02Pressure

Ma02TA of the Machines topic
• 01 Pressure modelMa02TAnugget01 Activity

Developing a model of pressure

What the activity is for

This activity provides a vital set of experiences and an introduction to pressure as a large scale measurement of the state of the particles in a fluid. You can also make the link that the force acting on the walls of the fluid container comes from the collisions of the particles with those walls. The higher the pressure, the more collisions a second, so the larger the force acting.

What to prepare

• a wooden tray with 20 marbles per group, with a wooden divider (best)

Or

• a kinetic theory demonstration (very much second best)
• a set of air filled syringes, sealed at one end

What happens during this activity

Shake the tray of marbles. Listen for two kinds of collisions – the snick of marbles against each other and the clack of marbles against the side. Point out that this can be a good model for a gas. You might ask in what ways it is a good model, depending on where the pupils are up to with their understanding of particles.

Once you have established that the marbles represent the particles in the gas, you can ask which of two collisions – snick or clack – will support forces on the walls. This is a very engaging model of the origins of pressure, so we urge you to use it.

You can take it further by asking what will happen to the forces acting on the walls if you squeeze all the particles closer together. Pupils can feel this by squeezing the air in the sealed syringe. Take a moment to imagine the particles at work there – can you hear them? No? OK, so your imagination will have to work overtime – this is physics, and so you need a good imagination.

Then return to the model and insert the wooden divider. Hear the increase in the number of collisions a second as you squeeze the particles into a smaller and smaller space. See if you can get pupils to relate this to the experience of compressing the air in the syringe. Try also to get them to relate the noises they hear – so the number of collisions a second – to the forces they exerted with their fingers. You might point out that the density of the gas has increased – but go on to suggest that physicists cannot relate the force acting on the sides of the container to the density alone.

Ask pupils to shake the trays harder and ask what has changed about the gas now? (Temperature). What has changed about the number of collision a second? (More). Then you can introduce pressure as the measure that summarises the effects of density (how many particles there are in a particular volume) and temperature (how fast these particles are whizzing around). If you know the pressure, then you can predict the force, because the pressure is a measure of the number of collisions per second.

• 02 Making the connection to force and areaMa02TAnugget02 Activity

Pressure, force and area

What the activity is for

Here you can show that there is a connection between the force generated, the area over which molecules bombard a surface and the pressure.

What to prepare

• this interactive object

• a video of a large soft tyre on a slow moving object

What happens during this activity

Show the video first. Draw attention to the support that the tyres give to the tractor. Ask what is in the tyres. Ask what happens to the tyre if more mass is put on the tractor. What about if the tyres are deflated a little? What will happen to the track left by the tractor in each case? These questions – you can add some more if you feel the need – all point towards the particles in the tyres supporting the mass of the tractor by bombarding the walls of the tyre. More mass, or lower pressure in the tyre, lead to a larger area of tyre in contact with the ground.

Now move to the second interactive and put the bike tyres in order of increasing pressure, given that it's the same rider in each case.

• 03 A pair of syringesMa02TAnugget03 Activity

Feeling the forces in a system, where fluids link two different pistons

What the activity is for

Here pupils can feel for themselves the change in forces that results from a change in area, relating this to the number of collisions of particles within the pistons.

What to prepare

• one large and one small diameter syringe, connected by plastic tubing (air works well enough as the linking fluid – water gets messy)

What happens during this activity

Introduce the apparatus. Tell the class that they are going to push down on the larger of the two plungers. Draw their attention to the different areas, and so the different numbers of particles colliding with the surface. Ask them how hard they'll have to push on the smaller plunger (choosing from: same force, less force, more force), to balance the two plungers. Then ask them to justify their assertion. Then hand out the apparatus and allow them to explore the effect and to relate their prediction to the forces they exert on the two pistons.

• 04 The hydraulic leverMa02TAnugget04 Activity

Trading force and distance

What the activity is for

Here pupils shift fluid from one cylinder to another, making predictions about the forces generated from the distances moved, and knowing that there is a trade off between force and distance.

What to prepare

• the interactive object running on a number of computers, so that two or three pupils can work together around the computer

• perhaps a single copy of the interactive running on a computer connected to a large display for a plenary activity

What happens during this activity

Pupils follow these simple instructions for each screen. There are three separate screens, so you might like to share out the screens amongst the class, so that results can be pooled at the end.

For each screen:

• Mark on the liquid heights before.
• Choose a force for the left hand piston.
• Move fluid from one side to the other.
• Mark on the liquid heights after.
• Adjust piston heights.
• Choose an appropriate force for the right hand piston.

Then ask: Can you explain what you have just done in a way that makes it obvious why the forces you have chosen are the right ones?

• 05 Modelling a hydraulic systemMa02TAnugget05 Activity

Building a model of a hydraulic system using VnR

What the activity is for

The purpose of this activity is to use a simple modelling tool to model the behaviour of a hydraulic system.

What to prepare

• the modelling program VnR running on a computer connected to a large display
• three prepared models
• this interactive object

Support sheet

What happens during this activity

Show the interactive object, drawing attention to the connections between the tube diameters and the forces. Then allow the pupils to work with the modelling tool VnR, using the support sheet to guide them. This leads them through three models closely related to the interactive objects – so it may pay to keep this on the large display – then challenge them to build one of their own.

• 06 Evesham pressure boxMa02TAnugget06 Activity

Using one volume of air for two pistons

What the activity is for

This activity shows how one volume of air can exert different forces on the two pistons. You can use it to discuss the microscopic model of pressure, relating the forces acting on the pistons to particle bombardment.

What to prepare

• an Evesham pressure box (schools who taught Nuffield Physics are likely to have one of these)
• six identical 1 kg masses

What happens during this activity

Show the apparatus, focusing on the construction of the box.

View clip

Make sure you point out that both pistons have the same volume of air in contact with them, but that one is four times the area of the other. Ask which piston will rise first if you inflate the bag. Inflate the bag (the large one rises first).

Check the pupils' answers against what happened. Now place a 1 kg mass on the smaller piston.

Ask how much mass needs to be added to the larger piston so that next time you inflate the bag the two pistons rise together (4 kg). Try it out.

Finish by reinforcing the point that the force exerted by the pistons depends on the area in contact with the air in the inflated bag. It's the same pressure everywhere in the bag, so the same number of collisions on each area of the bag. More area in contact with the bag produces more collisions, so leading to a larger force.

• 07 A pneumatic machineMa02TAnugget07 Activity

Building a pneumatic lever

What the activity is for

To allow some practical experience with using a pair of linked cylinders as levers.

To reinforce the idea that the forces exerted by fluids, resulting from the pressure in the fluid, are due to the collisions of particles.

What to prepare

From the Lego Pneumatics kit (or equivalent)

• Lego master pump
• Lego slave cylinder
• Lego switch and tubes
• Lego connecting board
• this support sheet

Support sheet

What happens during this activity

Pupils build a pneumatic machine, experiencing the spring of the air and finding by practical experience why liquids are preferred for most applications where linked cylinders are used as levers. A support sheet is used to guide them through the activity.

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