Time measuring instruments: from a tenth of a second to thousands of years

The stopwatch

A stopwatch measures how long an event lasts (Fig. 1.10). It has two needles, one for seconds and one for minutes.

Its dial usually has 30 big divisions, each split into 10 small ones. One small division is one tenth of a second, so 0.1 s is its least count.

One knob does everything: press to start, press to stop, then press again to send the needles back to zero. Digital stopwatches (Fig. 1.11) measure one hundredth of a second.

Table 1.7 lists other clocks. Each one suits a different length of time, from tiny flashes to ancient remains.

Pick the clock by the length of time: a stopwatch for seconds, a clock for hours, an atomic clock for 10⁻¹⁰ s, radioactive decay for thousands of years.

Key terms

Stopwatch

Measuring the duration of an event.

Digital stopwatch

Short intervals to a hundredth of a second.

Atomic clock

Very short time intervals.

Radioactive decay clock

The age of remains from thousands of years ago.

Short questions with model answers

1. Q1. A ticker-tape timer marks intervals of 0.02 s. How many intervals does it mark in one second?

   • 1 s ÷ 0.02 s
   • 1 ÷ 0.02 = 50

   50 intervals every second.

2. Q2. Your book says an electronic timer measures 1/10,000 of a second. Write it in scientific notation and compare it with a digital stopwatch.

   • 1/10,000 s = 1 × 10⁻⁴ s
   • 0.01 s ÷ 0.0001 s = 100

   1 × 10⁻⁴ s, a hundred times finer than 0.01 s.

Common mistakes

• ✗ Taking one big division of the dial as the least count.

  ✓ The least count is one small division: 1/10 s = 0.1 s.

• ✗ Saying the analogue stopwatch is more accurate than the digital one.

  ✓ Table 1.7: digital ±0.01 s beats analogue ±0.1 s.

• ✗ Choosing an atomic clock to date ancient remains.

  ✓ Atomic clocks time 10⁻¹⁰ s; ages in years come from radioactive decay.

MCQs

1. 1. The least count of the mechanical stopwatch in Fig. 1.10 is:

   1. (a) 1 s
   2. (b) 0.01 s
   3. (c) 0.1 s
   4. (d) 30 s
   Show answer

   (c) Each small division is 1/10 s.

2. 2. A digital stopwatch measures to an accuracy of:

   1. (a) ±0.01 s
   2. (b) ±0.1 s
   3. (c) ±1 s
   4. (d) ±0.02 s
   Show answer

   (a) Table 1.7; ±0.1 s is the analogue stopwatch.

3. 3. Which device measures intervals of about 10⁻¹⁰ s?

   1. (a) Ticker-tape timer
   2. (b) Pendulum clock
   3. (c) Digital stopwatch
   4. (d) Atomic clock
   Show answer

   (d) Table 1.7, p. 22.

4. 4. To find the age of remains from thousands of years ago, use:

   1. (a) An atomic clock
   2. (b) A radioactive decay clock
   3. (c) A sand clock
   4. (d) A stopwatch
   Show answer

   (b) Table 1.7: it measures ages in years.

5. 5. On the mechanical stopwatch, the needles return to zero when:

   1. (a) It is turned upside down
   2. (b) A second knob is turned
   3. (c) The same knob is pressed again after reading
   4. (d) Thirty seconds pass
   Show answer

   (c) One knob starts, stops and resets (p. 17).

Quick revision

• Mechanical stopwatch: 30 big divisions × 10 small; least count 0.1 s.
• Digital stopwatch ±0.01 s is finer than analogue ±0.1 s.
• Atomic clock for 10⁻¹⁰ s; radioactive decay clock for thousands of years.