Coulomb friction is a simplified way to measure the frictional force between two dry surfaces in contact. It assumes uniform surfaces and well-established coefficients of friction. Mechanical engineering often uses Coulomb friction formulas, but they are less accurate for semi-soft compounds. Coulomb’s law of friction is based on Amonton’s laws and is independent of the actual sliding speed of the bodies. The coefficient of friction is well known for many simple and pure materials and is given as a unitless number. Coulomb friction damping is the effect of friction always opposite to the direction of motion, and Coulomb friction torque involves rotational forces.
Coulomb friction is a simplified quantification of the frictional force that exists between two dry surfaces in contact with each other. All friction calculations are approximations and this measurement, developed in 1785 by Charles-Augustin de Coulomb as a refinement of Leonardo da Vinci’s classic model, depends only on the fundamental principles of motion. It is assumed that the contact surfaces are fairly uniform and that the coefficient of friction that must be overcome for motion to begin is well established for the materials in contact. It also takes into account the normal force involving gravitational pull, either in horizontal motion directed at the normal force or in a vector tilt.
Mechanical engineering calculations often use Coulomb friction formulas because of their simplicity and can be adapted to accommodate static friction of non-moving bodies or kinetic friction of bodies sliding against each other. This model assumes that the materials are rigid solids, with no lubricants or other liquids or gases between them. While Coulomb’s law of friction works well for these materials, involving semi-soft compounds such as rubber or polished metal surfaces, the calculations are less accurate.
Guillaume Amontons, a French inventor, perfected Leonardo da Vinci’s calculations of friction in 1699, and Coulomb used this to base his understanding of friction. For dry surfaces, three rules of physics apply, the first two known as Amonton’s laws and the third as Coulomb’s law. The first two state that the frictional force is directly proportional to the load and is independent of the visible contact area between the materials. Coulomb’s law states that the kinetic friction of moving bodies is independent of the actual sliding speed of the bodies.
Coulomb’s coefficient of friction is a static force slightly greater than the driving force when two materials are at rest while in contact with each other. This coefficient of friction is well known for many simple and pure materials and is given as a unitless number. For dry surfaces, the coefficient of friction for wood against concrete is 0.62, for polystyrene against steel 0.3 to 0.35 and for steel against Teflon® of 0.04. These numbers are used to calculate the force required to overcome static friction, known as the friction force, by multiplying the coefficient of friction by the normal force. The normal force is the mass of the materials multiplied by the gravitational pull, with vector calculations added as to whether the two surfaces are moving up or down an incline against, or toward, the gravitational pull.
Coulomb friction damping is the effect of friction always opposite to the direction of motion. It is expressed as the release of thermal energy between surfaces, which reduces the net kinetic energy of motion. Coulomb friction torque involves rotational forces when two materials are not moving linearly while in contact, and is another example of where the basic formulas are incorporated into more complex calculations of the actual friction taking place. These calculations take Coulomb’s formulas and expand upon them to include a variety of frictional environments, including viscous fluid friction, internal friction in materials where deformation occurs, and more.
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