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The speed of light in vacuum, c, is a universal physical constant that is exactly equal to 299,792,458 metres per second. Learn how c is defined in the SI system, how it relates to relativity and electromagnetism, and how it affects light propagation in different media.
Learn how fast sound waves travel in different media, such as air, water, and solids, and how the speed depends on temperature, frequency, and pressure. Explore the history of measuring the speed of sound and the basic concepts of sound transmission.
Miles per hour (mph) is a unit of speed used in the UK, the US, and some other countries. Learn which jurisdictions use mph, how it compares to other units, and see examples of speed limit signs.
A knot is a unit of speed equal to one nautical mile per hour, which is approximately 1.852 km/h or 1.151 mph. Learn about the history, definitions, conversions and usage of the knot in meteorology, maritime and aviation.
Wind speed is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. It is commonly measured with an anemometer, and affected by pressure gradient, Rossby waves, jet streams, and local weather conditions.
Escape velocity is the minimum speed needed for an object to escape from the gravitational field of a primary body. For Earth, it is about 11.186 km/s at the surface and decreases with altitude. Learn how to calculate it and see examples.
Naismith's rule is a rule of thumb for estimating the time needed to travel a distance, taking into account the slope and pace. It was devised by a Scottish mountaineer in 1892 and has various modern versions and adjustments.
Speed is the rate at which an object covers distance and is a scalar quantity. The SI unit of speed is the metre per second (m/s), but other units such as km/h, mph and kn are also common.