Linearly Proportional
When two quantities are linearly proportional their graph is a straight line with a slope of the constant of proportionality, which in the above case is k.
Two quantities y and x are linearly proportional to one another if they always obey an expression of the form:y = k x
where k is a constant. Thus if x doubles in value, y does too.
Many physical laws are expressed in the form of equations where two quantities are linearly proportional to each other. An example is the force F, and acceleration a, of a body of mass m, due to Newton's second law of motion:
F = m a
