Question

A uniform half metre rule can be balanced at the 29-0 cm mark when a mass 20 g is hung from its one end.

(a) Draw a diagram of the arrangement.

(b) Find the mass of the half metre rule.

Answer 1

(i)

(ii) The weight will act at 25 cm mark. Applying the principle of moments,

Let the weight of the half metre rule be x gf and it acts at the 25 cm mark (centre of gravity).

Now,

Acticlockwise moment $\text{= x×4}$

Clockwise moment $\text{= 20×21}$

In equilibrium,

Anticlockwise moment = Clockwise moment

$\text{4x=20×21}$

$\text{x=105 gf.}$

Thus, the weight of the metre rule is 105gf.

We know, one gf is the force acting on the body of mass 1 grams, thus mass of the body is $\text{105grams.}$ 105grams.