1 The idea

Every physical quantity has a definition that is agreed upon worldwide. For example, speed = distance ÷ time. We use letters for the three mechanical fundamental quantities:

• L for length, M for mass, T for time.

(Careful: here T means time dimension, not temperature.)

• When the definition of a quantity is written using these symbols we get its dimensional formula.
• Square brackets [ ] mean "the dimensions of":

[speed] = [distance] ÷ [time] = L ÷ T = LT⁻¹

In general, most derived quantities can be written as

[A] = L^a M^b T^c

where a, b, c are the dimensional exponents (positive, negative or zero).

Dimensional formula of a physical quantity: an expression showing how the quantity depends on the fundamental quantities L, M, T and the power of each.