Motion in a straight line

Speed, distance and time

Distance is how far an object moves. It does not include an associated direction, so distance is a scalar quantity.

Speed is the rate of change of distance - it is the distance travelled per unit time. Like distance, speed does not have an associated direction, so it is a scalar quantity.

Typical speeds

When people walk, run, or travel in a car, their speed will change. They may speed up, slow down or pause for traffic. The speed at which a person can walk, run or cycle depends on many factors including:

  • age
  • terrain
  • fitness
  • distance travelled

Some typical values for speed in metres per second (m/s) include:

Method of travelTypical speed (m/s)
walking1.5
running3
cycling6
car13-30
train50
aeroplane250

It is not only moving objects whose speed can vary. The speed of the wind and the speed of sound also vary. A typical value for the speed of sound in air is about 330 m/s.

Calculations involving speed, distance and time

The distance travelled by an object moving at constant speed can be calculated using the equation:

distance travelled = speed × time

s = v~t

This is when:

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

Example

A car travels 500 m in 50 s, then 1,500 m in 75 s. Calculate its average speed for the whole journey.

First calculate total distance travelled (s):

500 + 1,500 = 2,000 m

Then calculate total time taken (t):

50 + 75 = 125 s

Then rearrange s = v~t to find v:

v = \frac{s}{t}

v = 2,000 \div 125

v = 16~m/s