MODULE mod7A mod7B mod7C mod7D mod7E mod7F mod7G mod7H mod7I mod7J mod7K mod7L mod8A mod8B mod8C mod8D mod8E mod8F mod8G mod8H mod8I mod8J mod8K mod8L mod9A mod9B mod9C mod9D mod9E mod9F mod9G mod9H mod9I mod9J mod9K mod9L 9la1
9La1 Pressure on your body
You are going to find out the different pressures your weight can put on your body.
Apparatus
- Scales - Tape measure
- Calculator - Squared paper
Method
1 Find your weight in newtons. If you only have scales which show kilograms multiply your answer by 10 to give newtons (e.g. a 65 kg mass gives a weight of 650 N).
2 Measure the area of as many parts of your body as you think you could balance on by drawing around them on squared paper
and counting the squares. Parts of your body that you could measure include head, feet, tiptoes and hands.
Recording your results
1 Copy this table.
Calculate the pressure for each part of the body using the formula:
pressure = force ÷ area
2 Record your results in a bar chart like this:
Considering your results/conclusion
3 Copy and complete these sentences.
a The part of my body that would have the greatest pressure on it is _____________ .
b If I balance on one foot, instead of two, the pressure is ____________ (doubled/halved).
[ observing, considering ]
9La2 Pressure links
1 Cut out the boxes below.
2 Put together words which can link with each other.
3 Arrange them on a piece of paper and discuss your ideas with a friend.
4 When you are satisfied that you have found links stick them on the paper.
5 Say what the link is, like this:
force
newtons
[ knowledge, literacy ]
---------------------------------------------------------------------------------------------------------
small area of pin
low pressure for walking
pressure
weight
force/area
large pressure on a notice board
there is high pressure if the area is small
sharp edges cut easily
newtons per metre squared
1 newton per metre squared
large area of snow shoes
1 pascal
9La3 Pressure points
To work out the pressure you use the formula:
Use the formula to calculate the answers to the following questions.
1
Force = __________________________________
Area = ___________________________________
Pressure = force ÷ area
Pressure = ________________________________
2
Total force (from man and plank) = _____________
Area of plank = force ÷ pressure
Area of plank = ____________________________
3
Ground contact area = ______________________
Force (or weight) = area × pressure
Weight of bulldozer = _______________________
4
Force (or weight) = area x pressure
Weight of the groceries on the table = ___________
5
Force on head = ____________________________
Area of head = ____________________________
Pressure = force ÷ area ______________________
Force on point = __________________________
Area of point = ___________________________
Pressure on the point = _____________________
Which has the greater pressure? _______________
________________________________________
_________________________________________________________________________________
[ knowledge, numeracy ]
9La4 Pressure calculations
The formula for pressure is: pressure = force (in newtons)
area (in cm2 or m2)
1 A box of chocolate bars weighs 180 N.
Its base has an area of 0.09 m2.
What pressure does it exert on the ground?
2 This suitcase has sides measuring 75 cm long by 25 cm wide and 50 cm high.
The airline says that the maximum mass it can be is 20 kg.
a The suitcase is at the maximum mass. What will its weight be? (1 kg has a weight of 10 N.)
b What will its pressure be on the ground if it is standing on its side?
c What will its pressure be on the ground if it is on its end?
d What will its pressure be on the ground if it is lying flat?
e Which side puts the most pressure on the ground?
f How could you have answered part e without calculating any pressures?
3 A man is walking on a flat roof.
He weighs 880 N and each of his shoes has an area of 220 cm2.
a What is his pressure on the roof standing on both feet?
b If he uses a cat ladder with a weight of 120 N and an area of 5000 cm2 what will the pressure be now?
4 Ice skates are very narrow and the blades put a greater pressure on the ice than if you just wore shoes. This increased pressure melts the ice and reduces friction so you can move more quickly.
A skater wants to buy some super-fast ice skates that she has seen advertised.
She weighs 600 newtons. Her present skates are 30 cm long and the blade is 0.5 cm wide.
a What is the pressure of one blade on the ice if all her weight is on it?
b The new blades are 0.3 cm thick. What pressure will each blade have on the ice?
c Would you recommend her to buy the new blades?
d Explain your answer to c.
