Stopwatch & Light Gates (OCR A Level Physics): Revision Note

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

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

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

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