**Turning Effects**

The turning effect of a body is called the moment of that force. The turning effect produced depends on both the size of the force and the distance from the pivot. The moment of a force about a point is the product of the force applied and the perpendicular distance from the pivot (or turning point) to the line of action of the force. Hence, Moments of a force = Force × perpendicular distance from pivot.

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**The law of moments**

The law of moments states that “when a body is in balance or in equilibrium, the sum of the clockwise moments equals the sum of anti-clockwise moments”.

The SI units of the moments of a force is Newton metre (Nm).

**Examples**

- A uniform rod of negligible mass balances when a weight of 3 N is at A, weight of 3 N is at B and a weight of W is at C. What is the value of weight W?

- The following bar is of negligible weight. Determine the value of ‘ x’ if the bar is balanced.

**Solution**

The distance from the turning point to the line of action can be determined as,

**Clockwise moments** = 10 × 30 = 300 N cm, Anticlockwise moments = 10 × ‘x’ = 10 x. N cm. Using the principle of moments

Anti-clockwise moments = clockwise moments

10 x = 300, hence x = 30 cm.

- Study the diagram below and determine the value of X and hence the length of the bar.

**Solution**

Clockwise moments = 15x N + 5(X × 20) N

Anticlockwise moments = (20 × 10) + (60 × 10) N cm, = 800 N cm.

Anti-clockwise moments = clockwise moments

800 N cm = 15X + 5X + 100

800 n cm = 20X + 100

20X = 700

X = 35 cm.

Therefore, the length of the bar = 40 + 20 + 35 + 20 = 115 cm.

**The lever**

A lever is any device which can turn about a pivot or fulcrum .

The applied force is called the effort and is used to overcome the resisting force called the load. We use the law of moments in the operation of levers.

**Example**

Consider the following diagram. (The bar is of negligible mass). Determine the effort applied.

**Solution**

Taking moments about O. Then, clockwise moments = effort × 200 cm. Anticlockwise moments = 200 × 30 cm.

**Effort** = (200 × 30)/ 200 = 30 N.

See also:

RECTILINEAR PROPAGATION AND REFLECTION AT PLANE SURFACES. INTRODUCTION