OCR AS Physics

Revision Notes

1.3.4 Stopwatch & Light Gates

Stopwatch & Light Gates

  • A stopwatch or light gates are common physics instruments used for measuring time
    • For example, the time taken for a ball to fall a certain distance


Stopwatch, downloadable AS & A Level Physics revision notes

A stopwatch is used to measure time

  • A stopwatch is used to measure the time interval of an event
    • It is a type of watch that is much more precise and accurate, with a resolution of up to 0.01 s
  • A digital stopwatch is often preferred, as laps can also be tracked and the values are easier to read

sound-method-1, IGCSE & GCSE Physics revision notes

A stopwatch is used to measure the time interval between the clap and when the sound is heard

  • The disadvantage of keeping time manually using a stopwatch is there will be a large error in the reading
  • This is caused by:
    • Human reaction time (on average, about 0.25 s)
    • The mechanism of the stopwatch (older stopwatches may have a slight delay)
    • Accidentally pressing the start or stop button too many times
    • Consistently starting the stopwatch too late or too early
  • Therefore, repeat readings are very important for experiments that require timekeeping

Light Gate

  • A light gate is a digital switch-type sensor also used in time experiments
    • They consist of an infrared transmitter and receiver between the ‘gate’
    • When this signal is obstructed by an object, a timer can either be started or stopped depending on its configuration
  • If the distance between two light gates is known, the time interval between an object passes through both gates can then be used to measure its speed using the equation:

speed = distance ÷ time

  • This is assuming the object is not accelerating

light-gates, IGCSE & GCSE Physics revision notes

The first light gate starts a timer, and the second stops the timer when the flag passes between them. This is used to determine the speed of the object

  • A light gate is much more accurate than a stopwatch, as it removes the errors caused by human reaction time
  • They can also be connected to a digital timer or datalogger, which then output the time in which the signals are obstructed for data analysis

Exam Tip

If a manual or digital stopwatch is unavailable at school during an experiment, it is best to ask your teacher whether you can use the one built-in on your smartphone. However, the experiment is more reliable if everyone uses the same type of device, therefore this often may be discouraged.

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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