The same question as the latter,
Pressure and Force
Density and mass
Force and acceleration
Proportional linear relationship introduces a direct variation between the variables involved meaning when one increase the other also increases. The greater the amount of force, the greater is the pressure is an example.
It introduces the relationship between two variables and is called correlation. Proportionality or variation is state of relationship or correlation between two variables It has two types: direct variation or proportion which states both variables are positively correlation. It is when both the variables increase or decrease together. On the contrary, indirect variation or proportion indicates negative relationship or correlation. Elaborately, the opposite of what happens to direct variation. One increases with the other variables, you got it, decreases. This correlations are important to consider because you can determine and identify how two variables relates with one another. Notice x = y (direct), y=1/x (indirect)