Speed, distance and time

Speed is worked out by dividing the distance travelled by the time taken:

Speed, distance time equation

The formula can also be written in this form:

Speed = Distance \div Time

Question

If you travel 70~km in 2 hours, what is your average speed?

Speed = distance \div time

Speed = 70 \div 2 = 35~km/h

curriculum-key-fact
The distance in km and the time in h - so the units for speed are km/h.

Another suitable metric unit for speed is metres per second. Using metres per second would be suitable when measuring short distances and short periods of time. For example, an athlete might run the {400~m} race in {56} seconds. What is the average speed for the {400~m} race?

speed = distance \div time

= 400 \div 56

= 7.14~m/s ( {2} decimal places)

If you know the speed, you can work out the time or the distance by rearranging the equation:

speed = distance \div time or speed = \frac{distance}{time}

distance = speed \times time

time = distance \div speed or time = \frac{distance}{speed}

The triangle below is a good way to remember the equations:

Triangular diagram to show the relation between speed, distamce and time

To use the triangle, cover the value required and the formula that you need will be revealed by the location of the two values that you can see.

For example:

  • For time, you see distance above speed, so time = \frac{distance}{speed}
  • For distance, you see speed alongside time, so distance = speed \times time
Question

If you walk for 1\frac{1}{2} hours at an average speed of {4}~miles/h, how far will you have walked?

Speed = distance \div time

Distance = speed \times time

Distance = 4 \times 1.5 = 6~miles

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