5 You can buy many types of snowshoe, with different sizes of sole.
a Copy and complete this table.
Snowshoe
Weight of person
Area of sole
Pressure on snow
(N)
(m2)
(N/m2)
A
400
0.1
B
0.2
C
0.25
D
0.12
E
0.3
b Which snowshoe would you recommend and why?
9La5 Tiresome tyres
Tyre manufacturers spend a great deal of time and money trying to design tyres which will give maximum grip in all road conditions. In wet conditions water acts as a lubricant when it is between the road and the tyres. The patterns on tyres are designed so that they squeeze water out. The pressure of the air in tyres is also very important. Too little pressure or too much can be dangerous. Either of them reduces the amount of control the driver has.
1 How are tyres designed to give a good grip on a wet road? Explain why this is important.
2 Why do motorcycle and bicycle tyres have a tread on part of the sides?
3 A car weighs 10 000 N.
Each tyre has a pressure on the road of 50 N/cm2.
a What is the weight on each tyre? (Assume that it is spread evenly.)
b What is the area of each tyre in contact with the road?
c If the pressure is increased to 60 N/cm2 what will the area in contact with the road be?
d If the pressure is reduced will the area in contact with the road be reduced or increased?
e Why do you think that people sometimes reduce the tyre pressure in icy conditions? How is it done?
The first bicycles invented had solid tyres but they were heavy and gave a very uncomfortable ride. Riding a bike became much more comfortable when air-filled tyres were made. The disadvantage of these tyres is the inconvenience of punctures.
A new type of solid tyre has been developed, called a Greentyre. This is made from polyurethane and the process used to make the tyres is clean and releases no harmful toxins to damage the atmosphere. The process is also non-carcinogenic, which means that it does not cause cancer.
When rubber tyres are made, energy is needed to 'vulcanise' them. This is the process of heating and pressurising them with sulphur to improve their elasticity and strength. Processing Greentyres does not require fuels to burn. The product lasts longer than rubber and when the tyre eventually wears down it can be recycled.
4 What is the main disadvantage of using an ordinary bicycle tyre?
5 Explain what the vulcanising process is and what the disadvantage of using this process is.
6 What other advantages are there in using a Greentyre?
7 Design an advertising poster to encourage the use of Greentyres.
[ literacy, numeracy ]
9Lb1 Making a diving bell
Name _____________________________ Class ____________
A diving bell can be used to take people down into the ocean safely. It is usually hung from a ship, but a model diving bell can be made to move up and down using air pressure.
You are going to investigate some effects of pressure in air and liquids using a model diving bell.
- Measuring cylinder or large jar
- Balloon
- Test tube
- Plasticine
- Elastic band
1 Almost fill the measuring cylinder with water.
2 Cut the end off the balloon.
3 Make a model diving bell by rolling the Plasticine into a sausage shape and pressing it around the neck of the test tube.
4 Carefully drop the model into the measuring cylinder with the open end downwards.
5 If the model sinks completely take it out and remove some of the Plasticine. Change the amount of Plasticine until the model just floats near the top of the water.
6 Fix the cut balloon over the top of the measuring cylinder with an elastic band.
7 Push down on the balloon, then let go.
8 Record what happens.
When I pushed down on the balloon the model ___________________ to the bottom.
When I released the balloon the model ___________________ again.
Considering your results/conclusions
Complete these sentences using words from the box.
The pressure of my fingers on the ___________________ pushes on the water. This
___________________ on the air in the model diving bell and squeezes it. More water goes into
the model and it ___________________ . The harder I press on the balloon the more the
___________________ is squeezed. When I release the pressure the model goes
___________________ again.
air balloon pushes sinks up
9Lb2 Pressure effects
Polar bears
Polar bears are adapted for living in snowy places. They are well insulated, camouflaged and have large feet.
Crisp packets
Bags of crisps are sealed to keep the crisps fresh. Strange things happen to bags of crisps if you take them underwater or up a high mountain!
