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*