9l pressure and moments

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KS3 Physics

9L Pressure and Moments



Contents

Pressure in liquids

Moments

Pressure

Summary activities


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?


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.


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


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?


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.


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


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



Contents

Pressure in liquids

Moments

Pressure

Summary activities


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.


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


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.


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


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


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.


Hydraulics activity



Contents

Pressure in liquids

Moments

Pressure

Summary activities


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


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


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.


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


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


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


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


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



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


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.



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


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?


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?



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



At what distance can the loading platform carry each load safely?

Crane operator activity



Contents

Pressure in liquids

Moments

Pressure

Summary activities


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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Multiple-choice quiz

9l pressure and moments

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