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8/17/2019 Moments – Turning Forces
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Today’s lesson
• Know what the turning effect of a force is
• Know that the moment of a force depends on
force and the distance from the pivot
• Know the principle of moments
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Do you push near the hinges?
I can’t do
it!
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Do you push far from the hinges?
That’s
easier!
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The turning effect of a force depends
on two things;
The size of the force
Obviously!
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The turning effect of a force depends
on two things;
The distance from the pivot (axis of rotation)
Not quite so
obvious!
Axis of rotation
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Turning effect of a force
The turning effect of a force is called the
moment of the force
The moment is calculated by multiplying the
force by the distance from the pivot
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Turning effect of a force – moment of
a force
Moment (Nm) = Force (N) x distance from pivot (m)
Note the unit is Nm, not N/m!
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• How is the turning force or moment affected by the
distance from the pivot?
• As the perpendicular distance increases the turningforce increases
• How is the moment affected by the force (effort)exerted?
• As the force increases the moment increases
What do you think?
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A simple example!
nut
spanner
(wrench)
50 N
0.15 m
Moment = Force x distance from pivot
Moment = 50 N x 0.15 m
Moment = 7.5 Nm
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What do you do if the nut won’t move
and you can’t push harder?!
nut
spanner
(wrench)
50 N
0.15 m
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Get a longer spanner!
nut
spanner
(wrench)
50 N
0.25 m
Moment = Force x distance from pivot
Moment = 50 N x 0.25 m
Moment = 12.5 Nm
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More than one force
Take an uneven see-saw for an example
Do you think we’ll
be safe in this power point?
It’s not
looking
good!
pivot
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If the see-saw is balanced, what must be the
weight of the dog on the left?
pivot
1.2 m 2.2 m
110 N? N
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The force on the right is trying to turn the see-
saw clockwise about the pivot
pivot
1.2 m 2.2 m
110 N? N
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If the see-saw balances, the turning effect
anticlockwise must equal the turning effect
clockwise
pivot
1.2 m 2.2 m
110 N? N
Anticlockwise moment clockwise moment=
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Anticlockwise moment = clockwise moment
? X 1.2 = 110 x 2.2
? X 1.2 = 242? = 242/1.2
? = 201.7 N
pivot
1.2 m 2.2 m
110 N? N
Anticlockwise moment clockwise moment=
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Is Moment a vector quantity?
• Does the direction matter?
Moments can either be
clockwise or anticlockwise .
Calculate the moments below. In which direction are theyacting? Clockwise or Anticlockwise?
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Experiment -Testing the principle of moments
• When forces act in a different direction, yet still balance, the total
turning effect in each direction will be the same:
• sum of clockwise moments = sum of anti clockwise moments
• What did you notice aboutthe anticlockwise momentand the clockwise moment?
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Examiner’s Tip:
• An easy way to remember the order of the componentsin a particular class of levers is the mnemonic;
– ‘1, 2, 3, - FLE’
• Where 1, 2, 3 refers to the class of the lever and F, R, Erefers to the middle component.
1. F – as the middle component2. L
3. E
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Three
Classes of
Levers
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“First Class Lever”
• A first-class lever is a lever in which
the fulcrum is located between the
input effort and the output load.
• In operation, a force is applied (by
pulling or pushing) to a section ofthe bar, which causes the lever to
swing about the fulcrum,
overcoming the resistance force on
the opposite side.
• The fulcrum may be at the center
point of the lever as in a seesaw or at
any point between the input and
output.
• This supports the effort arm and the
load.
Examples:
•Seesaw
•Scissors (double lever)
Classes of Levers
http://upload.wikimedia.org/wikipedia/commons/0/03/LeverFirstClass.svg
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Fulcrum is between EF (effort) and RF (load)
Effort moves farther than Resistance.Multiplies EF and changes its direction
The mechanical advantage of a lever is the ratio of the length of the lever
on the applied force side of the fulcrum to the length of the lever on the
resistance force side of the fulcrum.
First Class Lever fulcrumEffort
Resistance
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Common examplesof first-classlevers include
– crowbars
–scissors
– pliers
– tin snips
– and seesaws.
Examples of first class levers
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RF (load) is between fulcrum and EF
Effort moves farther than Resistance.
Multiplies EF, but does not change its direction
The mechanical advantage of a lever is the ratio of the distance
from the applied force to the fulcrum to the distance from the
resistance force to the fulcrum.
Second Class Lever
EffortResistance
Explanation
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Three Lever Classes
p o
• Second class lever – Resistance is located between the effort force and the fulcrum. – Always multiplies a force
– Example: Wheelbarrow
R
F
E
Always multiplies a force.
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“Second Class
Lever”
In a second class lever the input effort
is located at the end of the bar and the
fulcrum is located at the other end of
the bar, opposite to the input, with the
output load at a point between thesetwo forces.
Examples:
•Paddle
•Wheelbarrow
•Wrench
Examples of Second class levers
http://upload.wikimedia.org/wikipedia/commons/2/22/LeverSecondClass.svg
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• Examples of
second-class
levers include:
• nut crackers
• wheel barrows
•
doors
• and bottle
openers.
Examples of second-class levers
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EF is between fulcrum and RF (load)
Does not multiply force
Resistance moves farther than Effort.
Multiplies the distance the effort force travels
The mechanical advantage of a lever is the ratio of the distance
from the applied force to the fulcrum to the distance of the
resistance force to the fulcrum
Third Class Lever
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• For this class of levers, the input
effort is higher than the output load,
which is different from second-class
levers and some first-class levers.
• However, the distance moved by the
resistance (load) is greater than the
distance moved by the effort.
• In third class levers, effort is applied
between the output load on one end
and the fulcrum on the opposite end.
“Third Class Lever” Examples:
•Hockey Stick
•Tweezers
•Fishing Rod
Classes of Levers
Explanation
http://upload.wikimedia.org/wikipedia/commons/5/54/ThirdClassLever.svg
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Three Lever Classes
• Third class lever – Effort force located between the resistance and the fulcrum. – Effort arm is always shorter than resistance arm – MA is always less than one
– Example: Broom
R
F
E
There is an increase distance
moved and speed at the other end.
Other examples are baseball bat orhockey stick.
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Examples of Third Class Levers
•
Examples ofthird-class
levers include:
– tweezers
–arm hammers
– and shovels.
Third class lever in human body.