1. 18.3 Friction and Gravity
Friction is a contact force and gravity is a force that can act at a distance.

2. The source of friction
Friction occurs b/c the surface of any object is rough… even smooth objects have microscopic hills and valleys.
Amount of friction 2 factors :
Roughness of the surfaces in contact
Force pushing the surfaces together
Heavier objects = more friction than lighter objects

3. Types of Friction
Kinetic = moving. Kinetic friction = friction between moving surfaces.
Sliding kinetic friction - surfaces slide against each other
Rolling kinetic friction - a surface rolling on another surface.

4. Examples:
sliding kinetic friction = brakes on a bicycle, writing with chalk, scratching a bug bite
Rolling kinetic friction = anything that has wheels
Skates, bicycles, cars, trains, etc

5. Static Friction Static means not moving.
Static friction – force applied to an object but it DOES NOT move.
Static friction can be overcome by applying a large enough force… then kinetic friction takes over.

6. Friction: Harmful and Helpful
Harmful – can cause wear and tear –
on jeans, the earth, on machines, etc…
Helpful – creates traction –
To walk, to drive on icy roads, hold objects, etc…

7. Gravity: Force at a Distance
Sir Isaac Newton – made the connection between unbalanced forces and called that force – Gravity
Newton’s ideas = law of universal gravitation
Applies to all objects in the Universe

8. Law of Universal Gravitation
Gravitational force increases as mass increases
Elephant vs Cat
Elephant = more mass, heavier to pick up, more force
Cat = less mass, easier to pick up, less force
Gravitational force decreases as distance increases
Small distance = strong force
Large distance = decreasing force, less force

9. Forces at a Distance Electric force – force between charged objects
Opposite charges pull together – Like charges push apart
Ex. Opposite charge = static eletricity
Magnetic force – force pulling or pushing magnets
2 poles = opposite magnetic qualities
Like poles repel – Opposite poles attract