Free Body Force Diagrams and Newtons Third Law

Free Body Force Diagrams and 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 ? Normal Force • 1) what is exerting the force of the Earth on the man • 2) on what the force is exerted 750 N • 3) the type of force • The magnitude can also be labelled near the arrow. 750 N Gravitational force of the Earth on the man

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

Identifying Newton 3 rd 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?

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

Try another

Identify the Newton 3 rd 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 Normal Force of the Earth on the table Gravitational force of the table on the Earth

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

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

Horse pulling a cart

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

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

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