# Direct proportionality

Two quantities a and b are said to be directly proportional, if the quotient $\frac{a}{b}$ is constant. This is written as $a \propto b$.

## Constant of Proportionality

The constant $c = \frac{a}{b}$ is called constant of proportionality. Therefore a proportionality can be written as:

$a = c \times b$

## Famous Proportionality

$C \propto r$
Here, the constant is
$E \propto m$
Here, the constant is , the square of the speed of light.

## Misunderstandings

The role of the quantities and of the constant are not interchangeable. It would be wrong to say in the examples above that $C \propto 2 \pi$ or $E \propto c^2$.