How to use: Drag the Hanging mass slider up and down slowly.
Every time you release the slider at a new mass, a point is added to the graph on
the right. The graph plots extension against force. After a few
points, look at the shape. Then change the spring constant and repeat —
what changes? (Hint: the graph clears when you change the spring, because it's a
different spring.)
Notice: the points form a straight line through the origin — extension is proportional to force. That's Hooke's law: F = kx. A stiffer spring (bigger k) gives a shallower line, because the same force stretches it less.