A student measures only the length of the thread, Ls, in a pendulum experiment, and forgets the bob…

Step 1. The value of g. The slope is $\dfrac{4\pi^2}{g}$, so $g = \dfrac{39.48}{4.02\ \text{s}^2/\text{m}} = 9.82\ \text{m/s}^2$.

Step 2. The left-out distance. The intercept is $\text{slope} \times r$. We want $r$, so $r = \dfrac{0.060\ \text{s}^2}{4.02\ \text{s}^2/\text{m}}$ $= 0.0149\ \text{m} = 1.5$ cm.

Step 3. The cut on the length axis. Put $T^2 = 0$: $L_s = -r = -1.5$ cm.

What to do. Add the radius of the bob to the length of the thread before the graph is drawn. Then the line passes through the origin.