# Proportionality Two varying quantities are said to be in a relation of **proportionality**, when they are **multiplicatively** connected to a constant. That is, when either their *ratio* or *product* yields a *constant*. Consider two variables, $x$ and $y$, where their ratio is equal to a constant, $k$: $\frac{y}{x} = k \longrightarrow y = kx$  This case is known as **direct proportionality**, and makes up a [linear relationship](Linearity.md). ### Examples * The circumference of a circle is directly proportional to its diameter, with a proportionality constant of $/pi$. * The force, acting on a small object with a small mass by a nearby object with large mass, is directly proportional to the objects mass: $F = m a$. The constant of proportionality between the force and the mass is known as *gravitational acceleration*. --- References: * https://en.wikipedia.org/wiki/Proportionality_(mathematics)