Speed: time, t, in seconds, s

The idea

Time is the duration between two events and is measured in seconds (s), the SI base unit. In speed calculations, time is the denominator: average speed = distance ÷ time. You must read time from a stopwatch, timer or light gate and record it in seconds, converting minutes to seconds by multiplying by 60 and milliseconds to seconds by dividing by 1000. For example, a runner taking 2 minutes 30 seconds has t = 150 s. When timing oscillations, time several cycles and divide by the number of cycles to reduce reaction-time error. Always give the unit s with the value, and remember that a smaller time for the same distance means a greater speed.

What earns the marks

• State that time is measured in seconds (s), the SI base unit, and that 1 minute = 60 s and 1 millisecond = 0.001 s.
• Convert a mixed time such as 3 minutes 20 seconds into 200 s before substituting into speed = distance ÷ time.
• Read a stopwatch or timer correctly, including times greater than 60 s, and record the value with the unit s.
• Explain that timing several oscillations and dividing by the number of oscillations reduces the effect of human reaction time.
• Use time as the denominator in speed calculations and rearrange speed = distance ÷ time to give time = distance ÷ speed when required.

Common mistakes and how to put them right

• Writing time in minutes or milliseconds without converting to seconds.

  Put it right by multiplying minutes by 60 or dividing milliseconds by 1000 before calculating.

• Confusing the symbol t with temperature or with time in hours.

  Put it right by checking the unit and context, since in speed equations t always means time in seconds.

• Forgetting to divide the total time by the number of oscillations when timing repeated motion.

  Put it right by dividing the measured total by the number of cycles to find the time for one cycle.

Exam tips

• Always write the unit s after a time value and convert any minutes or milliseconds before substituting into an equation.
• Show the conversion step, for example 2 min 15 s = (2 × 60) + 15 = 135 s, so the examiner can award conversion credit.
• In practical questions, state that repeating the timing and calculating a mean improves reliability, and identify reaction time as a source of uncertainty.