Turning Effect of force

Chapter 4
Turning Effect
of Force
By
Khizar yousaf

Moment or
Moment of
force

2

The moment of a
force is the turning
effect produced
when a force is
exerted on an
object

Examples of the turning
effect of a force are:

➢A child on a see-

saw

➢Turning the

handle of a
spanner

3

Examples of the turning
effect of a force are:

➢A door opening

and closing

➢Using a crane to

move building
supplies

4

Examples of the
turning effect of
a force are:

5



Using a screwdriver
to open a tin of paint



Turning a tap on and
off

Examples of the turning
effect of a force are:

6

 Picking up a wheelbarrow

 Using scissors

Scenes of rotation:

7

Anticlockwise
Rotation:

It is taken as a
positive.

Clockwise
Rotation;

It is taken as a
negative.

Consider the hands of a clock when deciding if an object will
rotate in a clockwise or anti-clockwise direction

The moment equation:
8

The turning effect of force about some
pivot is called the moment of force.

We calculate the moment of force by
using the following formula

𝑀𝑜𝑚𝑒𝑛𝑡𝑜𝑓𝑓𝑜𝑟𝑐𝑒 = 𝐹𝑜𝑟𝑐𝑒 × 𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒𝑓𝑟𝑜𝑚𝑝𝑖𝑣𝑜𝑡𝑡𝑜𝑡ℎ𝑒𝑙𝑖𝑛𝑒𝑜𝑓𝑎𝑐𝑡𝑖𝑜𝑛
𝑜𝑓𝑓𝑜𝑟𝑐𝑒

𝑀𝑜𝑚𝑒𝑛𝑡𝑜𝑓𝑓𝑜𝑟𝑐𝑒(𝑇) = 𝐹 × 𝑑

 Moment is measured in Newton

meters (Nm).

Factors on which moment of force depends upon:
9

The moment of force depends on the
following factors:

1. The size (magnitude) of the force

2. The perpendicular distance between
the line of action of the force and the
turning point is called the pivot.

Factors on which
moment of force
depends upon:

10

 If you try to push open a door

right next to the hinge, it is
tough, as it requires a lot of
force

 If you push the door open at

the side furthest from the
hinge, then it is much easier,
as less force is required

Principle of
moments:

11

The principle of moments
states that:

If an object is balanced, the
total clockwise moment about a
pivot equals the total
anticlockwise moment about
that pivot

 The principle of moments

means that for a balanced
object, the moments on both
sides of the pivot are equal

 clockwise moment =

anticlockwise moment

Derivation of the Principle of moments:

12

In the example, the forces and distances of the objects on
the beam are different, but they are arranged in a way that
balances the whole system

In the diagram:

➢Force F 1 causes an anticlockwise moment of F 1 X d 1
about the pivot.

➢Force F 2 causes a clockwise moment of F 2 X d 2
about the pivot.

➢Force F 3 causes an anticlockwise moment of F 3 X d 3
about the pivot.

Principle of
moment:

13



Collecting the clockwise and
anticlockwise moments:



Sum of the clockwise

moments = (𝐹2 × 𝑑2)



Sum of the anticlockwise

moments = (𝐹1 × 𝑑1 + 𝐹3 × 𝑑3)



Using the principle of moments,
the beam is balanced when:

sum of the clockwise moments = sum
of the anticlockwise moments

Equilibrium:

14

 In physics, the term

equilibrium means:

A state of balance or
stability

 In other words, a

system in equilibrium
keeps doing what it’s
doing without any
change

Centre of
gravity:

15

The centre of gravity of an
object is defined as:

The point through which the
weight of an object acts

For a symmetrical object of
uniform density, the centre of
gravity is located at the point of
symmetry

For example, the centre of
gravity of a sphere is at the
centre

Stability:

16

•
The centre of
gravity of a
symmetrical object is
along the axis of
symmetry

•
The position of the
centre of gravity affects
the stability of an
object

•
An object
is stable when its
centre of gravity lies
above its base

Low centre of
gravity:

17

 If the centre of gravity

does not lie above its
base, then an object
will topple over

 The most stable objects

have a low centre of
gravity and a wide base

High centre of
gravity:

18

Taller objects with a narrow
base have a higher centre of
gravity and are less stable

This is why lorries and buses
are advised not to use
motorways and bridges on
very windy days

Moments and
stability:

19

 If the line of action of the weight

force lies outside the base of the
object, there will be a resultant
moment, and the body will topple

 Tall objects with a narrow base will

topple easily

 Ten-pin bowling pins are designed

specifically to topple easily

 The stability of objects can be

increased by widening the base

 High chairs are designed with a

wide base so that they do not
topple

 Bunsen burners have a wide base to

ensure they do not topple

The car can be titled to 60° without toppling, but the
bus will topple at 45°

Investigating
Centre of
Gravity:

20

Punch 3 holes near the outer edges of
the plane lamina in different locations

Create a loop of thread and hang the
plane lamina from the clamp

Use a plumb line (a weighted thread)
aligned with the hanging thread to show
the line of action of the weight force

Use a ruler and pencil to mark the line of
action of the weight force onto the plane
lamina

Repeat the process until 3 lines have
been drawn

The point at which the lines cross is the
position of the centre of gravity