1 of 44© Boardworks Ltd 2008. 2 of 44© Boardworks Ltd 2008

1 of 44 © Boardworks Ltd 2008



What is pressure?

The elephant has a larger weight than the lady, but the contact area between its feet and the floor is far larger.

Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot?


A force spread over a larger area produces a lower pressure.

High pressure and low pressure

Which type of footwear would be better for standing on a muddy field – the flat-soled shoes or high-heeled boots?


The shoes have flat soles, so they spread the person’s weight over a large surface area.

These boots exert a low pressure on the ground.

High pressure and low pressure

In contrast, the soles of the high-heeled boots have a smaller surface area and so exert a higher pressure.

These shoes are likely to sink into soft ground.


Pressure is measured in newtons per square metre (N/m2), which are also called pascals (Pa).

pressure =area

force

Pressure is the force per unit area and is calculated using this formula:

Calculating pressure

f

p a

This can also be represented in a formula triangle.

Pressure can also be measured in newtons per square centimetre (N/cm2).


How much pressure is the woman exerting on the floor?

Pressure =

Pressure calculation

Weight of woman = 500 N

Area of contact with floor = 20 cm2

=

= 25 N/cm2

Area

Force

20

500


A force spread over a large area produces low pressure. This is used in skis and snowboards.

The large surface area of the board means the snowboarder exerts very little pressure on the snow.

Using low pressure

This means he slides over the top of the snow and does not sink into it.


A force concentrated on a small area produces high pressure. This is used in things like pins and sharp knives.

The narrow blade of a knife means that it exerts a high pressure, making it easier to cut fruit and vegetables.

The force applied to the tiny surface area of the end of a pin creates a high enough pressure to puncture even wood.

Using high pressure


Investigating pressure


Pressure – highest to lowest



high pressure

low pressure

Pressure in a liquid acts in all directions and increases with depth.

Pressure in a liquid

You can investigate this by using a needle to make holes at different heights in a plastic bottle, before filling it with water. The water escaping from the lower holes will travel further because it is at a higher pressure.


When entering deep water, a diver will experience pressure from all sides.

At greater depths, this pressure becomes too much for the body to handle.

So how do we overcome this pressure?

Overcoming pressure underwater

Submarines use extremely strong materials to withstand the huge forces. This means some can dive to the very deepest points of the ocean.


Hydraulic systems use the principle that pressure is transmitted throughout a liquid.

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 which carries special oil called hydraulic fluid.

Pressure inside all parts of the hydraulic system is the same.

force applied

here

force transferred

here


All hydraulic brake systems (like those used in cars) use a small master piston and bigger slave pistons.

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 brakes


Hydraulic car brake – labelling the parts


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 is greater than the force exerted by the driver on the brake pedal.

The pressure is transferred to the slave pistons, so the force exerted by the slave piston can be calculated using:

pressure = force exerted

area of slave piston

force exerted = pressure × area of slave piston


What causes pressure in gases?


Pressure: summary



What happens to the see-saw when a force is applied on the left-hand side?

pivot

Force and rotation

A force acting on an object can cause it to turn about a pivot.

Does the see-saw turn? If so, clockwise or anticlockwise?

100 N


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

100 N


A spanner is a lever that can be used to unscrew a nut.

force

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.

pivot

distance from force to

pivot

force


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


pivot


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


pivot



moment = force (N) × distance from pivot (cm or m)

The moment of a force is given by the formula:

Moments are measured in newton centimetres (Ncm) or newton metres (Nm).

Moment equation

moment

f d

This can also be represented in a formula triangle:


1 m

The counterweight on the trebuchet weighs 300 N and is attached to the short arm. It is 1 m from the pivot. It exerts a clockwise moment. What is the size of this moment?

moment = 300 × 1 = 300 Nm

Moment calculation

300 N

3 m

100 Npivot


Principle of moments

The lead shot on the trebuchet weighs 100 N and is attached to the long arm. It is 3 m from the pivot. It exerts a clockwise moment. What is the size of this moment?

moment = 100 x 3 = 300 Nm

3 m

300 N100 Npivot

1 m



When something is balanced about a pivot:total clockwise moment = total anticlockwise moment

If the anticlockwise moment and clockwise moment are equal, then the trebuchet is balanced. This is known as the principle of moments.

3 m

300 N100 Npivot

1 m


1m

100 N

3 m 1m

What happens if the counterweight is increased to 1000 N?


1000 N

The moments will no longer be balanced, so the trebuchet will be able to fire.


The principle of moments can be investigated using 10 g masses with this balance. 10 g exerts a force of 0.1 N.

Anticlockwisemoment

Clockwise moment

Both moments are equal, so the see-saw is balanced.


= 0.1 × 7= 0.7 Ncm

= (0.1 × 3) + (0.1 × 4) = 0.7 Ncm


Two girls are sitting on opposite sides of a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. How far from the pivot must her 250 N friend sit if the see-saw is to balance?

Principle of moments – calculation

total clockwise moment = total anticlockwise moment

200 N × 1.5 m = 250 N × distance

200 × 1.5 = distance250

= 1.2 m

200 N1.5 m ? m

250 N


Tower cranes are essential at any major construction site.

Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

How do tower cranes work?

load arm

loading platform

counterweight

tower

trolley


moment of = moment of load counterweight

If the crane below is balanced, how heavy is the load?

6 m

3 m

10,000 N?

Moment calculation – crane


6

= 30,000

Moment calculation – crane

10,000 N

?

6 m 3 m

Counterweightmoment

Moment of load

Moment of load

? × 6

= load × distance from tower

= ? × 6

= counterweight × distance

= 10,000 × 3

= 30,000 Nm

= moment of counterweight

= 30,000

=?

?

5,000 N

5,000 N


Crane operator activity



Glossary


Anagrams


Pressure – true or false?


Match the definition


Multiple-choice quiz

Documents

1 of 44© Boardworks Ltd 2008. 2 of 44© Boardworks Ltd 2008