KS3 Physics

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KS3 Physics
9L Pressure and
Moments

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9L Pressure and Moments
Contents
Pressure in liquids
Moments
Pressure
Summary activities

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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 called
pascals (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 = 10N = 2N/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 = 2N/cm2
x 50cm2
= 100N
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 500N and stands on one end of a seesaw.
She is 0.5m from the pivot.
What moment does she exert?
moment = 500 x 0.5
= 250 Nm
0.5m
500N
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
= 70gcm
moment (right) = (10 x3)+(10x4)
= 70gcm
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 200N and is 1.5m from the pivot. Where
must her 150N friend sit if the seesaw is to balance?
When the see-saw is balanced:
Principle of moments – calculation
total clockwise moment = total anticlockwise moment
200N x 1.5m = 150N x distance
200 x 1.5 = distance
150
distance of second girl = 2m

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Tower cranes are essential at any major construction site.
load arm
trolley
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 10,000N counterweight is three metres from the
tower, what weight can be lifted when the loading
platform is six metres from the tower?
6m
3m
10,000N?
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

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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). 1Pa = 1newton 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