Magdeburg spheres
In 1654 in Magdeburg, Germany, Otto van Guericke demonstrated the enormous effect of air pressure. He had two large metal bowls made, which he placed together and pumped the air out. Two teams of horses were attached to the bowls and they were unable to pull them apart. Once the air was allowed back in the bowls came apart easily.
The words and phrases at the bottom of the page may help you to write your answers to these questions.
1 a Which adaptation of polar bears helps them to walk on snow?
b Explain how this adaptation helps, using ideas about forces and pressure.
2 Explain why Otto van Guerike's trick worked, using ideas about pressure, forces and particles.
3 Explain why the bag of crisps looks different in different places. Use ideas about pressure and particles in your answer.
Pressure increases with depth.
If a force is spread out over a large area, the pressure is lower.
Pressure in gases is caused when moving particles hit something.
Air pressure depends on the weight of air above.
Pressure gets less as you go higher.
If the air pressure is low, there are fewer particles to hit an object.
If a bag is sealed, the number of air particles inside it will not change.
Pressure in liquids depends on the weight of liquid above.
The force on an object from high-pressure air is greater than the force from low-pressure air.
If you take the air out of something, there are fewer particles and the pressure will be lower.
9Lb3 A question of pressures
Pressure is the amount of force pushing on a certain area. Air has weight, so it can put a pressure on surfaces. Air can be compressed (squashed) easily, but liquids cannot. Pressure in both liquids and gases comes from all directions. The pressure increases with depth.
1 These diagrams show water coming out of a can with holes in its sides. Only one of these diagrams is correct. Tick the correct diagram.
2 Complete this sentence.
Water tanks are placed in the roof space in a house because the height of the water _______________ (increases/decreases) the water pressure to the taps.
3 This diagram shows a model of a car braking system. The cylinders and tubing are filled with water. The pressure is the same in all parts.
a What is the pressure on piston A? Use the formula:
The pressure on piston A is _______________ N/cm2.
b This pressure is transferred to the second cylinder. What is the force on piston B? Use the formula:
force = pressure × area
The force on piston B is _______________ N.
4 Add arrows to the diagram to show water pressure on the dam, the boat and the fish. Remember that pressure increases with depth.
Use long arrows to show large pressures and short arrows to show small pressures. A few arrows have been drawn for you.
5 This is a diagram of a hydraulic brake system. The diagram shows only one wheel but there are actually four tubes from the brake pedal, one leading to each wheel.
Use the words from the box to complete the sentences below.
Hydraulic _______________ make use of pressure in liquids. When the car brake pedal is
pressed a _______________ puts pressure on the brake _______________ in cylinder A.
This increases the pressure in the trapped _______________ fluid.
The fluid in cylinder B has the same _______________ , and pushes on the piston in
_______________ B. The force on the piston in this cylinder pushes the brake pads against
the _______________ attached to the wheel. The _______________ on the wheel slows
it down.
brake brakes cylinder disc fluid friction piston pressure
[ knowledge, literacy, numeracy ]
9Lb4 Atmospheric pressure
Air exerts a pressure equally in all directions. The pressure of the atmosphere is due to the weight of the air above us. At sea level atmospheric pressure is 100 000 N/m2 or 100 000 pascals. This is a large number, so we can also say 100 kN/m2 or 100 kPa or 1 atmosphere of pressure. This air pressure is about the same as a double-decker bus standing on every square metre. We are not crushed by the enormous pressure of air because the pressure inside our bodies exactly matches the air pressure outside our bodies.
As we go higher in the atmosphere there is less air pressure. This is because there is less weight of air above us, and there are fewer air molecules to put pressure on surfaces.
Variation of atmospheric pressure with height above sea level.
1 What causes atmospheric pressure?
2 Give three different units that can be used to measure air pressure.
3 What is the air pressure at sea level?
4 Why are we not crushed by air pressure?
5 This table shows how air pressure changes with height above the Earth.
Height (m)
Air pressure (kilopascals)
0
100
2000
75
5000
50
10 000
25
15 000
10
a Plot a graph to show this information.
b Use your graph to find the air pressure at 3000 m.
