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