# TURNING EFFECT OF A FORCE

TURNING EFFECT OF A FORCET:urning 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.**

**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?*

*Solution *

Taking moments about the fulcrum, O then

Anti-clockwise moments = (3 × 1) + (3 × 3)

= 3 + 9 = 12 Nm

Anti-clockwise moments = clockwise moments

3 W = 12 Nm

W = 4 N

*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,

60 cm 150^{0}

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.

ALL PHYSICS NOTES FORM 1-4 WITH TOPICAL QUESTIONS & ANSWERS

PRIMARY NOTES, SCHEMES OF WORK AND EXAMINATIONS