Practical: Balancing a lever
A lever arm is the distance from the pivot (axis of rotation) to the point where the weight hangs. A lever is balanced when the left mass × its arm equals the right mass × its arm: m1 × l1 = m2 × l2. In other words the ratio of masses is the inverse ratio of the arms: m1 : m2 = l2 : l1. So placing a load near the pivot allows a small force to balance it. The same holds for forces: F1 × l1 = F2 × l2. That is why a crowbar is long and scissors’ handles are long. In the experiment we use a ruler on a pencil and identical coins: since each coin has the same mass we can use the number of coins instead of mass.
Practical: place a 30 cm ruler on a round pencil at the 15 cm mark. On the left put 2 identical coins 10 cm from the pivot; on the right move 1 coin to find the balance point (20 cm). Record m1, l1, m2, l2, m1 × l1 and m2 × l2 in a table. Safety: do not work at a table edge, do not put coins in the mouth and do not use a sharp pencil.