Question 13
A rod is suspended at its centre. On one side a weight of 20 N hangs 0.2 m from the centre. What weight must hang 0.8 m from the centre on the other side to balance the rod?
The right answer is the one marked above.
5 N. The moment on one side is 20 N × 0.2 m = 4 N m. For equilibrium the other side must also give 4 N m, so x × 0.8 = 4 and x = 5 N.
Choosing 20 N, on the idea that balance needs equal forces, is the trap. The distance is four times as great, so the force must be a quarter as large.