Science at key stage 3    (Year 9)

• Elicit pupils' ideas about what it would feel like to lie on a bed of nails and whether they would rather have their foot trodden on by an elephant or by someone wearing stiletto heels. Draw the conclusion that the area over which a force is concentrated makes a difference to its effect. State the relationship: pressure = force÷area (perpendicular to direction of force).
• Ask pupils to work out the lowest and highest pressure they can exert on a part of their body. This requires them to weigh themselves in newtons and measure the cross-sectional area of any part of their body on which they are capable of balancing, eg head, tiptoes, hands, full body.
• Ask pupils to contribute to a list of devices that rely on low pressure being exerted, eg snowboards, snowshoes, camels' feet, and a list where high pressure is required, eg blades, drawing pins. Demonstrate or show video clips of some of these.

• Ask pupils to take the role of a sales manager, select one item and make a spoof written report of a 'new, improved' version where the item in question exerts high pressure instead of low, or vice versa. They then have to 'sell' it, explaining its features, eg the smallest snowshoes in the world, guaranteed to sink as far and as fast as possible; or the drawing pin with a new, improved thick pin.

• Introduce the word 'pneumatics'; ask about experiences of gases under pressure, eg bicycle tyre ('pneu' - French for tyre). Remind pupils of the nature of gases and liquids as described by the particle model of matter, and elicit their ideas about why gases - unlike liquids - are compressible. Offer pupils empty syringes to show this, if required. Spray aerosol air freshener and elicit pupils' ideas as to how the freshener gets out - gas under pressure acts as a propellant for the liquid. Establish why the pressurised container can explode when on the fire.
• The use of pressure to provide controlled movement could be demonstrated with a model steam engine, or pupils could find out about the work of, eg Hero of Alexandria, Thomas Newcomen.

• Elicit pupils' ideas about the effect of exerting pressure on a container of liquid. Offer them balloons or syringes filled with water to determine whether they can reduce the volume.
• Show a diagram of a simple hydraulic system where the liquid is transferred from a reservoir of unit area to one of 10 times unit area, and ask pupils to predict the force with which it will lift the plunger when a force of 10N is applied. Take votes on the outcome. Most will intuitively predict that the force is diminished rather than magnified. Reference to the distance moved by the applied force may help the understanding of this counter-intuitive outcome.
• Demonstrate, eg using video, the ability of a hydraulic jack to lift large loads, eg a car, with little effort, or model this with water-filled tubes.

• Show pupils a video clip or a photograph, eg of a paramedic running alongside a patient's hospital trolley and holding a saline drip in the air, and ask why the drip has to be raised. Establish the importance of the 'head of liquid'.
• Ask for pupils' experiences of swimming underwater at the deep end of the pool. Many will report discomfort in their ears. Associate this with the pressure exerted by the height of water above their heads.
• Elicit pupils' ideas as to why watches used by swimmers and divers are 'pressurised to three atmospheres'.

• Provide pupils with resources on relevant devices or situations and ask them to write summaries which link all the key ideas about force and pressure in solids, liquids and gases.

• Ask pupils how to remove the lid from a tin of paint. Offer a range of simple levers for pupils to try, and in each case ask them to identify the pivot and the position of the effort. Encourage pupils to appreciate use of a long lever arm, eg ask pupils to cut paper with the tips of scissors and then to cut thick cardboard. They will intuitively put the card nearer the pivot to cut it. Demonstrate a torque wrench and a wheel brace.
• Ask pupils to find out about devices in the home that rely on levers.

• Ask two pupils to arm wrestle (gently) to demonstrate how arms act as levers. Elicit pupils' ideas about how longer arms are an advantage.
• Show or ask pupils to make a model of the arm with the biceps and triceps attached, as an example of antagonistic muscles. Identify the elbow joint as the pivot and ask pupils to explain how the forearm operates as a lever, both up and down.

• Link to the previous section, with illustrations of gymnastic balance, eg photographs or video clips of Olympic gymnasts. Get pupils to try some balancing activities, eg
• stand with their heels touching a wall and try to pick up an object placed about 0.5m away, on the floor. Ask why this is impossible
• stand about three foot-lengths from the wall and try to touch the wall with their noses. This may be possible for some pupils and not for others
Ask whether there are any patterns.
• Draw out ideas about muscles and levers and 'counterbalance'.
• Demonstrate a toy crane or show photographs of cranes, explaining the role of the counterbalance weight.
• Make a model of the human back to demonstrate the tension in the muscles when we bend. Tie string to a mop about one quarter of the way down from the head. Pivot the mop at the other end by drilling a hole through it to take a rod. Now try to balance the mop as it tilts by holding the string at an angle of about 10 degrees. This will show pupils that a large force is needed.

• Describe the turning effect of a force as its moment. Ask pupils to make a mobile, or show them a mobile, eg for a child, and ask them to write notes about the mobile's construction using the terms 'pivot', 'balance' and 'moment'. Introduce the see-saw balance, eg a pivoted metre rule.

• Ask pupils to plan an investigation to find the rule of balancing. Encourage them to plan how to collect data systematically to determine the pattern.
• Derive the principle of moments from their patterns and use them to calculate moments about a pivot. Help pupils to find anomalies in their readings and to identify the origins of these, eg incorrect measurement, poor balance.

• Identify a range of situations where the turning effect of a force is used, eg tools, gymnastics.
• Ask pupils a range of questions related to pressure and the principle of moments, requiring both numerical and descriptive answers. Discuss their answers with the pupils, asking them to explain their methods and the reasoning used.