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

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

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.

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

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.

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…

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

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

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