Introduction
All forces in physics are derived from a few basic units (cannot be reduced to other units)
Base Units:
These are the SI units upon which every other unit is based on and derived from.
Quantity Name Quantity Abv. Unit Name Unit Abv. Definition
Distance d Meter m Now defined as length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of second
Mass M Kilogram kg Originally defined as the mass of 1000cm3 of water. Grams is the base unit.
Time t Second s Now defined as 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.
Charge Q Coulomb C A coulomb of charge is defined as the charge of about 6.2 x 1018 electrons
Temperature T Kelvin K Now defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
Kinematics Units:
These units are used for calculations involving mechanics and motion.
Quantity / Abv. / Units / Abv. / Definition / Equation / Basic Definition
Speed None Distance / Time m / s
Acceleration a None Distance / Time2 m / s2
Force F Newtons N Mass x Acceleration M x a (kg x m) / s2
Energy (Work) E Joule J Force x Distance F x d or
N x m (kg x m2) / s2
Power (Work Rate) P Watt W Energy / Time E / t or
(N x m) / s or
J / s (kg x m) x m / s3
Pressure Pascals Pa Force / Area F / Area or
N/m2 [(kg x m) / s2] / m2
Note: Power is also measured in Horsepower (hp).
n 1 hp = 746 Watts
n 1 Watt = 0.00134 hp
Note: Pressure is often measured in Pounds per Square Inch (psi)
n 1 Pa = 0.000145 psi
n 1 psi = 6891 Pa
Note: Pressure can also be measured in atmospheres (atm) or barometric pressure (bar), which is essentially the same thing. 1 ATM = 1 bar
n 1 ATM = 14.7 psi
n 1 ATM = 101300 Pa
Electrical Units:
These units are used when working with electrical components.
Quantity / Abv. / Units / Abv. / Definition / Equation
Voltage (Potential Difference) V Volts V Energy / Charge V = E / Q or V = J / C
Current I Amperes A Charge / Time I = Q / t or
A = C / s
Resistance R Ohms ? Voltage / Current R = V/I or
? = V/A
Energy (electrical) E Joule J Voltage x Charge E = V x C or
J = V x Q
Power (electrical) P Watts W Voltage x Current P = V x I or
W = V x A or
W = (J / C) x (C / s)
Capacitance C Farads F Charge / Voltage C = Q / V
F = C / V
Note: Another quantity that may be used in electrical systems is Frequency. Frequency, the unit for which is the Hertz (Hz) and it is equivalent to s-1 or a �per second�
Note: Another unit that may be used is the amp-hour (Ah). A one amp-hour battery can store enough electricity to give out 1 amp for 1 hour.
SI Prefixes
These prefixes may be tacked onto the beginning of the abbreviation of any unit. Examples are MILLI-meter (mm), KILO-watt (kW), MEGA-hertz (mHz), NANO-meter (nm), MILLI-amp (mA), MICRO-farad (?F)� etc.
Power of 10 / Prefix Name / Prefix Code / English Meaning / Original Language
+18 exa E outside Greek
+15 peta P spread Greek
+12 tera T monster Greek
+9 giga G giant Greek
+6 mega M large Greek
+3 kilo k thousand Greek
+2 hecto h hundred Greek
+1 deka D ten Greek
0
- 1 deci d ten Latin
- 2 centi c hundred Latin
- 3 milli m thousand Latin
- 6 micro ? small Latin
- 9 nano n dwarf Greek
- 12 pico p tiny Italian
- 15 femto f fifteen Old Norse
- 18 atto a eighteen Old Norse
Note: Niels Bohr, a Dane, is responsible for the addition of the femto- and atto-prefix names to the lexicon of the metric system