It is not only moving objects that have varying speed. The speed of sound…

The idea

Speed is the rate of change of distance with time, and it is not restricted to solid objects travelling along a track. Sound travels through a medium as a wave, and its speed depends on the medium and its temperature; in air at about 20 °C the speed is roughly 330 m/s, but in warmer air it is slightly greater and in water it is much greater. Wind speed also varies with time and place, from calm conditions to gales, and is measured with an anemometer. So when you calculate speed using speed = distance ÷ time, you must recognise that the value can change during a journey or a measurement.

For example: Timing thunder and lightning gives a rough distance because the sound speed is not fixed.

What earns the marks

• State that speed is the distance travelled per unit time and is calculated using speed = distance ÷ time.
• Explain that sound is a wave travelling through a medium, so its speed depends on the medium and on temperature rather than being a universal constant.
• Give the typical speed of sound in air as about 330 m/s and note that it increases in warmer air and is greater in liquids and solids.
• Describe wind as moving air whose speed varies with time and location, and state that it is measured with an anemometer.
• Apply the idea of varying speed to a context, such as estimating the distance to a lightning strike from the time delay before thunder.

Common mistakes and how to put them right

• Treating 330 m/s as the exact speed of sound in all conditions.

  Put it right by saying it is a typical value for air near room temperature and that temperature and medium change it.

• Assuming that only solid moving objects can have a speed.

  Put it right by explaining that waves such as sound and moving fluids such as wind also travel at measureable speeds.

• Using speed = distance × time instead of speed = distance ÷ time.

  Put it right by rearranging the equation and checking that the units work out as m/s.

Exam tips

• Quote the equation speed = distance ÷ time and substitute values with units before calculating.
• When a question mentions sound or wind, comment on why the speed may not be constant rather than giving a single fixed number.
• Show the rearrangement clearly if you are asked for distance or time, and give the unit with your answer.