Forces and Motion

Practical Activity
for 14-16

**Class practical**

This is an introduction to the language of measurement, including concepts of range, reproducibility, mean value, true value, accuracy, instrument resolution and, most important, measurement uncertainty.

Apparatus and Materials

*For each student or student group*

- Stopwatch or stopclock
- String
- Statistics board (see technical notes)
- Masses, 50 g, 5 or 6
- Cones/track markers, 10 OPTIONAL
- Video camera OPTIONAL
- Tape measure, long (at least 10 m) OPTIONAL

Health & Safety and Technical Notes

If working outside, students must be appropriately supervised.

If a trolley is used in the lab, ensure that the trolley cannot land on anyone's feet or legs.

Read our standard health & safety guidance

A statistics board is made from a piece of wooden board about 0.5 m square. Ten slotted channels are glued to it and metal (or other suitable material) discs are cut so that they fit into the channels. The board is supported vertically.

Assign values to each channel. Students drop in a disc for the value they achieve. The distribution of results grows as results are added.

Procedure

- One student runs a distance of 100 metres. You, and other students, all independently time the run.
- Compare all of the measurements. What is their
range

(the difference between the highest and the lowest measured values)? What does this tell you about the reproducibility

of the measured values of time?
- What is the
mean

of all the measurements? A mean is a kind of average. Work this out by adding them all together and then dividing by the number of measurements. How closely do you think the mean value agrees with the true value of the run time? In other words, estimate the accuracy of the mean value.
- Did everybody use stopwatches with the same
resolution

? For example, were everyone’s stopwatch time indications

in tenths of seconds or hundredths of seconds? (0.1 s econd is a tenth of a second; 0.01 s econds is a hundredth of a second).
- Try to estimate the reaction times involved in pressing a stopwatch to both ‘start’ and ‘end’ the run. The sum of these reaction times is very likely larger than the resolution of the stopwatch.
- How certain can you be about the actual time taken for the run? You can’t be perfectly certain! There must be some uncertainty in the measurements. The mean measurement could be 14.8 s econds. Perhaps you think that the ‘true’ time for the run is in between 14.6 s econds and 15.0 s econds. Then you can say that the uncertainty is ± 0.2 s econds.

Teaching Notes

*This experiment was safety-checked in January 2007*