## Speed

**Speed is defined as a measure of the distance an object travels in a given length of time.**

The speed of an object can be determined from the equation below:

Speed = *Distance travelled (meters)* **Time taken (seconds)**

or,

**v = dt**

*Note: the symbol v can be used to denote speed or velocity, here it is used for speed.*

Typically the unit for speed is given as metres per second (m/s). However, other units such as kilometres per hour for large distances and centimetres per second (cm/s) for small distances can be used.

### Rearranging the equation

The equation for speed can be rearranged to give two other useful equations:

Distance travelled = Speed x Time taken

And,

Time taken = *Distance travelled* **Speed**

Using the equation in the distance travelled (d) = Speed (v) x Time taken (t), we can put it in an equation triangle.

### Average Speed

The equation for average speed is the same as that given for speed above:

Average speed = *Distance travelled (meters)* **Time taken (seconds)**

Thus, if a car travels 100 meters in 5 seconds, its average speed can be calculated as:

Average speed = *100***5** = 20 meters per second

Average speed is used to give the speed of an object over a given interval of time; if however the speed of an object is required for a particular moment then the instantaneous speed is used.

**Instantaneous speed is the speed of an object at a given moment.** In this case the equation is similar to that of average speed but the time taken is a much smaller interval. A speedometer in the car gives an instantaneous speed as it gives the speed of the moving car at that specific time, whereas the average speed would be used as a measure of speed for the whole journey.