The metre (Commonly spelt meter by parking inspectors and other ignoramuses...)
The meter is the basic unit of length in the SI system of units. The meter is defined to be the distance light travels through a vacuum in exactly 1/299792458 seconds (*)
. 1 m is about 39.37 inches. Meters are abbreviated using a lower case "m" in measurements.
(*) aka a bees dick.
RPM.
According to the International System of Units (SI), rpm is not a unit. This is because the Revolution is a semantic annotation rather than a unit. The annotation is instead done in the subscript of the formula sign if needed. Because of the measured physical quantity, the formula sign has to be f for (rotational) frequency and ? or ? for angular velocity. The corresponding basic SI unit is s?1 or Hz. When measuring angular speed, rad·s?1 can also be used as unit.
1 rad·s?1 = 60/2? rpm = c.9.55 rpm = 1/2? Hz.
Even though angular velocity, angular frequency and hertz all have the dimensions of 1/s, angular velocity and angular frequency are not expressed in hertz, but rather in an appropriate angular unit such as radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2? rad/s or 1 Hz, where the former measures the angular velocity and latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in hertz and an angular velocity ? measured in radians per second are:
\omega = 2 \pi f\,\,\! and \,\,f = \frac {\omega} {2 \pi}\text{.}\,\!