6 a Atmospheric pressure is 100 000 N/m2. What is the force exerted by the air on the top of a wall if the area of the top is 50 m2?
b What is the answer in kilonewtons?
7 As you suck on a straw, liquid moves up the straw.
a When you suck you make your mouth bigger. What happens to the air pressure inside your mouth?
b Explain why the liquid moves up the straw.
9Lb5 Manometers and barometers
A manometer is used to measure gas pressures.
The U tube contains a liquid. One end is connected to the gas supply. The height difference between the liquid in the two arms is a measure of the gas pressure.
If a manometer is used to measure large pressure differences the liquid is usually mercury. Mercury is 13.6 times more dense than water, so the pressure cannot support as much of it. The height difference is 13.6 times smaller using mercury instead of water, and so a smaller manometer can be used.
A mercury barometer can also be used to measure pressure. A tube is completely filled with mercury so that all the air is removed. It is then turned upside down and placed in a tank of mercury.
Atmospheric pressure pushes down onto the mercury in the tank, and stops the mercury falling out of the tube. There is a column of mercury left in the tube with a vacuum above it. If there is any change in the atmospheric pressure the height of the mercury column in the tube will change. At a pressure of one atmosphere or 100 kPa there is 76cm of mercury in the tube.
1 What is a manometer?
2 If you are measuring the gas pressure at the gas taps in the lab, which liquid would you use in the manometer? (Hint: There is likely to be only a small difference between air and gas pressure.) Explain your answer.
3 Why would you use a mercury barometer rather than a water one to measure atmospheric pressure?
4 a What is the height of mercury in the tube at a pressure of one atmosphere?
b How much less dense is water compared with mercury?
c What would the height of water be at one atmosphere? (Hint: Multiply your last two answers together.)
The barometer on the previous page is too large to have on the wall at home or in the office. An aneroid barometer is much smaller than a mercury barometer, but is less accurate. Aneroid barometers are easier to read and much less liable to damage.
Aneroid barometer.
An altimeter is a special type of aneroid barometer used by pilots and mountaineers. It has a scale which gives the height above sea level. This is calculated from the difference between air pressure where the aeroplane or mountaineer is, and the normal sea level pressure (1 atmosphere).
5 Construct a table to compare three ways of measuring gas or atmospheric pressure. Include advantages and disadvantages in your table.
6 Which type of barometer would a helicopter pilot use?
7 Try to find out what aneroid means. Explain why it is called an aneroid barometer.
[ knowledge, literacy, numeracy, research ]
9Lc1 Investigating levers 1
How does the length of a lever affect the force you need to lift a load?
Is it better to have a long or a short lever if you want to lift a heavy load?
- Metre rule - Masses
- Triangular block of wood (pivot)
- Sand bag
1 Set up the apparatus as shown. Make sure the pivot is under the 30cm mark on the ruler.
2 Put the sand bag on the end of the ruler, at 0 cm.
3 Put some masses on the side opposite the sand bag, 15 cm away from the pivot (the 45 cm mark on the ruler). Keep adding masses until the ruler just balances.
4 Write down the number of masses used in the table.
5 Now move the masses along to the 60 cm mark. They are 30 cm away from the pivot. Add masses or take them away until the ruler just balances. Write the number of masses in the table.
6 Repeat step 5 with the masses at the 90 cm mark (60 cm away from the pivot).
Mark on ruler (cm)
Distance from pivot (cm)
Number of masses needed
45
15
60
30
90
Complete these sentences using words from the box. You may use the words once, more than once, or not at all.
The load was the __________________ each time, and so was the distance between the
__________________ and the pivot.
When the masses I used to __________________ the ruler were close to the pivot, this was like
using a __________________ lever. When the masses were a __________________ way from the
__________________ , this was like using a long lever.
I needed the __________________ masses when I used a long lever.
Long levers make it __________________ to lift heavy loads.
balance easier fewest harder load long most pivot same short
9Lc2 Investigating levers 2
- Metre rule - Slotted masses
- Triangular block of wood
How does the length of a lever affect the force you need to lift a load? Is it better to have a long or a short lever?
