The orginal question should be interpreted this way: a uniform metre rule (or ruler) means a beam of 1 metre in length; a 4N weight (a separated heavy object) being hanged from one end, exerting a downward force; the rule is being hanged 0.1m from the end with the 4N weight, exerting an upward force (the tension); uniform metre rule means the CG (centre of gravity) is 0.5m from either end, and the weight of the rule is producing another downward force; Let W = weight of the rule, T = the tension to "hang" the rule taking moment about the end with the "4N weight", then (0.1)(T) = (0.5)(W) all upward forces = all downward forces, then T = W + 4 by solving the simultaneous equations W = 1 so the weight of the rule is 1 N Hope my words help. Cheers.