The coefficient of friction (COF) tester is designed to measure the static and kinetic coefficients of friction of plastic film, sheeting and similar materials.

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What is Coefficient of Friction?

Coefficient of friction, the ratio of the frictional force resisting the motion of two surfaces in contact to the normal force pressing the two surfaces together.

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Coefficient of Friction Measurement

Coefficient of friction (COF) is a dimensionless number that is defined as the ratio between friction force and normal force. Materials with COF smaller than 0.1 are considered lubricous materials. COF depends on the nature of the materials and surface roughness.

The formula for the coefficient of friction is μ = F_{f} / F_{N}, where μ is the coefficient, F_{f} is the friction force, and F_{N} is the normal force.

Materials with COF smaller than 0.1 are considered lubricous materials. COF depends on the nature of the materials and surface roughness. Usually, ASTM D1894 is the most widely used method for COF measurement.

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Static vs Kinetic Friction

Typically friction can be distinguished into static friction and kinetic friction. Static friction force F_{fs} is the friction force prior the initiation of motion. Kinetic friction (also known as dynamic, or sliding) force F_{fk} is the friction force developed during the motion. It should be noted that these forces are different with a following relation holding in most situations:

F_{fk} ≤ F_{fs}

Correspondingly, there are two friction coefficients, static μ_{s} and kinetic friction μ_{k} coefficients.

Static friction coefficient is calculated according to the following equation:

μ_{s} = F_{fs} / F_{N}

Kinetic friction coefficient is calculated according to following equation:

μ_{k} = F_{fk} / F_{N}