Two related quantities x and y are called proportional (or
directly proportional) if there exists a constant non-zero
number k such that - y = kx In this case, k is called the
proportionality constant of the relation. If y are proportional,
we often write - y ~ x. For example, if you travel at a constant
speed, then the distance you cover and the time you spend are
proportional, the proportionality constant being the speed.
Similarly, the amount of force acting on a certain object from
the gravity of the Earth at sea level is proportional to the
object's mass To test whether x and y are proportional, one
performs several measurements and plots the resulting points in
a Cartesian coordinate system. If the points lie on (or close
to) a straight line passing through the origin (0,0), then the
two variables are proportional, with the proportionality
constant given by the line's slope. The two quantities x and y
are inversely proportional if there exists a non-zero constant k
such that - y = k/x For instance, the