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# What are types of friction?

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There are four general types of friction: static, kinetic, rolling and fluid. Friction is defined as the force that resists the motion of two objects sliding against each other.

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Static friction occurs when an object is forcefully moved along a surface and no movement takes place. The direction of the force is in the direction opposite of the motion. Kinetic friction takes place when an object just starts to move across another surface. Rolling friction is defined as the force that slows down the movement of a rolling object. Fluid friction is the restrictive force that slows down the movement of an object in liquid.

## Related Questions

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Examples of rolling friction are a roller skate slowing down on pavement or the wooden surface of a roller-skate rink and a basketball rolling across the surface of the court. When one surface rolls across another, small indentations on both surfaces interfere with each other, reducing speed.

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To create a science fair project about wheels reducing friction, the exhibitioner could give live demonstrations of how difficult it is to push identical objects across a surface with and without wheels. Friction is the resistance of a body to motion in contact with another body and is directly proportional to the contact area between two surfaces; wheeled bodies move more smoothly over surfaces because their contact areas are reduced.

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Ball bearings reduce friction by using smooth balls lubricated with oil or grease that freely roll between a smooth inner and outer surface. The main concept of the ball bearing is that objects that roll past each other produce less friction than if the objects were sliding against each other.