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Uncertainties in timing a tennis ball hitting the ground

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Matthieu Robin                                                                                                13/09/08


11th Grade

Uncertainties in timing a tennis ball hitting the ground


In a group of four, the aim was to record the time of a tennis ball being dropped from a certain point on the third floor of the Great Portland Place school campus until it hit the ground of the ground floor; and then record the uncertainties of the experiment.

In this experiment the following apparatus was used;

  • two stopwatches which measures to 2 decimal places (1/100 of a second – centiseconds)
  • 4 ordinary tennis balls
  • Utensils to record the time of each tennis ball try

 There was 21 tries done using 4 different tennis balls, in order to gain sufficient data to provide a measurement of time taken. In order to improve the accuracy of the time of one try, time was recorded from the third floor and the ground floor simultaneously. These measures were all mainly relied upon our auditory and visionary perceptions.


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  (all the data is rounded to the 2nd decimal place)

 The measures recorded from the ground and Third floor were combined to create a big set of data. The data is sorted in ascending order.

The average of all these times is approximately 1.28 seconds.

The lowest measure (1.12 sec) is 0.16 seconds below the average, while the highest measure (1.65 sec) is 0.37 seconds above the average.

The average of these two is approximately 0.27 seconds and so the time could be written as 1.28 ± 0.27 seconds.

This is a way which is too broad and imprecise in trying to find an accurate measure. There needs to be refinement in the data because if one looks at the data of the times measured from the third and ground floor one will find that there are many random errors with too many measures which are too big and too small. These errors in timing were most likely caused by the person’s reaction when timing. These errors can be easily be deciphered on a graph (next page).


These errors increase inaccuracies of the final result and therefore the data has to be reduced to a more accurate set of data.

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When narrowing down my data the uncertainty decreased as well, this therefore means that my technique of manipulating my data was successful. All I did is to keep subtracting all the anomalous data from the previous data. I f I had continued the process I would have gotten and even more precise answer with an uncertainty of ± 0.01 seconds. Still that answer would still not be fully accurate.

In order to gain to strongest accuracy in results the following should be suggested:

  • Use a more precise apparatus to measure the times such as an electrical watch which measures to more than a 1/100 of a second.
  • Collect as much data as possible
  • Improve the reaction time when letting go of the ball on time and timing the impact of the ball as it hits the ground, so there would be less uncertainties
  • Do the experiments in some other place or environment because the space in which the ball was thrown was very narrow and decreased the ability of using vision as a sense to help measure time. Maybe if this experiment was done in an open space it would be easier to use on senses, away from other distractions.
  • Using only one Tennis ball would reduce the errors in the results if ever these different balls had different air resistance.

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