Friction , force that opposes the motion of an object when the object is in contact with another object or surface.
Friction results from two surfaces rubbing against each other or moving relative to one another. It can hinder the motion of an object or prevent an object from moving at all. The strength of frictional force depends on the nature of the surfaces that are in contact and the force pushing them together. This force is usually related to the weight of the object or objects. In cases involving fluid friction, the force depends upon the shape and speed of an object as it moves through air, water, or other fluid.
When friction affects a moving object, it turns the object’s kinetic energy, or energy of motion, into heat. People welcome the heat caused by friction when rubbing their hands together to stay warm. Frictional heat is not so welcome when it damagesmachine parts, such as car brakes.
Different kinds of motion give rise to different types of friction between objects.
Static friction occurs between stationary objects, while sliding friction occurs between objects as they slide against each other. Other types of friction include rolling friction and fluid friction. The coefficient of friction for two materials may differ depending on the type of friction involved.
Static friction prevents an object from moving against a surface. It is the force that keeps a book from sliding off a desk, even when the desk is slightly tilted, and that allows you to pick up an object without the object slipping through your fingers.
This force depends on the coefficient of static friction (µ s ) between the object and t he surface and the normal force (N) of the object.
A book sliding off a desk or brakes slowing down a wheel are both examples of sliding friction, also called kinetic friction. Sliding friction acts in the direction opposite the direction of motion. It prevents the book or wheel from moving as fast as it would without friction. When sliding friction is acting, another force must be present to keep an object moving. In the case of a book sliding off a desk, this force is gravity.
The force of kinetic friction depends on the coefficient of kinetic friction between the object and the surface on which it is moving (µ k ) and the normal force (N) of the object.
Rolling friction hinders the motion of an object rolling along a surface. Rolling friction slows down a ball rolling on a basketball court or softball field, and it slows down the motion of a tire rolling along the ground. Another force must be present to keep an object rolling. For example, a pedaling bicyclist provides the force necessary to the keep a bike in motion.
Rolling friction depends on the coefficient of rolling friction between the two materials (µ r ) and the normal force (N) of the object.
Objects moving through a fluid experience fluid friction, or drag. Drag acts between the object and the fluid and hinders the motion of the object. The force of drag depends upon the object’s shape, material, and speed, as well as the fluid’s viscosity( Viscosity is a measure of a fluid’s resistance to flow).