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Primary Newsletter 31 Under pressure | |
 Hang
on a minuteForum comment So we turn instead to a widely published demonstration using balloons and a wire coat hanger (Fig.1). Nothing could be simpler, especially the apparatus - two balloons, a coat hanger and some string or a couple of clothes pegs. Fig.1 - Coat hanger balance? One balloon is blown up to as large a diameter as possible. It is then suspended from one arm of the coat hanger. The uninflated balloon is placed on the other arm. Balance the hanger by the hook and ‘Hey Presto’ we have a beam balance. Showing what exactly? The theory is that the balloon with air will be the heavier and will drop dramatically. We have tried this many times with a pack of supermarket balloons and a wide variety of coat hangers. We can report little success. There’s no dramatic movement, and no ‘Wow’ factor (to use a current cliché). However, if you have had success with this we’d be delighted to hear from you (he wrote, not without irony). Then, using an electronic balance we weighed an ‘empty’ balloon and another inflated to almost bursting point. This exercise was repeated and the usual difference was about 0.3 g. This is not a difference detectable on the types of balance usually found in primary schools. There are other worthwhile investigative activities of this type. They all rely on the use of a fairly sensitive, electronic balance and also involve the use of special air pumps and other types of container. As such, they are thus much better suited to work at the secondary stage. Simple methods, such as detailed above, with its misguided use of a coat hanger, often ignore confounding factors. In this case, these include such matters as water vapour and carbon dioxide in the balloon - should exhaled breath be used to ‘fill’ it? It also ignores the effects of buoyancy and the fact that each balloon will displace its own volume of air (good ol’ Archimedes). Therefore, it’s not faulty physics but flawed experimental design, which is the problem here. Primary teachers and their pupils deserve better. They can make their own mistakes and they don’t need or want help from authors and publishers to make more. Where do we go from here? Well we’ve argued theoretically to show that a column of air does have mass. How could we show this? Forum comment Title : Balloon experiment
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