Q:

# What kinds of friction are there?

A:

There are four different types of friction: static, kinetic, rolling and fluid. Each of these involves relative motion between two bodies, with the exception of static friction, which takes place even when relative motion between the two bodies is absent.

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Static friction is considered when a body is forced to move against a surface, but no actual motion is initiated. Static friction is equal and opposite to the applied external force. The magnitude of this friction force is the product of the net normal reaction of the object on the body and the static coefficient of friction, which is determined empirically measured for two different materials. An example of static friction in action is a block of wood resting on a rough, inclined plane.

Kinetic friction takes place when a body just starts moving. When the applied force is sufficient to move the body, the drag force in the opposite direction to the direction of body movement is the product of the normal reaction and the dynamic coefficient of friction. The coefficient of kinetic friction is always lower than the coefficient of static friction. Kinetic friction is kinematically derived by solving the equations of motion when the body moves at constant velocity, indicating that the thrust is equal to the drag.

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