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Friction

The resistence present to motion between contacting materials or media.
It occurs in all types of media: solids, liquids + gases.

Types of friction
- Static friction: the force that prevents motion between dry + clean contacting solid surfaces, to start motion between the surfaces static friction must be overcome.
- Sliding or kinetic friction: the resitive force between dry and clean solid surfaces sliding over each other. Static friction is always greater than sliding friction. It takes less effort to keep a sliding object moving than it does to set it in motion.
- Rolling friction: the force that resists relative motion between two solid objects when one or both roll over the surface of the other, example: a ball or wheel rollingover a hard surface.
- Fluid friction: the force that resists the flow of liquids and gases.

Classical laws of friction (between solid objects)

Friction depends on the physical and material properties of the contacting surfaces. It is:
-always in a direction opposite the motionor a force attempting to produce motion;
-directly proportional to the load;
-indipendent of the surface area;
-indipendent of the sliding speed.
The load is the applied force perpendicular to the contact surface that presses them together.
The load on a horizontal surface is equal to the weight of the object.

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