Forces and Turning Effects

TURNING

EFFECT OF

FORCES

MOMENTS

Lesson Objectives

Identify what are the forces that will give rise

to clockwise / anticlockwise moments

Calculate clockwise / anticlockwise moments

Use the principle of moments to solve

problems.

MOMENT

DEFINITION

A force can cause many things to move or stop. When a force causes an object to turn, this turning effect is called moments.

Example: A person sitting on a see-saw.

MOMENTS

A force can cause many things to move or stop. When a force causes an object to turn, this turning effect is called moment.

Example: A person sitting on a see-saw.

How many types do we have?

There are 2 types of moments:

Clockwise moment

Anticlockwise moment

How many Types do we have? There are 2 types of moments:

Clockwise moment

Anticlockwise moment

Clockwise Moment

When a force causes an object to turn in a clockwise

direction, it is called a CLOCKWISE MOMENT.

To determine if the force causes a clockwise or

anticlockwise moment, we have to ignore all other forces

acting on the object.

When the girl sits on the see-saw, her weight is the force

exerted on the see-saw.

weight

Clockwise Moment

When a force causes an object to turn in a clockwise

direction, it is called a CLOCKWISE MOMENT.

To determine if the force causes a clockwise or

anticlockwise moment, we have to ignore all other forces

acting on the object.

When the girl sits on the see-saw, her weight is the force

exerted on the see-saw.

weight

ANTI-Clockwise Moment

When a force causes an object to turn in an anti-clockwise

direction, it is called a ANTI-CLOCKWISE MOMENT.

To determine if the force causes a clockwise or

anticlockwise moment, we have to ignore all other forces

acting on the object.

When the girl sits on the see-saw, her weight is the force

exerted on the see-saw.

weight

ANTI-Clockwise Moment

When a force causes an object to turn in an anti-clockwise

direction, it is called an ANTI-CLOCKWISE MOMENT.

To determine if the force causes a clockwise or

anticlockwise moment, we have to ignore all other forces

acting on the object.

When the girl sits on the see-saw, her weight is the force

exerted on the see-saw.

weight

CALCULATING MOMENTS

Moment = force perpendicular distance between force

and pivot

In symbols: Moments = F d

Weight => F

d

Unit for moments: Newton-metre (Nm)

F = 20N

d = 3m

Example 1:

A cat of weight 20N stands on one end of a see-saw and the distance

between the cat and the pivot is 3m, find the moment.

Solution: In this case the cat is causing a clockwise moment.

Clockwise moment = F x d

= 20 x 3

= 60 Nm

Example 2:

A duck stands on one end of a see-saw, 5m away from the pivot.

If the weight of the duck is 10N, find the moment.

Solution: The ducks weight is causing an anticlockwise moment.

Anticlockwise moment = F x d

= 10 x 5

= 50 Nm

F = 10N

d = 5m

Examples which is the distance? Where is the force & pivot?

F

F d

d

d F

F

d

F

weight weight

Principle of Moments

For an object to be in equilibrium(stable/not moving), the

total clockwise moment must be equal to the anticlockwise

moment about the same pivot point.

pivot It is the fixed(non moving) point

SOLVING PROBLEMS RELATED TO PRINCIPLE OF MOMENTS

Step 1:

Identify what are the forces that will give rise to clockwise / anticlockwise moment

Step 2:

Find the clockwise / anticlockwise moment

Step 3:

Equate the clockwise and anticlockwise moments

10N 30N

6m d

Find the value of d.

WORKED EXAMPLE

Step 1:

Identify what are the forces that will give rise to clockwise / anticlockwise moment

10N 30N

6m d

Find the value of d.

WORKED EXAMPLE

Anticlockwise moment

Clockwise moment

Step 2:

Find the clockwise / anticlockwise moment

10N 30N

6m d

Clockwise moment = Force x distance between force and pivot

= 30 x d

= 30d Nm

WORKED EXAMPLE

Anticlockwise moment

Clockwise moment

Anticlockwise moment = Force x distance between force and pivot

= 10 x 6

= 60 Nm

Find the value of d.

Solution:

Step 3:

Equate the clockwise and anticlockwise moments

10N 30N

6m d

Find the value of d.

Solution:

Clockwise moment = Force x distance between force and pivot

= 30 x d

= 30d Nm

Anticlockwise moment = Force x distance between force and pivot

= 10 x 6

= 60 Nm

Using the principle of moments,

Clockwise moment = Anti-clockwise moment

30d = 60

d = 60 30

d = 2 m

WORKED EXAMPLE

Anticlockwise moment

Clockwise moment

Points to note:

1. The unit for force must be in Newtons, the unit for distance must be in metres.

2. The distance must be measured from the force to the pivot.

In Conclusion . . .

Let us look at examples A & B on page 54

Discussion questions: Page 55 #1-3

Home work: Study page 55 (conditions for

equilibrium) then answer question 4.