Friction can be considered a nuisance since it resists motion and slows sliding objects to an eventual stop, but without friction, you would not be able to walk, bicycles would not be able to move and you would slide right off your chair when you sit down. The force due to friction is dependent on a value called the coefficient of friction, which varies between different materials. As an object slides, it will experience a force in opposition to its movement proportional to the weight of the object.
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Determine the normal force, denoted N, of the surface against the object. If the surface is horizontal, then this force is equal but opposite to the force of gravity, which equals 9.8 times the mass of the object. For example, a 5kg block has a normal force of (9.8)(10) = 98 Newtons.
Look up the coefficient of friction of the surface the object is sliding on and the object itself. This can be found in the appendix of most introductory physics textbooks. This coefficient is denoted u. For example, the coefficient for a dry piece of wood on brick is 0.6.
Multiply the normal force by the coefficient of friction. This will give you the resulting force of friction. For example, the 10kg piece of wood sliding on concrete has a force of friction of uN, or (0.6)(98) = 58.8 Newtons.
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