Density

AQA RP8: Investigate, using appropriate apparatus, the densities of regular and irregular solid objects and liquids, making and recording appropriate measurements.

OCR PAG1: Use of measurements to determine densities of solid and liquid objects

Edexcel 4.3: Investigate the densities of solid and liquids (such as an investigation that uses irregularly shaped objects and a density bottle)

There are lots of misconceptions here – the well known idea of comparing the weight of a kilo of feathers and a kilo of bricks shows this. We conflate density and weight in common conversation, quite apart from mass! Students have often encountered the ideas when learning about buoyancy, which is frequently taught in primary school science. You may have shown them a ball bearing floating on liquid mercury.

Predictably, there are lots of great ideas at Practical Physics; in particular I recommend reading the teaching guidance which suggests a narrative approach before introducing the maths. Considering typical misconceptions is also worthwhile. There are many practical activities which provide opportunities for various skills such as PRODME discussion, measurement and data recording. If you provide differently-sized lumps of plasticene to students and have them plot mass against volume the points should give a nice straight-line relationship – time to discuss proportionality! The same would be true of varied pieces of (the same) plywood or water balloons if you’d like more data. You might also like to discuss the difference between the name of a quantity and the unit, or the symbol and the abbreviated unit.

As a follow-up, there’s a virtual practical at PhET which could be set as homework. This, like any simulation, gives an opportunity to discuss how we collect data, which lead to models, which make predictions, which can be compared to more observations. These animations give data which are unusually neat and clear compared to the untidiness of real life. (For real data, see the A-level Nuffield data book available on the eLibrary.) The density column as described at the American TeachEngineering resource site would be a great starter in a later lesson to check understanding. And how about using the Density notes at BBC Bitesize to create a ‘mistakes’ exercise which students can correct in class?

If you want videos there are unsurprisingly many options. Bang Goes The Theory created instructions to build your own Cartesian Diver. If you’d like something more theoretical, the ever-reliable Veritasium or Sixty Symbols might be good. You may of course have access to something like BrightStorm or BrainPOP as well.

If you’ve any ideas or improvements, please add them in the comments. This is the first post of a planned series supporting teachers with the new Physics GCSE ‘required’ practicals.

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