Speed, distance and time

Distance is a measure of how far an object moves. Distance refers only to how far an object moves - it does not include an object's direction. This means that distance is a scalar quantity. Smaller distances can be measured with a ruler, a tape measure or a trundle wheel. Larger distances can be measured with GPS or aerial photography.

Speed is the rate of change of distance - it is the distance travelled per unit time. Like distance, speed is also a scalar quantity, as it does not refer to direction. To measure speed in the laboratory, a distance value and a time value are needed. The time value can be measured accurately with light gates, although a stopwatch can also be used.

Measuring distance, time and speed

To calculate speed, use the equation:

speed = \frac{distance}{time}

This is when:

  • speed is measured in metres per second (m/s)
  • distance is measured in metres (m)
  • time is measured in seconds (s)

Example

A toy car rolls down a ramp. The car takes 0.4 s to complete the final 30 cm of the ramp. Calculate the speed of the car as it rolls down the final 30 cm of the ramp.

First convert the distance from centimetres to metres:

30 cm = 0.3 m

Then substitute the values into the equation:

speed = \frac{distance}{time}

speed = 0.3 \div 0.4

speed = 0.75~m/s