High School Physics/Si units

The SI Unit System
If physical quantities are measured using different systems of units, the magnitudes would be different. It would become inconvenient and difficult to compare results. It has been also observed that the units of some physical quantities related to heat, light, current electricity, etc. have complicated relations with some basic physical quantities and can be expressed using them. The resulting units are therefore impractical. Taking these problems into account, the International Bureau of Weights and Measures recommended a new system of units, International System of Units (SI). SI units provide and international standard of measurement and are used widely.

So, there are seven base SI units.

Base units

 * Meters (Distance)
 * Describes the space between two locations. Think of the length of a highway or the distance between two cities. The definition of the meter has changed in the last few years. The meter was originally defined as one ten millionth of the distance from one of the Earth's poles to the equator. The meter is now defined as the length of the path travelled by light in vacuum during a time interval of $$1/299,792,458$$ of a second.


 * Kilograms (Mass)
 * Describes how much matter an object has. Think of the difference between a small female gymnast and large male football player. The two athletes' bodies move differently because of their difference in mass. The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.


 * Seconds (Time)
 * Describes the passage of time or aging of the universe.The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.


 * Kelvin (Thermodynamic temperature)
 * The kelvin is the fraction $$1/273.16$$ of the thermodynamic temperature of the triple point of water.


 * Mole (Amount of substance mole)
 * 1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol."
 * 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.


 * Ampere (Electric current)
 * The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 newton per meter of length.


 * Candela (Luminous intensity)
 * The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Derived units

 * Force $$\rm N$$
 * The Newton is a product of mass with acceleration, representing a force.
 * $$1.0\,\rm N=0.24\,\rm lb$$ of force, defining lbs using Earth's value of gravity


 * Velocity $$\frac{\rm m}{\rm s}$$
 * The motion of an object based on time, with a direction. Velocity is always specified with a direction, such as “12 miles per hour north”.
 * If the direction is not specified, it is called speed.


 * Acceleration $$\frac{\rm m}{\rm s^2}$$
 * The rate at which an object is changing velocity.


 * Momentum $$\rm kg\cdot\frac{\rm m}{\rm s}$$
 * The product of mass and speed. Note that heavy or fast objects have high momentums.


 * Energy $$\rm J$$
 * The Joule is a product on force and distance.

Conversions
Though best learned in chemistry, conversion are simply ways in which you can represent a relative quantity.