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KS3 Physics
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© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 37
KS3 Physics
9L Pressure and Moments
© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20052 of 37
9L Pressure and Moments
Contents
Pressure in liquids
Moments
Pressure
Summary activities
© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20053 of 37
Pressure is exerted whenever a force is applied over an area.
If the same force is applied in each picture, which arm exerts the highest pressure on the board?
1. 2.
What is pressure?
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The arm applies a force to the board via a fingertip.
The force acts over a small area and so produces a high pressure.
1.
High and low pressure
The same force is now acting over a larger area – the palm has a greater surface area than the fingertip.
A lower pressure is produced.
2.
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Pressure is measured in:Newtons per square metre (N/m2), which are also calledpascals (Pa).
Pressure can also be measured in:Newtons per square millimetre (N/mm2);Newtons per square centimetre (N/cm2).
pressure =area
force
p x a
f
Pressure is the force per unit area and is calculated using this formula:
Calculating pressure
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The same force spread over a larger area means a lower pressure.
Which type of pressure?
Which type of shoes would be best for walking over a muddy field – flat soles or heels?
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The boots have flat soles and spread the person’s weight over a large surface area.
These boots exert a low pressure on the ground.
Which type of pressure?
In contrast, the heeled shoes have a smaller surface area and so exert a higher pressure.
These shoes are likely to sink into soft ground.
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A force spread over a large area means low pressure, e.g. skis and snowboards.
The large surface area of the board means the skier exerts very little pressure on the snow.
This means he slides over the top of the snow and does not sink into it.
Using low pressure
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A force concentrated on a small area means high pressure, e.g. high heeled shoes, needles, ice skates, sharp knives.
The narrow blade of a knife means that it exerts a high pressure and makes it easier to cut fruit and vegetables.
The high pressure of the blade of an ice-skate melts the ice and helps the skater slide across the surface.
Using high pressure
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9L Pressure and Moments
Contents
Pressure in liquids
Moments
Pressure
Summary activities
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Pressure in a liquid:
acts in all directions;
increases with depth.
Pressure in a liquid
A liquid can be used to transmit pressure from one place to another.
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high pressure
low pressure
The relationship between pressure and depth is shown by a water bottle with holes along its length.
Pressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3)
The pull of gravity
The greater the depth, the higher
the pressure
The denser the liquid, the heavier it is.
Pressure in a liquid
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Hydraulic systems use the principle that pressure is transmitted throughout a liquid.
Force applied
here
Pressure inside all parts of the hydraulic system is the same
Force transferred
here
Hydraulics
They are used to transfer movement from one part of a machine to another without linking the parts mechanically.
All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid.
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All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston.
The master piston is used to apply a force. This puts the liquid under pressure. The pressure is transmitted to the pistons on all four wheels of the car.
Hydraulic brake
foot pedal
master piston
slave pistons
hydraulic fluid
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The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation:
pressure = force applied
area of master piston
Hydraulic brake – pressure equations
The slave piston has a larger area than the master piston. So, the force exerted by the slave pistons on the brakes is greater than the force exerted by the driver on the brake pedal.
The pressure is transmitted to the slave pistons and so the force exerted by the slave piston can be calculated using:
pressure = force exerted
area of slave piston
force exerted = pressure x area of slave piston
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The master piston of a car has an area of 5cm2.
Hydraulic brake – calculations
Calculations:
1. At the master piston, p = f = 10 N = 2 N/cm2
a 5cm2
1. If a force of 10N is applied to the master piston, calculate the pressure created in the brake pipes.
2. At the slave piston, f = p x a = 2 N/cm2 x 50cm2 = 100 N
So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston.
2. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc.
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Hydraulics activity
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9L Pressure and Moments
Contents
Pressure in liquids
Moments
Pressure
Summary activities
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5N
A force acting on an object can cause it to turn about a pivot.
What happens to the see-saw when a force is applied on the left-hand side?
Does the seesaw turn? If so, clockwise or anti-clockwise?
pivot
Force and rotation
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pivot
The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn.
The turning effect of a force is called a moment.
Force and rotation – a moment
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A spanner is a lever that can be used to unscrew a nut.
force
pivot
distance from force
to pivot
Using moments
If the moment is big enough it will unscrew the nut.
If not, there are two ways of increasing the moment.
The spanner exerts a moment or turning force on the nut.
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1. Increase the distance from the force to the pivot – apply the force at the end or use a longer spanner.
Using moments – increasing the moment
force
If the same force is applied over a greater distance, a larger moment is produced.
pivot
distance from force
to pivot
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2. Increase the force applied – push/pull harder or get someone stronger to do it!
Using moments – increasing the moment
force
If a greater force is applied over the same distance, a larger moment is produced.
pivot
distance from force
to pivot
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moment = force (N) x distance from pivot (cm or m)
The moment of a force is given by the equation:
Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).
moment
f x d
Moment equation
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Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot.
What moment does she exert?
moment = 500 x 0.5
= 250 Nm
0.5 m
500 N pivot
Moment calculation
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Principle of moments
The girl on the right exerts a clockwise moment, which equals...
The girl on the left exerts an anti-clockwise moment,which equals...
her weight x her distance from pivot
her weight x her distance from pivot
pivot
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Principle of moments
When something is balanced about a pivot:
total clockwise moment = total anticlockwise moment
If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments.
pivot
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The principle of moments can be investigated using 10g masses with this balance.
moment (left) = 10 x 7 = 70 gcm
moment (right) = (10 x 3) + (10 x 4)
= 70 gcm
Both moments are equal and so the see-saw is balanced.
Principle of moments
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Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the seesaw is to balance?
When the see-saw is balanced:
Principle of moments – calculation
total clockwise moment = total anticlockwise moment
200 N x 1.5 m = 150 N x distance
200 x 1.5 = distance150
distance of second girl = 2 m
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Tower cranes are essential at any major construction site.
load armtrolley
loading platform
tower
Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?
counterweight
Why don’t cranes fall over?
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Using the principle of moments, when is the crane balanced?
moment of = moment of load counterweight
If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?
6 m
3 m
10,000 N?
Why don’t cranes fall over?
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moment of counterweight
distance of counterweight from tower
=
= 10,000 x 3= 30,000 Nm
counterweight x
moment of load
=
= ? x 6
load x distance of load from tower
moment of load = moment of counterweight ? x 6 = 30,000
? = 3,000 6
? = 5,000 N
Why don’t cranes fall over?
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At what distance can the loading platform carry each load safely?
Crane operator activity
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9L Pressure and Moments
Contents
Pressure in liquids
Moments
Pressure
Summary activities
© Boardworks Ltd 20041 of 20 © Boardworks Ltd 200535 of 37
Glossary
counterbalance – A weight used to balance another weight.effort – The force applied to use a lever.hydraulics – The use of liquid to transmit pressure from
one place to another.lever – A simple machine that moves about a pivot and
makes work easier by increasing the size of a force.load – The force moved when using a lever.moment – The turning effect of a force. It equals the force
multiplied by the distance from the pivot.pascal – A unit of pressure (Pa). 1 Pa = 1 newton per square
metre (N/m2).pivot – The point around which a lever turns.pressure – The force pushing on a certain area. It equals
the force divided by area and can be measured in pascals (Pa).
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Anagrams
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Multiple-choice quiz