# 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

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

#### 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.

### Author: Ashika

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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