When two
objectsIn physics, a physical body or physical object is a collection of masses, taken to be one...
touch, a certain portion of their surface areas will be in contact with each other.
Contact area refers to the fraction of this area that consists of the
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of one object being in contact with the atoms of the other object. Because objects are rarely perfectly flat, the actual contact area (on a
microscopicMicroscopic or Micro is a term used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
scale) is usually much less than the contact area apparent on a
macroscopicMacroscopic is a word commonly used to describe physical objects that are measurable and observable by the naked eye.When applied to phenomena and abstract objects, it describes existence in the world as we perceive it, often in contrast to experiences or theories considering objects of geometric...
scale. Contact area may depend on the
normal forceIn physics, the normal force is the component, perpendicular to the surface of contact, of the contact force exerted by, for example, the surface of a floor or wall, on an object, preventing the object from entering the floor or wall...
between the two objects due to deformation.
The contact area depends on the geometry of the contacting bodies, the load, and the material properties.
When two
objectsIn physics, a physical body or physical object is a collection of masses, taken to be one...
touch, a certain portion of their surface areas will be in contact with each other.
Contact area refers to the fraction of this area that consists of the
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of one object being in contact with the atoms of the other object. Because objects are rarely perfectly flat, the actual contact area (on a
microscopicMicroscopic or Micro is a term used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...
scale) is usually much less than the contact area apparent on a
macroscopicMacroscopic is a word commonly used to describe physical objects that are measurable and observable by the naked eye.When applied to phenomena and abstract objects, it describes existence in the world as we perceive it, often in contrast to experiences or theories considering objects of geometric...
scale. Contact area may depend on the
normal forceIn physics, the normal force is the component, perpendicular to the surface of contact, of the contact force exerted by, for example, the surface of a floor or wall, on an object, preventing the object from entering the floor or wall...
between the two objects due to deformation.
The contact area depends on the geometry of the contacting bodies, the load, and the material properties. The contact area between two parallel cylinders is a narrow rectangle. Two, non-parallel cylinders have an elliptical contact area, unless the cylinders are crossed at 90 degrees, in which case they have a circular contact area. Two spheres also have a circular contact area.
Friction and Contact Area
It is an
empiricalThe word empirical denotes information gained by means of observation, experience, or experiment. A central concept in science and the scientific method is that all evidence must be empirical, or empirically based, that is, dependent on evidence or consequences that are observable by the senses...
fact for many materials that F = μN, where F is the
frictional forceFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:...
for sliding friction, μ is the coefficient of friction, and N is the
normal forceIn physics, the normal force is the component, perpendicular to the surface of contact, of the contact force exerted by, for example, the surface of a floor or wall, on an object, preventing the object from entering the floor or wall...
. There isn't a simple derivation for sliding friction's independence from area.
Methods for Determining Contact Area
One way of determining the actual contact area is to determine it indirectly through a physical process that depends on contact area. For example, the
resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
of a wire is dependent on the cross-sectional area, so one may find the contact area of a metal by measuring the
currentElectric current can mean, depending on the context, a flow of electric charge or the rate of flow of electric charge ....
that flows through that area (through the surface of an
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
to another electrode, for example.)