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InkFlaskCambridge IGCSE

Measure time · Biology · Chemistry · Physics

Stop-watch

Stop-watch, with labels: display, start/stop button, reset button.

What it is for

A stop-watch measures a time interval: press start as the event begins and stop as it ends. A digital one shows minutes, seconds and hundredths of a second.

Range
0 s to an hour or more
Resolution
0.01 s on the display
Record to
what the display shows, e.g. 42.37 s, unless the question asks for less; one precision in a table

Why choose it

For timing anything you can see begin and end: a colour change, a reaction going cloudy, 20 swings of a pendulum. Each press adds a reaction time of about 0.2–0.3 s, so a light gate or data logger times short, fast events far more precisely; for a short swing, time 20 and divide by 20.

How to read it

0:42.37min : srecord what it shows: 42.37 sbut your reaction time is ≈ 0.2 s
  1. Reset it to 0.00 before you start.
  2. Watch the event, not the stop-watch, and press start the moment it begins.
  3. Press stop the moment it ends; for a short swing, time 20 and divide.
  4. Write down the time shown, with the unit: 42.37 s.

Exam-style question

A student times 20 complete swings of a pendulum. The stop-watch shows 28.43 s.

  1. How certain is this time?
  2. Calculate the period of the pendulum.
  3. Why time 20 swings rather than one?
Show what a good answer says
  1. Record 28.43 s as shown, but each press adds a reaction time of about 0.2 s, so the true time could be a few tenths of a second different.
  2. Period = 28.43 ÷ 20 = 1.42 s.
  3. The reaction-time error is about the same for one swing or twenty, so shared over 20 swings it is twenty times smaller for each swing.

Teaching points for checking your own answer, not official Cambridge marks.

Common errors

  • Watching the stop-watch instead of the event, so you press late.
  • Trusting the hundredths: the display shows 0.01 s, but your reaction time of about 0.2 s makes one timing far less certain, so repeat it and take a mean.
  • Timing one short swing or event, so the reaction-time error is large compared with the time.
  • Saying one as you press start, so only 19 swings are timed: start the count at zero.

Where you use it here

From the Practical Test apparatus lists of the Cambridge IGCSE Biology 0610, Chemistry 0620 and Physics 0625 syllabuses for 2026–2028, and the apparatus the experiments on this site use.

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