Planning
Factors to be investigated could include: the distance between the pivot and the load, the size of the load, the distance between the pivot and the effort. Change one factor and keep the others the same.
1 Describe how you would carry out an experiment to investigate one of these factors.
2 Explain how you will make your experiment a fair test.
3 To get more accurate results you must take at least five different measurements.
4 Your table of results could look like this:
5 Show your results on a line graph.
6 Write a conclusion to say what you have found out. Should the distance from the effort to the pivot be long or short to reduce the force needed to lift the load?
Evaluation
7 Did any of your results not fit the general pattern?
8 Suggest one way to improve or extend the investigation.
[ planning, observing, presenting, considering, evaluating ]
9Lc3 Levers reverseword
Here is a completed crossword.
1 E
F
2 F
U
2 L
R
4 M
O
V
T
H
5 L
6 P
I
N
1 Make up a clue for each word and write the clues in your book. You can use a science book or a dictionary to help you.
2 Cut off the blank grid below.
3 Show the clues to a friend and ask them to complete the puzzle.
6
9Lc4 Levers 1
Levers are simple machines. A machine makes work easier by changing the size or direction of the force that you put in.
Levers work by moving objects around a pivot or fulcrum. An effort is applied when you push down on one side and this makes the load move up.
1 Draw a line to match the word to its meaning.
lever the force you put in
pivot a simple machine
effort the force that you move
load the point a lever turns around
2 On each of the pictures below, label the load, the pivot and the effort.
[ knowledge ]
9Lc5 Levers 2
Levers work by moving objects around a pivot or fulcrum. An effort is applied when you push down on one side, and this makes the load move.
1 Copy these diagrams, and mark the pivot, load and effort on each one.
a
b
2 These drawings show two pairs of scissors. One is for doing embroidery, when you need to cut thin threads very close to a piece of material. The other pair is for use in the kitchen, and can cut through thin bones like chicken bones.
a Which drawing shows the kitchen scissors? Explain how you worked out your answer.
b You could use the kitchen scissors for embroidery. Explain why the embroidery scissors are better.
3 Janet is trying to lift a log.
Describe two things that Janet could do to make lifting the log easier.
4 If you push a door closed, it is much easier if you push it near the handle instead of pushing it near the hinges. Explain why this is so.
9Lc6 Wheelchairs
Someone who has suffered a spinal cord injury can get around as quickly in a wheelchair as an uninjured person can walking. Active people who are interested in sport can use a wheelchair to participate in marathons, basketball and other sports. Someone with arthritis can use a wheelchair to get around outside the home.
Just like shoes, a proper fit is essential if people are to feel comfortable using their wheelchairs. Designers should take into account a person's needs and interests. Wheelchairs come in many sizes and shapes to meet the needs of users with differing levels of physical function and varying interests.
The most popular type of wheelchair is the lightweight manual wheelchair. Lightweight chairs provide independence of movement with a minimum of effort.
This is achieved by having:
- lightweight metal construction
- push rims around the wheels
- wheels which are wider apart at the bottom than the top
- a lever to put on the brake
- parking brakes (wheel locks) which can be mounted at various heights to fit the user
- pneumatic tyres.
1 Why is it important to have a wheelchair which is made to fit the user?
2 a Why is a lever used to apply the brake?
b What might happen if the brake were applied strongly going downhill?
3 Explain what a pneumatic tyre is and what the advantages and disadvantages are. How are the wheelchair's wheels different to car wheels? (Hint: 'pneu' is Greek for breath.)
4 Why do you think the wheels are not parallel? (Imagine going round a corner.)
5 If the wheels had a larger diameter, what effect would this have on the effort needed to push the wheelchair?
6 The diameter of most wheels is 60 cm.
a If one strong push on the rim gives five revolutions, how far does the wheelchair go? (Remember the circumference of the wheel can be worked out using π × the diameter.)
b Where in your school would it be easy to go this distance?
c If the diameter of the wheels is 50 cm, how far would the wheelchair go with five revolutions of the wheels?
d Which diameter would be best for a race? Explain your answer. What other factors might be important?
e Look around your school. How easy is it for wheelchairs to move around the school?
