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PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure

Physics 6.2 Precision and accuracy: what’s the difference? — Timing Gates and Reaction Time

5 min read 6 parts Lesson 2 of 19

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Lesson summary

Physics 6.2 Precision and accuracy: what's the difference?

  • Why precision matters
  • Precise but inaccurate
  • Reaction time
  • Timing with a stopwatch
  • Automatic timing
  • False starts

Section 1 of 6Precision

1Precision1 point
= the number of significant figures to which a value is given.
  • Significant figures are the digits in a number, so 24.3 has 3 and 24 has 2.
2Why precision matters3 points
  • More significant figures can separate values that a less precise measurement cannot.
  • Yohan Blake and Justin Gatlin had times that looked the same to 2 significant figures.
  • Three significant figures are needed to say who is faster.

Section 2 of 6Accuracy

  • Accuracy = how close a measurement is to its true value.
  • Precise but inaccurate
    • A measurement can be very precise without being accurate.
    • A time recorded as 9.73 s instead of the true 9.63 s has the same precision but is inaccurate.
    • A stopwatch reading to 0.01 s that is started late is precise but not accurate.

Section 3 of 6Reaction time

1Reaction time1 point
= the time it takes the brain to process information.
  • It is typically about 0.2 seconds.
2Timing with a stopwatch3 points
  • There is a delay between hearing the starting gun and starting the stopwatch.
  • The judge at the finish line can anticipate the athlete, so there is less delay when stopping the watch.
  • The measured times were therefore shorter than they should be by about 0.2 s.

Section 4 of 6Automatic timing

  • Automatic timing
    • Introduced in the 1970s, it removes human reaction time.
    • The starter's gun triggers the start of the timer.
    • The athlete breaks a light beam as the body crosses the line.
    • A light sensor automatically stops the clock.
  • Effect
    • When it was first used, measured 100 m times got longer, because stopwatch times had been too short.
  • False starts
    • Sensors in the starting blocks detect a pressure change; if it comes too soon after the gun for a human to react, a false start is registered.

Section 5 of 6When reaction time matters

1Long times2 points
  • Over a long period reaction time makes little difference.
  • A marathon takes hours, so 0.2 s is negligible.
2Very short times3 points
  • For fast events reaction time has a big effect.
  • 0.2 s is 2% of a 10 s sprint.
  • A ball falling 1 m takes about 0.45 s, so a 0.2 s error is very large.

Section 6 of 6Light gates in the laboratory

  1. Place the first light gate where the object will start; the timer starts when the object breaks its beam.
  2. Place the second gate below; the timer stops when the object breaks the second beam.
  3. Read the time, which has no reaction-time error.
  4. The timers can be accurate and precise to 3 significant figures.

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