Science at key stage 3    (Year 7)

• Review what pupils know about forces by presenting them with a number of quick activities, eg
• weighing objects with a forcemeter
• pushing an ice cube and a wooden block across a smooth surface
• tying or untying a knot in plastic and natural string
• explaining how the time taken for a piece of paper to fall varies with the amount it is crumpled up
• pulling strong magnets apart
• stretching a spring
and questions, eg
• What force is acting here?
• What is its direction?
• Why are the forces changing?
• Discuss observations and answers with pupils.

• Present pupils with floating objects which they can push down to feel the upthrust pushing up. Extend the range of objects and ask pupils to predict which will float, and to weigh them in air and immersed in water. Compile a table of results and ask pupils to identify patterns and draw conclusions from these. Ask pupils to record their results and explanations, eg by annotated drawings showing the forces acting on the objects. Discuss pupils' ideas with them, referring to the work of Archimedes. Encourage generalisations, eg light for size, and show how to calculate density. Displacement of water can be used to measure volume (refer again to Archimedes).

• Show pupils pictures either of people floating in the Dead Sea or of similar situations. Tell them of claims that it is easier to float in sea water than in fresh water. Ask them to suggest how they could test this. Remind them of the previous activity, provide them with samples of fresh water and sea water of varying salinity and ask them to investigate the claim and to produce a brief account using a combination of methods, eg drawings, tables, bullet points. Ask pupils to explain to others what they did, why they chose to do it that way, what conclusions they drew and to ask questions about other pupils' methods. Draw together the outcomes and discuss the methods chosen, eg depth of immersion, number of measurements.

• Remind pupils that when an object floats its weight is balanced by the upthrust. Ask them to suggest why we cannot float in air. Demonstrate a number of situations in which there are easily identifiable balanced forces on a stationary object, eg a tug of war, a paper clip or magnet held up by magnetic attraction or repulsion, a helium balloon floating at a fixed height, and ask pupils to identify the forces, and the directions in which they are acting. Extend to other contexts, eg a book resting on a table, an object suspended from a string. Test pupils' understanding by showing them diagrams in which forces of given magnitude are represented by arrows and ask them questions, eg
• Are the forces balanced?
• Will it begin to move? In which direction?
• What force would be needed to stop the object sinking?
• Why can't we float in air?

• Ask pupils about work they did on stretching materials in key stage 2. Establish that they understand that the greater the force pulling down on a material the more it stretches. Ask pupils to explore this in more detail by adding weights to a spring, measuring the extension and predicting the extension when additional weights are added. Help them to plot a suitable graph of their results.
• One way of supporting graph plotting is to get each pupil to stand in line at a height representing the value. Two other pupils, suitably marked, represent the axes, and lie on the ground and stand to the left. (For more details see Getting to grips with graphs (AKSIS, ASE, 1999).)
• Ask pupils to repeat the experiment with a rubber band and to plot a similar graph. Before they draw a curve through the points suggest they repeat their readings once or twice. Plot all readings and help pupils to decide what line to draw. Show examples of the graphs for both the spring and the rubber band. Ask pupils to describe the differences between them and to explain what they show about the differences in stretch.

• Review the way the direction of a force can be represented by an arrow. Tell pupils they can also be scaled to show size. Give representations of the floating, sinking and stretching situations. Ask pupils to draw arrows to show that the forces are balanced. Discuss how the elastic force in a spring/rubber band increases to balance the increased weight of an object.

• Ask pupils to 'weigh' objects using scales marked in both grams or kilograms and newtons. Use bathroom scales for pupils themselves if possible. Collect data and help pupils distinguish mass (in g/kg) as a measure of 'stuff' in an object and weight as a force measured in newtons.
• Alternatively provide a range of labelled masses and ask pupils to weigh them with forcemeters. Ask pupils to draw a graph of mass against weight and use this to work out the weight of other masses shown on their scale. Discuss these results and establish that the relationship shown is (approximately) 1 kilogram (1000 grams) mass has a weight of 10 newtons. Provide pupils with household items whose mass is marked and ask them to work out the weights.

• Remind pupils of the experiments they did at the beginning of the unit and ask them why the wooden block and ice cube or string and plastic thread behaved differently. Carry out some quick demonstrations, eg oiling a wheel, to illustrate the importance of lubricants, and ask pupils to explain how the lubricants work.

• Ask pupils to suggest what other factors might affect friction between an object, eg a wooden block or weighted margarine tub, and a surface. List the suggestions and tell pupils you are going to show them how to set up an investigation. Do so making deliberate mistakes, eg changing the surface area of the block when investigating the effect of weight; saying the same person must always measure the time taken to slide down a ramp. Ask pupils to point out any mistakes you make and to tell you what you should do.
• Help pupils to plan what to investigate and how to do it, ensuring that the plan will result in data that can be represented on a graph.
• Ask them to use their graphs to predict a value not measured and then test this value experimentally. Compile a class set of information about factors affecting friction.

• Present pupils with a set of cards with a series of statements, eg friction always slows things down, shoelaces stay tied because of friction, friction is useful to gymnasts, matches light because of friction, cars need friction to keep moving, and ask pupils to say if they are true/partly true/false. Discuss answers with the pupils and draw out the idea that friction is often helpful. Emphasise the importance of friction in walking and for wheeled vehicles, eg by showing a video clip of cars, people on ice or of vehicles stuck in sand. Show pupils pictures/examples of the treads on tyres and ask them to suggest how they work, or ask them to write a story of imagination, 'A world without friction'. Ask pupils to produce an information card, eg 'Ten things you never knew about friction'.

• Remind pupils of the importance of friction in driving cars forward and ask them to describe what happens when cars/bicycles try to stop suddenly on greasy/wet/smooth roads and which factors affect stopping distances.
• Show pupils the speed/stopping distance data in the Highway code and ask for interpretations. Some pupils could plot stopping distances against speed and be asked to describe the trend shown.
• Draw out pupils' understanding of what speeds actually tell them. Explore pupils' qualitative understanding of speed by posing questions, eg How would you find out who in the class runs fastest?
• Extend this by giving pupils distance/time graphs, eg of a journey to school partly on foot and partly on a bus, a journey home on a tricycle, a trip in a lift up a high building, and asking them to 'tell the story' of the journey. Discuss with pupils and tell them a story of a journey and ask them to turn it into a graph.

• Present pupils with a context, eg a bicycle or a picture in which there is a variety of examples of friction. Ask them to identify these and explain which are useful and which are not.
• Draw together pupils' knowledge of the key ideas in the unit by asking them to draw a concept map using appropriate terms, eg balanced, friction, upthrust, gravity, weight, mass, movement, speed. Discuss pupils' maps with them. It may be helpful to agree a class summary map.