Timing a trolley on a slope

Class practical

Graphical presentation of data helps to build concepts. Here, time, distance and velocity can all be measured and displayed, and the idea of acceleration can be introduced.

Apparatus and Materials

For each student or student group

• Runway
• Dynamics trolley
• Stopwatch or stopclock

Health & Safety and Technical Notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a string is tied across the bottom of the runway, to prevent the trolley falling onto anyone.

Read our standard health & safety guidance

Relatively light runways can be made using planks with lengths of angle-iron along each side to produce rigidity. Alternatively, MDF window board, available from builders merchants, is relatively rigid and makes suitable runways. Long lengths are quite heavy, and slopes do require midway support to prevent excess sagging.

For this activity, lengths of 1.5 m are adequate. For other activities, longer boards of about 2.5 m are required.

The boards also need to be about 30 cm wide and 2.5 cm thick so that they do not flex.

Procedure

1. Use books or wooden blocks to support one end of the runway and make the board slope. The slope should be big enough for the trolley to gently increase in speed as it travels down it. (About 1 in 10)
2. Let the trolley start from rest (standstill). Time the trolley, as well as you can, over the first 25 cm it travels. Time it over the second 25 cm, over the third 25 cm, and so on.
3. Repeat the measurements twice again. See how much variation there is in your measurements.
4. Write all your measurements in a table. Work out the average (mean) time for the first 25 cm, for the second 25 cm, and so on.
5. Make bar charts to show the information about distance and time.
6. Divide the distance by the time. That will tell you the mean velocity of the trolley during each part of its journey. Make bar charts of this velocity information.
7. Note the patterns you see in the changes that take place to the time, the distance, and the velocity.

Teaching Notes

• Here, we use the word distance rather than the more correct displacement. At more advanced levels, the distinction between scalar quantities like distance, and vector quantities like displacement, becomes important.
• The experiment shows that the times to cover successive 25 cm distances decrease. It also demonstrates difficulties in measuring short time intervals of less than a second, as well as measuring continuous time.
• Students could decide on their own format for graphic data presentation.
• More than three students working at a board will lead to "passengers" in the group.

This experiment was safety-tested in November 2004