The student first calibrates the spring: (i) The length of the spring without any load is measured as $l_0=3.0cm$. (ii) With a load of $100g$ suspended from it, the stretched length is $l_1=8.0cm$. (iii) Calculate the extension $e_1$ of the spring using the equation $e_1=l_1-l_0$. (iv) Determine the spring constant $k$ using the equation $k=\frac{m}{e_1}$, where $m=100g$, and state its unit.

The apparatus is then set up such that the metre rule is resting on a bench at its zero mark (pivot) and is supported at the other end by the spring. The rule is at a small angle to the bench. With only the rule in place, the student measures the length of the stretched spring as $l_2=4.5cm$. (i) The student then places a metal block with its centre at the $40.0cm$ mark on the metre rule. Explain briefly how the student can ensure that the block is in the correct position. (ii) With the block in position, the student measures the new stretched length of the spring as $l_3=8.1cm$. Determine the change in extension $e_2$ due to the block using the equation $e_2=l_3-l_2$. (iii) Calculate the mass $M$ of the block using the equation $M=k\left(\frac{e_2}{0.40}\right)$.

Suggest two practical causes of inaccuracy in this experiment.