Direct proportion is one of the most important relationships in physics, and a graph shows it clearly. Two quantities are directly proportional when their graph is a straight line that passes through the origin. This means when one quantity is zero, so is the other, and doubling one doubles the other.
The straight line through the origin is the test. A straight line that does not pass through the origin shows a linear relationship, but not direct proportion, so the two ideas must not be confused.
The gradient of the line is the constant of proportionality, linking the two quantities in the form y equals a constant times x. Reading this gradient gives you the value of that constant along with its unit.
Many KSSM relationships, such as extension against load within the elastic limit, are directly proportional, so recognising the straight-line-through-origin pattern is a valuable and frequently rewarded skill.
What examiners award
Marks are awarded for identifying direct proportion from a straight line through the origin and for finding the constant of proportionality as the gradient.
Common mark losses
In short
Two quantities are directly proportional when their graph is a straight line through the origin. Doubling one doubles the other, and the gradient is the constant of proportionality.
A routine that works
Practise the mechanics until they are automatic: label both axes with units, choose a scale that fills more than half the grid, plot points with a sharp pencil, draw a single best-fit line, and take the gradient from a large triangle with its coordinates and unit shown. One clean example beats re-reading the theory.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)