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)
-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.