Force Notes:

Free Body Diagrams | Weight | Normal Forces

Types of Forces◦ Contact Forces:

Frictional force: Ff

Tensional force: FT

Normal force: FN

Air resistance force: Fa

Applied force (push or pull): Fapp

◦ Action-at-a-Distance-Forces: Gravitational force: Fg=

W Electrical force: Fe

Magnetic force: Fb

Free Body Diagrams

Draw a force diagram on the book at rest on the table:

This normal force will always be perpendicular to the surface and may differ from the object’s weight.

The table pushes back up on the book with as much force as the downward force of gravity on the book.

You can imagine it like a spring!

If you push down on a spring and compress it, the spring pushes back up on you!

As a book lying on the table

compresses the atoms in the table, the microscopic atoms in the table act as little springs that squeeze upward on the book.

They produce the normal, or support force.

Normal Forces

FN

Fg

Balanced or Unbalanced?BALANCED: UNBALANCED:

•Forces are equal and opposite•NO net force•NO acceleration (either at rest or constant velocity

Larger force in one direction than the other

•Net force is the sum of all forces, taking direction into accountObject accelerates (speeds up or slows down) in direction of net force

Scenario:

◦ A book is at rest on a table top

◦ A car is coasting to the right and slowing down

◦ An egg is free-falling from a nest in a tree (ignore air resistance)

Drawing Free Body Diagrams FBD: Balanced/

Unbalanced:

◦ x: no forces◦ y: balanced

◦ x: unbalanced◦ y: balanced

◦ x: no forces◦ y: unbalanced

FN

Fg

Ff

Fg

FN

Fg

Drawing Free Body Diagrams

Try the next four on your own!

Stop Notes

A spring mechanism gets squeezed between the force of gravity pulling down on you and the atoms in the floor pushing back up with an equal and opposite force.

How does a bathroom scale work?

FN

Fg

Rory (9.30 kg) sits in his 3.70 kg high chair

A. Draw a free body diagram for Rory, and find the normal force exerted by the chair on Rory.B. Draw a free body diagram for the chair, and find the normal force exerted by the floor on the chair.

Example (do on OH)

Note: We are interested (for now) only in the magnitude of the force of gravity, therefore we will use 9.8 m/s2 for “g” and assign directions (positive or negative) later.

Mass versus Weight• Mass = measure of inertia• Same at any location

• Mass = m• Units of m: Kilograms (kg)

• Weight = measure of the force of gravity• Depends on your mass and location

• The pull of earth’s gravity (Fg) • Weight = W = mg = Fg • Units of Fg: Newtons (N)

Kerry, the ballet dancer, has a mass of 45.0 kg.

◦ What is Kerry’s weight on Earth? W = mg W = (45)(9.8) W = 441 N

Now Kerry takes a trip to Jupiter where the acceleration due to gravity is 25 m/s2.

◦ What is Kerry’s mass on Jupiter? 45 kg

◦ What is Kerry’s weight on Jupiter? W = mg W = (45)(25) W = 1125 N

Example:

Mass vs. Weight

http://www.youtube.com/watch?v=_Z0X0yE8Ioc

You Try it Problems