9Ld1 Balance it 1
A lever can be used to apply a force which turns something around a pivot. This is called the turning effect or moment of the force. The moment is calculated by:
moment of the force = force × distance from the pivot
(newton metres, Nm) (newtons, N) (metres, m)
If there are several forces on an object, it is in equilibrium if the moments of all the forces are balanced.
- Metre ruler - 2p coins, or masses - Triangular block of wood
1 Balance the ruler at the centre.
2 Put a coin on one side.
3 Add two coins together to the other side so that the ruler balances.
4 Record the numbers of coins in the table.
5 Measure the distances and record them in the table.
6 Repeat with different distances and coins.
Left side
Right side
Number of coins
Number × distance
Complete the following sentences using words from the box. You can use some words more than once.
The moment caused by the weight of the coins is equal to the force (weight) × the
__________________ from the pivot.
My results show that when the __________________ are the same the ruler balances.
The rule is: the force times the __________________ must be the same on both sides to
__________________ the ruler.
balance distance moments
9Ld2 Balance it 2
moment of the force = force × perpendicular distance from
(newton metres, Nm) (newtons, N) the pivot (metres, m)
When the distances are small, we use centimetres instead of metres. The moment is then in newton centimetres (Ncm).
When moments are balanced no movement takes place. The object is in equilibrium.
- Metre ruler - Identical slotted masses - Triangular block of wood
1 Use the apparatus above to design an experiment to demonstrate the principle of moments. Describe what you would do.
2 To get more accurate results you must take a range of measurements (at least five).
3 Your table of results could look like this:
4 Can you plot a graph of your results?
5 What do you notice about the clockwise and the anticlockwise moments when the ruler balances?
Evaluating
6 a How easy was it to get accurate measurements?
b Is there anything you could have done differently?
9Ld3 Revision questions 1
Each of these pictures shows where a turning force would be used.
Label the pivot, and draw an arrow to show where you would apply a force. The first one has been done for you.
7
9Ld4 Momentary questions 1
How much is the moment in each of these cases?
You can use ideas about moments to work out if a seesaw will balance.
anti-clockwise moment clockwise moment
= 600 N × 2 m = 300 N × 2 m
= 1200 Nm = 600 Nm
The seesaw will not balance.
Use the example to help you to work out if these seesaws will balance.
9Ld5 Revision questions 2
1 a A stilt walker is hired to entertain children at a summer fair. Why might he have trouble walking on the grass?
b If his weight is 800 N, and the total area of the bottom of his stilts is 50 cm2, what pressure does he put on the ground?
2 Town A gets its water from a reservoir in the hills above the town. The reservoir is 300 m above the town. Town B is built on flat land, and the houses get their water from a water tower which is 50 m higher than the town.
a Will water come out of the taps faster in Town A or Town B?
b Explain your answer.
3 This diagram shows part of a hydraulic system used to lift heavy weights.
a Would the heavy weight be above piston A or piston B? Explain your answer.
b How could the sizes of the pistons be changed so that the system could lift even heavier weights?
c If the area of piston B is 10 cm2, how big must piston A be for the system to magnify the force 20 times?
d How far will piston A move compared to piston B?
4 Joe is holding one end of a see-saw that his little sister is sitting on.
a What is the moment of Joe's sister?
b The see-saw is in equilibrium. How much force is Joe using to hold up
the end of the see-saw?
5 Aeroplanes are usually pressurised. What does this mean, and why is it necessary?
9Ld6 Momentary questions 2
1 What is the moment for each of the following?
c
d
If the object is in equilibrium then
the anticlockwise moment = the clockwise moment.
The anticlockwise moment = the clockwise moment
500 × 0.5 = the distance from the pivot × 250 N
250 = the distance from the pivot × 250 N
250 ÷ 250 = distance from pivot
1 m = distance from pivot
2 Use the example to help you to work out the unknown quantities if the following seesaws are in equilibrium (balanced).
a 3 m × 400 N against 2 m × ? N
b 2 m × ? N against 1.5 m × 400 N
c ? m × 300 N against 3 m × 450 N
d 0.75 m × 300 N against ? × 450 N
3 Some people steer their bikes by holding the handlebars close to the centre. Explain why this is hard to do, using ideas about moments.
