SI unit
Summary
SI units are the international standard metric units: meter, kilogram, second, ampere, kelvin, mole and candela. Other units, such as the newton and joule, are combinations of these. Testable point: convert to SI before calculating, especially kg for mass and K for temperature.
Detail
Meaning: SI (Système International) is the standard metric system. Its base units are the meter (m, length), kilogram (kg, mass), second (s, time), ampere (A, current), kelvin (K, temperature), mole (mol, amount) and candela (cd, rarely tested). Derived units combine them: newton (N = kg·m/s²), joule (J = N·m), watt (W = J/s), pascal (Pa = N/m²), coulomb (C = A·s).
Equation: F = ma, where F is force in newtons (kg·m/s²), m is mass in kg, and a is acceleration in m/s².
On the MCAT: Passages often give data in g, cm, L, atm, mmHg, °C or eV. Convert to SI before substituting, and check units to catch errors. Know the prefixes: kilo 10³, centi 10⁻², milli 10⁻³, micro 10⁻⁶, nano 10⁻⁹. With R = 8.314 J/(mol·K), pressure goes in Pa and volume in m³.
Common confusion: The base unit of mass is the kilogram, not the gram, so putting grams into F = ma or KE = ½mv² gives an answer 1000 times too small in N or J. In gas-law problems, temperature must be in K. A temperature change ΔT is the same size in °C and K, but a temperature reading is not.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway and Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.