Forces and Turning Effects
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Transcript of Forces and Turning Effects
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TURNING
EFFECT OF
FORCES
MOMENTS
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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.
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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.
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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.
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How many types do we have?
There are 2 types of moments:
Clockwise moment
Anticlockwise moment
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How many Types do we have? There are 2 types of moments:
Clockwise moment
Anticlockwise moment
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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
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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
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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
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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
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CALCULATING MOMENTS
Moment = force perpendicular distance between force
and pivot
In symbols: Moments = F d
Weight => F
d
Unit for moments: Newton-metre (Nm)
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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
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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
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Examples which is the distance? Where is the force & pivot?
F
F d
d
d F
F
d
F
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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
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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
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10N 30N
6m d
Find the value of d.
WORKED EXAMPLE
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Step 1:
Identify what are the forces that will give rise to clockwise / anticlockwise moment
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10N 30N
6m d
Find the value of d.
WORKED EXAMPLE
Anticlockwise moment
Clockwise moment
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Step 2:
Find the clockwise / anticlockwise moment
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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:
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Step 3:
Equate the clockwise and anticlockwise moments
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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
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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.
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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.