9Ld7 Tower cranes
Tower cranes are used for lifting large heavy loads. They are so tall that there is a danger that they could topple over. To help prevent this they carry ballast to help them balance. The ballast is also called a 'counterbalance' or 'counterweight'.
Tower cranes are a common feature at most major construction sites. They can be seen rising over a hundred metres into the air and can reach out just as far. The construction crew uses the tower crane to lift steel, concrete, large tools (like acetylene torches and generators) and a wide variety of other building materials.
All tower cranes have four main parts.
- A base which is bolted to a large concrete pad. This helps to stabilise the crane.
- The base is connected to the mast or tower, which gives the tower crane its height.
- The mast has a slewing unit at the top, which contains the gears and a motor to help the crane rotate.
- The jib is the working arm of the slewing unit.
The jib is the part of the crane that carries the load. A trolley or sliding pulley block runs along the jib to move the load in and out from the crane's centre. This changes the load that the crane can lift. The shorter horizontal machinery arm contains the crane's motors and electronics as well as the large concrete counterweights. These help the crane to balance.
When a load is lifted, there is a turning force on the tower. The counterweights provide a moment in the opposite direction to stop the crane falling over. The counterweights can be moved to keep the crane stable.
There is a maximum load that a crane can lift, but the crane cannot lift that much weight if the load is positioned at the end of the jib. The closer the load is positioned to the mast, the more weight the crane can lift safely. For example, if the crane is built to withstand a moment of 300 000 newton metres, this means that if the operator positions the load 30 metres from the mast, the crane can lift a maximum of 10 000 newtons.
The largest tower crane in the world is 120 metres high and lifts loads up to 1 million newtons. It has a rating of 10 million newton metres.
1 Where do you find tower cranes and what are they used for?
2 How is the crane kept stable?
3 How does the crane demonstrate the principle of moments?
4 a This tower crane has a sliding pulley block which is 20 m from the mast. The counterweight has a weight of 250 000 N and it is 5 m from the mast. What is the maximum weight the crane can lift?
b What is the mass of this load?
c The load moves 10 m along the jib towards the mast. Will the maximum weight that can be lifted increase or decrease? Explain your answer.
d If the counterweight moves to 6 m from the mast what is the maximum weight that can be lifted if the load is 20 m from the mast? Explain your answer.
e For health and safety reasons the load lifted should be less than the maximum. Why do you think this is?
5 Look up the word 'slewing' in a dictionary and explain what a slewing unit is.
[ literacy, numeracy, research ]
9L Summary Sheets
Pressure and moments
Pressure on solids
The thumb is putting a force onto the head of the pin. The force is transferred to the point of the pin. This is a very small area, so there is a very large pressure on the board, and the pin goes in.
The thumb is putting a force on the board. The area of the thumb is much larger than the area of the pin point, so there is only a small pressure on the board. The thumb does not go into the board.
Examples of a small area giving a large pressure:
Sharp knife.
Ice skates.
Examples of a large area giving a small pressure:
Snow shoes.
Camel on sand.
We can work out the pressure on something by using this formula:
Pressure can be measured in:
- newtons per square metre (N/m2)
- newtons per square centimetre (N/cm2)
- pascals (Pa).
1 Pa = 1 N/m2
Pressure in liquids and gases
Both gases and liquids are fluids. Fluids can flow. Pressure in fluids acts in all directions. The particles in fluids are moving all the time and hitting the walls of containers or other things they come into contact with. The force of the collisions causes pressure which acts in all directions.
The swimmer is floating because pressure in the water provides a force called upthrust, which balances the force of gravity. As you go deeper into the sea, pressure increases because there is more water above you pressing down. Dams are made with thicker walls at the bottom to withstand the pressure.
Uses of pressure in liquids and gases
Gases can be compressed. The pressure in a compressed gas is higher because there are more molecules moving around and hitting the walls of the container. Pneumatic tyres contain compressed air and this keeps the tyre inflated and helps to soften a bumpy ride.
