Every day people and vehicles travel at different speeds and are exposed to large accelerations and forces.

When people walk, run, or travel in a car, their speed changes. 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 include:

Method of travel | Typical speed (m/s) | Typical speed (mph) |
---|---|---|

Walking | 1.5 | 3.4 |

Running | 5 | 11 |

Cycling | 7 | 15 |

Car | 13 - 30 | 29 - 67 |

Train | 56 | 125 |

Plane | 250 | 560 |

It is not only moving objects that have varying speed. The speed of the wind and the speed of sound also vary. A typical value for the wind is about 12 m/s and the speed of sound in air is about 330 m/s.

It is useful to be able to convert between everyday units such miles per hour (mph) to the units used in physics, such as metres per second (m/s). The table below shows some common conversions.

Unit | Conversion | Example |
---|---|---|

mph to km/h | 1 mile = 1.6 km. Multiply by 1.6 to change mph into km/h. | A car travels at 30 mph. In kilometres per hour this is: 30 × 1.6 = 48 km/h. |

km/h to m/s | 1 km = 1,000 m. First turn km/h into m/h by multiplying by 1,000. Next, as there are 3,600 seconds in 1 hour, divide by 3,600 to find m/s. | A cyclist travels at 25 km/h. In m/h this is: 25 × 1,000 = 25,000 m/h. In m/s this is: 25,000 ÷ 3,600 = 6.9 m/s. |

km/h to mph | 1.6 km = 1 mile. Divide by 1.6 to change km/h into mph. | A bus travels at 55 km/h. In miles per hour this is: 55 ÷ 1.6 = 34 mph. |

m/s to km/h | 1,000 m = 1 km. Divide by 1,000 to find km/s. Then, as there are 3,600 seconds in 1 hour, multiply by 3,600 to turn seconds into hours. | A sound wave travels at 330 m/s. In km/h, this is: 330 ÷ 1000 = 0.33 km/s. 0.33 × 3,600 = 1,188 km/h. |

Different calculations can often be needed to work out rates such as speed, and sometimes units may need to be converted. For example, if the diameter of a bicycle wheel is 50 cm, and the wheel makes four complete revolutions in 0.8 s, the speed of the bicycle can be calculated in the following way:

First, work out the circumference of the wheel:

circumference = × diameter (in metres, as speed is measured in m/s)

× 0.5 = 1.57 m

Then work out the total distance travelled by the wheel:

Four revolutions of the wheel is a total distance of: 4 × 1.57 = 6.28 m

Then use the formula for calculating speed, and substitute in the values that are known: