man
on the Earth

6. 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?

7. What do notice? 750N 750N 750N Same line of action
Gravitational force
of the Earth
on the man
Normal Force
of the Earth
on the man
750N
Same line of action
Act on different bodies
They are the same type
They are equal and opposite
Always concerns two forces (pair)
750N
750N
750N
Normal Force
of the man
on the Earth
Gravitational force of the man
on the Earth

8. Try another

9. Identify the Newton 3rd Law Pairs
Normal Force
of the table
on the cat
Gravitational force
of the Earth
on the cat
Normal Force
of the table
on the Earth
Normal Force
of the cat
on the table
Gravitational force
of the cat
on the Earth
Gravitational force
of the table
on the Earth
Gravitational force
of the Earth
on the table
Normal Force
of the Earth
on the table

10. Tangential Forces What causes tangential forces? Friction
Frictional Force
of the table
on the cat
Normal Force
of the table
on the cat
Gravitational force
of the Earth
on the cat

11. Demo: Driven wheels vs. Trailing wheels
Which pencil
will fall?

12. Horse pulling a cart

13. The horse Applied Force of the cart on the horse Applied Force
of the horse
on the cart
Normal Force
of the Earth
on the cart
Gravitational force
of the Earth
on the cart
Frictional Force
of the Earth’s surface on the horse
Normal Force
of the Earth
on the horse
Frictional Force
of the Earth’s surface on the cart
Gravitational force
of the Earth
on the horse

14. The Earth What do you notice about the sum of the forces? Normal Force
of the horse
on the Earth
Normal Force
of the cart
on the Earth
Frictional Force
of the cart on the Earth’s surface
Frictional Force
of the horse on the Earth’s surface
Gravitational force
of the cart
on the Earth
Gravitational force
of the horse
on the Earth

15. Force on a car Are the horizontal forces balanced? Air resistance
of the air
on the car
Normal Force
of the Earth
on the car
Frictional Force
of the Earth’s surface on the car
Frictional Force
of the Earth’s surface on the car
Gravitational force
of the Earth
on the car