Liquids cannot be compressed. Liquids are used in hydraulic systems which can be used to increase the size of a force. Hydraulics are used in car braking systems.
Example
The pressure on the water is 25 N .
5 cm2
This is 5 N/cm2.
The area at the end of the other syringe is 12 cm2.
Force = pressure × area
The output force is 5 N/cm2 × 12 cm2 = 60 N.
Levers
Forces can be used to turn objects around pivots. A pivot is also known as a fulcrum.
Levers work by magnifying the force that is put in or the distance it moves.
The hammer is acting as a force multiplier.
Moments
A turning force is called a moment. Moments are measured in newton centimetres (N cm) or newton metres (N m).
Small moment. Big moment.
The longer the distance the greater the moment. It is easier to turn the long spanner than the short one.
When an object is balanced, the anticlockwise moment = the clockwise moment.
In the example above:
the anticlockwise moment = 300 N × 2 m
= 600 Nm
the clockwise moment = 400N × 1.5 m
The clockwise and anticlockwise moments are the same, so the seesaw is balanced or in equilibrium.
Cranes use the principle of moments. The moment from the load is balanced by the moment from the concrete blocks to stop the crane toppling over.
9L Target Sheet
Topic
Targets
Before the unit
I have learned this
I have revised this
9La
Understand how pressure depends on force and area.
Know some ways of changing pressure.
Know some units for pressure.
Be able to use the equation for calculating pressure.
9Lb
Know why there is pressure from liquids and gases.
Know how water pressure changes with depth.
Know what happens when a gas is compressed.
Know what hydraulic systems are.
9Lc
Know what a lever is.
Understand the words 'load', 'effort', 'pivot'.
Know some everyday examples of levers.
Know how the length of a lever affects the effort needed.
9Ld
Know what a moment is.
Know how to calculate the moment of a force.
Understand the meaning of 'in equilibrium'.
Be able to apply the principle of moments.
9L Word Sheets
Word sheets that include new words from the 'Focus on:' pages are available on the Exploring Science website.
9La - Under pressure
Word
Pronunciation
Meaning
pascal (Pa)
A unit for pressure. 1 Pa = 1 N/m2.
The force on a certain area, measured in newtons per square metre (N/m2), newtons per square centimetre (N/cm2), or pascals (Pa).
9Lb - Pressure all around/Hydraulic systems/Changing pressure
altitude sickness
An illness caused by very low air pressure. It can be fatal.
bends
Another name for decompression sickness.
compressed
Squeezed together.
decompression sickness
Bubbles in the blood caused if divers come to the surface too quickly. It can be fatal.
hydraulic
hi-draw-lick
A system which works by transmitting pressure through pipes containing a liquid.
pneumatic
new-mat-ick
Containing air or gas under pressure, eg tyres.
9Lc - Levers
antagonistic muscles
Two muscles that work a joint by pulling in opposite directions, eg biceps and triceps.
biceps
by-seps
Muscle found at the front of the arm between the shoulder and elbow.
contracting
Making something smaller or shorter.
effort
The force put on a lever to put a force on something else.
force multiplier
A lever used to turn a small force into a larger one.
fulcrum
A point around which something turns. Another name for a pivot.
lever
A simple machine which can increase the size of a force.
load
The weight or force on something.
machine
Something which alters the size or direction of a force.
pivot
Another name for a fulcrum.
radius bone
The bone in the forearm that the biceps muscle pulls on.
triceps
try-seps
Muscle found at the back of the arm between the shoulder and elbow.
9Ld - Moment by moment
anticlockwise moment
The moment of a force in an anticlockwise direction around a pivot.
clockwise moment
The moment of a force in a clockwise direction around a pivot.
exert
Push on something.
in equilibrium
In balance.
moment
The turning effect of a force. It is calculated using: moment = force x distance of force from pivot.
newton metre (Nm)
The unit for the moment of a force.
principle of moments
The principle of moments states that when something is in equilibrium (in balance), the clockwise moment is equal to the anticlockwise moment.
turning effect
The moment of a force. The way in which a force turns something around a pivot.