Moments – Turning Forces

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.

Moments – Turning Forces

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