Unit of power, the rate at which energy is supplied. One watt is the power which supplies 1 joule per second, 1 kilowatt = 1000 watts. A grown human climbing stairs (e.g.) supplies about 100 watt; 1 horsepower = 736 watt. Named for James Watt, inventor of the modern steam engine.


A disturbance spreading in space, obeying a certain "wave equation." Sound waves, ocean waves and electromagnetic waves are some of the examples; other, more complicated types of waves can spread in plasmas.


(Often denoted by l, the Greek letter letter lambda.) The distance between two crests of a propagating wave of a single frequency n . If v is the velocity at which the wave advances, v=ln.

Wave number

A term used for the inverse of the wavelength, i.e. for 1/l


General name for processes in the atmosphere--winds, rain, thunderstorms etc.--driven by the heating of Earth by sunshine. That heat has to be returned to space, and weather processes are the result. See troposphere and stratosphere.


The force exerted on mass by gravity.


(or "zero gravity") the condition when no force (such as weight) is sensed. Occurs in orbit or free fall, when gravity already produces its full acceleration and can produce no further effect.


The overcoming of a resisting force over a distance. The work performed when a force F overcomes an equal resisting force along a distance x in the same direction equals Fx, i.e. F times x.
If the force is not in the direction of the motion, only the vector component of F in that direction enters the calculation.
Energy can be defined as the ability to perform work.