These cost about £20 + VAT from education suppliers, one of which is Timstar (as of 2020-08-03 Timstar appear to have stopped selling the G Ball. We’ve found other UK suppliers: Philip Harris, Better Equipped, Breckland Scientific. Search for ‘G Ball’ to find others. There appear to be at least two manufacturers, Unilab and Mollic).
The g-ball starts timing when released and stops timing as soon as it hits the floor. Like most stopclocks, it measures to a resolution of 0.01 seconds. The switch release can limit accuracy, but overall the g-ball is a quick way to collect a large number of data points. In the film, Alom and Christina use an L-shaped bracket clipped to a metre rule to press the release switch, to aid a clean release.
Questions to ask your students:
- How is this better than using a stopwatch?
- How can they ‘average’ their data?
Your students could just repeatedly drop the g-ball from the same height and average all their data, then substitute their average \(t\) into \(s = \frac{1}{2}gt^2\) to find \(g\). Much better would be to exclude anomalous values first, and better still would be to plot a graph of \(h\) against \(t^2\) to spot those anomalies.
If you’re going to plot a graph, however, you might as well have two meaningful variables. In the film, Christina and Alom drop the ball from different heights and collect a lot of (not very good!) data quickly.
This approach:
- Helps to spot anomalies visually – an indication of random error.
- Shows that \(g\) is (more-or-less!) constant: the graph is a straight line.
- Allows \(g\) to be determined from a gradient, an important skill.
- Might lead on to a discussion about systematic errors (should the graph pass through the origin?).
Systematic errors can often be eliminated by plotting a graph. For example, if the height is measured 1 cm too short each time, the line on the graph will be shifted downwards. The gradient will be unchanged, and the systematic error should be easily detected.
Another approach, incidentally, would be to plot \(2s\) against \(t^2\), giving a simpler gradient of \(g\). This is a matter of personal preference.
Language:
Random error: a measurement error due to results varying in an unpredictable way.
Systematic error: A measurement error where results differ from the true value by a consistent amount each time.