# Radian

### From Wikipedia, the free encyclopedia

The **radian** is a unit of plane angle, equal to 180/π (or 360/(2π)) degrees, or about 57.2958 degrees, or about 57°17′45″. It is the standard unit of angular measurement in all areas of mathematicsbeyond the elementary level.

The radian is represented by the symbol “rad” or, more rarely, by the superscript c (for “circular measure”). For example, an angle of 1.2 radians would be written as ” 1.2 rad ” or ” 1.2^{c} ” (the second symbol is often mistaken for a degree: ” 1.2° “). However, the radian is mathematically considered a “pure number” that needs no unit symbol, and in mathematical writing the symbol “rad” is almost always omitted. In the absence of any symbol radians are assumed, and when degrees are meant the symbol ° is used.

The radian was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit. The SI unit of solid angle measurement is the steradian.

## Contents[hide] |

- 1 Definition
- 2 History
- 3 Conversions
- 4 Advantages of measuring in radians
- 5 Dimensional analysis
- 6 Use in physics
- 7 Multiples of radian units
- 8 See also
- 9 References
- 10 External links

## [edit]Definition

One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. More generally, the magnitude in radians of such a subtended angle is equal to the ratio of the arc length to the radius of the circle; that is, *θ* = *s* /*r*, where *θ* is the subtended angle in radians, *s* is arc length, and *r* is radius. Conversely, the length of the enclosed arc is equal to the radius multiplied by the magnitude of the angle in radians; that is, *s* = *rθ*.

It follows that the magnitude in radians of one complete revolution (360 degrees) is the length of the entire circumference divided by the radius, or 2π*r* /*r*, or 2π. Thus 2π radians is equal to 360 degrees, meaning that one radian is equal to 180/π degrees.

## [edit]History

The concept of radian measure, as opposed to the degree of an angle, should probably be credited to Roger Cotes in 1714.^{[1]} He had the radian in everything but name, and he recognized its naturalness as a unit of angular measure.

The term *radian* first appeared in print on 5 June 1873, in examination questions set by James Thomson (brother of Lord Kelvin) at Queen’s College, Belfast. He used the term as early as 1871, while in 1869, Thomas Muir, then of the University of St Andrews, vacillated between *rad*, *radial* and *radian*. In 1874, Muir adopted *radian* after a consultation with James Thomson.^{[2]}^{[3]}^{[4]}

## [edit]Conversions

### [edit]Conversion between radians and degrees

As stated, one radian is equal to 180/π degrees. Thus, to convert from radians to degrees, multiply by 180/π. For example,

For Example:

Conversely, to convert from degrees to radians, multiply by π/180.

For Example:

Radians can be converted to revolutions by dividing the number of radians by 2π.

### [edit]Conversion between radians and grads

2π radians are equal to one complete revolution, which is 400^{g}. So, to convert from radians to grads multiply by 200/π, and to convert from grads to radians multiply by π/200. For example,

The table shows the conversion of some common angles.

Units | Values | |||||||
---|---|---|---|---|---|---|---|---|

Revolutions |
0 | 1/12 |