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 รท 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 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.