Proportional and linear relationships

Proportional

directly proportional (y = mx)

For example using Ohm's Law, potential differece is proportional to the current through an Ohmic resistor at a fixed temperature.

Intercept should be zero if you graph: y vs x

inversely proportional (y = m/x)

For example when investigating drag, the velocity squared is inversely proportional to the surface area.

Do not graph the inverse relationship y vs x, graph y vs 1/x to get a directly proportional relationship.

Intercept should be zero if you graph: y vs 1/x

Linear

(y = mx + c)

For example the voltage generated across a solar panel with changing temperature is linear, in the form: V(a) = k ΔT + V(r)