A uniform metre rule balances horizontally on a knife-edge placed at the $58 \mathrm{cm}$ mark when a weight of $20 \mathrm{gf}$ is suspended from one end.
(i) Draw a diagram of the arrangement. (ii) What is the weight of the rule?

The diagram below shows a uniform bar supported at the middle point $O$. A weight of $40 \mathrm{gf}$ is placed at a distance $40 \mathrm{cm}$ to the left of point $O$. How can you balance the bar with a weight of $80 \mathrm{gf}$?

A uniform metre rule of mass $100 \mathrm{g}$ is balanced on a fulcrum at mark $40 \mathrm{cm}$ by suspending an unknown mass $m$ at the mark $20 \mathrm{cm}$.

(i) Find the value of $m$.
(ii) To which side the rule will tilt if the mass $m$ is moved to the mark $10 \mathrm{cm}$? 
(iii) What is the resultant moment now?
(iv) How can it be balanced by another mass of $50 \mathrm{g}$?