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