Question

A uniform bar 4 metres long and weighing 2 N is resting on a knife edge at 0.8m from one end. What weight suspended from the end to keep the bar horizontal?

Answer 1

The weight 2N of the bar acts at its midpoint D. Let C be the position of the knife edge and W be the weight suspended at the end A which keeps the bar horizontal. 

Taking moments about C

W × AC = 2 × CD

W × 0.8 = 2 × 1.2

W = \(\frac{2\times 1.2}{0.8}\)

= 3 N.

To keep the bar horizontal the weight to be suspended is 3 N.