Stopwatch & Light Gates (OCR AS Physics): Revision Note
Exam code: H156
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
Examiner Tips and Tricks
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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