Figure 1: Simulated blocks with fractal rough surfaces, exhibiting static frictional interactions
Free-body diagram for a block on a ramp. Arrows are vectors indicating directions and magnitudes of forces. N is the normal force, mg is the force of gravity, and Ff is the force of friction.
When the mass is not moving, the object experiences static friction. The friction increases as the applied force increases until the block moves. After the block moves, it experiences kinetic friction, which is less than the maximum static friction.
Angle of friction, θ, when block just starts to slide.

When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat).

- Friction

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Measure of its resistance to deformation at a given rate.

A simulation of liquids with different viscosities. The liquid on the left has lower viscosity than the liquid on the right.
Illustration of a planar Couette flow. Since the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. The relative strength of this force is a measure of the fluid's viscosity.
In a general parallel flow, the shear stress is proportional to the gradient of the velocity.
Viscosity, the slope of each line, varies among materials.
Common glass viscosity curves
In the University of Queensland pitch drop experiment, pitch has been dripping slowly through a funnel since 1927, at a rate of one drop roughly every decade. In this way the viscosity of pitch has been determined to be approximately 230 billion (2.3) times that of water.
Honey being drizzled

Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion.

Mechanical energy

Sum of potential energy and kinetic energy.

An example of a mechanical system: A satellite is orbiting the Earth influenced only by the conservative gravitational force; its mechanical energy is therefore conserved. The satellite's acceleration is represented by the green vector and its velocity is represented by the red vector. If the satellite's orbit is an ellipse the potential energy of the satellite, and its kinetic energy, both vary with time but their sum remains constant.
A swinging pendulum with the velocity vector (green) and acceleration vector (blue). The magnitude of the velocity vector, the speed, of the pendulum is greatest in the vertical position and the pendulum is farthest from Earth in its extreme positions.

In all real systems, however, nonconservative forces, such as frictional forces, will be present, but if they are of negligible magnitude, the mechanical energy changes little and its conservation is a useful approximation.

Kinetic energy

The first to publish the relation for kinetic energy . In relativistic mechanics, this is a good approximation only when v is much less than the speed of light.

The cars of a roller coaster reach their maximum kinetic energy when at the bottom of the path. When they start rising, the kinetic energy begins to be converted to gravitational potential energy. The sum of kinetic and potential energy in the system remains constant, ignoring losses to friction.

On a level surface, this speed can be maintained without further work, except to overcome air resistance and friction.


Synthetic fluoropolymer of tetrafluoroethylene that has numerous applications.

Advertisement of the Happy Pan, a Teflon-coated pan from the 1960s
Advertisement for Zepel, the trade name used to market Teflon as a fabric treatment
Teflon thermal cover showing impact craters, from NASA's Ultra Heavy Cosmic Ray Experiment (UHCRE) on the Long Duration Exposure Facility (LDEF).
Logo of Teflon, the commonly known brand name of PTFE-based compositions manufactured by Chemours
PTFE is often used to coat non-stick pans as it is hydrophobic and possesses fairly high heat resistance.
PTFE-jacketed (white) shielded twisted-pair cables
Two teflon jars
PTFE tapes with pressure-sensitive adhesive backing
Teflon is also used as the trade name for a polymer with similar properties, perfluoroalkoxy polymer resin (PFA).

PTFE has one of the lowest coefficients of friction of any solid.


Figure 1: Simulated blocks with fractal rough surfaces, exhibiting static frictional interactions

Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact.

Conservative force

Force with the property that the total work done in moving a particle between two points is independent of the path taken.


Gravitational force is an example of a conservative force, while frictional force is an example of a non-conservative force.


Mechanical phenomenon whereby oscillations occur about an equilibrium point.

One of the possible modes of vibration of a circular drum (see other modes).
Car suspension: Designing vibration control is undertaken as part of acoustic, automotive or mechanical engineering.
Simple mass spring model
Mass–spring–damper model
How a 1 Hz square wave can be represented as a summation of sine waves (harmonics) and the corresponding frequency spectrum. Click and go to full resolution for an animation
Frequency response model
Two degrees of freedom model

Such vibrations could be caused by imbalances in the rotating parts, uneven friction, or the meshing of gear teeth.

Charles-Augustin de Coulomb

French officer, engineer, and physicist.

Portrait by Hippolyte Lecomte (1894 copy)
Coulomb's torsion balance
Mémoires, 1884

He also did important work on friction.


Mechanical device that inhibits motion by absorbing energy from a moving system.

Disc brake on a motorcycle
Rendering of a drum brake
Single pivot side-pull bicycle caliper brake
Brake booster from a Geo Storm.
Brake lever on a horse-drawn hearse

Most brakes commonly use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed.


Science and engineering of interacting surfaces in relative motion.

Tribological experiments suggested by Leonardo da Vinci
Dynamic and static coefficient
Open system tribology - wheel-rail contact in winter

It includes the study and application of the principles of friction, lubrication and wear.