Conversion from Meters per second to Knots

Information about the Meter per second (m/s)

The metre per second is the SI unit of speed. Its symbol is m/s. It is a derived unit, formed from the base units of length and time, and it is the speed at which an object covers one metre in one second.

Physics uses nothing else. Every equation of motion, every conservation law and every result in mechanics assumes speeds in metres per second, because mixing in an hour or a mile would force conversion factors into formulas that are otherwise clean. The speed of light in vacuum is exactly 299,792,458 metres per second, a defined value since 1983, and that definition is what fixes the length of the metre.

Outside the laboratory the unit is less comfortable. Road speeds in metres per second would be small numbers with awkward decimals, which is why almost every country signs roads in kilometres per hour instead. Where the metre per second does appear in daily life, it is usually because the quantity is short-lived or physical rather than navigational: wind speed in meteorological reports, the speed of sound at about 343 metres per second in air at twenty degrees Celsius, and lift or conveyor speeds in engineering specifications.

Sport uses it for anything measured over a short course. Sprint velocities, swimming splits and ball speeds are often analysed in metres per second even where the published result is in kilometres per hour, because it makes acceleration and stride analysis straightforward.

A handful of speeds in this unit are worth carrying in memory because so much else is measured against them. Sound travels through dry air at about 343 metres per second at room temperature, through water at 1480 and through steel at some 5000, which is why a distant lightning strike is timed at roughly three seconds to the kilometre. Light in vacuum moves at exactly 299,792,458 metres per second, a number now fixed by definition rather than measured, and the metre itself is derived from it. Between those extremes sit the everyday values: a brisk walk is 1.4 metres per second, a motorway car 33, and a passenger jet about 250.

One metre per second equals 3.6 kilometres per hour, about 2.23694 miles per hour, or about 1.94384 knots.

Information about the Knot (kn)

The knot is a unit of speed equal to one nautical mile per hour, or exactly 1.852 kilometres per hour. Its symbol is kn, and the older abbreviation kt remains common. It is the speed unit of shipping and aviation worldwide, used by every nation regardless of what it does on land.

The name is literal. Before mechanical logs, a ship's speed was measured with a chip log: a weighted board on a long line, thrown astern to sit still in the water while the line ran out. The line carried knots at fixed intervals, and a sandglass measured a fixed time, usually twenty-eight seconds. Counting the knots that passed through the hand in that interval gave the speed directly, and the count was recorded in the ship's log, which is where the word for that record comes from.

Its survival is not sentimental. The nautical mile is one minute of arc of latitude, so a speed in knots converts directly into change of latitude per hour on a chart. A vessel making ten knots due north advances ten minutes of latitude every hour, a calculation a navigator can do without a table.

Aviation adopted the same unit for the same reason, and airspeed indicators, air traffic control instructions and flight plans worldwide use knots. Meteorology uses it for wind aloft and in marine forecasts, and the barbs on a weather map's wind symbols each represent ten knots.

Wind at sea is described in the same unit, and the Beaufort scale is essentially a table of knots dressed as descriptions. Force 4, a moderate breeze, is 11 to 16 knots; force 8, a gale, is 34 to 40; and force 12, a hurricane, begins at 64. A shipping forecast that says severe gale force nine is telling a listener that the wind is between 41 and 47 knots without giving a number at all. Aviation keeps the unit for the same practical reason as shipping: an aircraft's speed through the air, its speed over the ground and the wind that separates the two are all quoted in knots, so a navigation triangle can be solved without converting anything.

One knot equals exactly 1.852 kilometres per hour, about 1.15078 miles per hour, or about 0.514444 metres per second.