Speed
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 |
| Method of travel | Walking |
|---|---|
| Typical speed (m/s) | 1.5 |
| Typical speed (mph) | 3.4 |
| Method of travel | Running |
|---|---|
| Typical speed (m/s) | 5 |
| Typical speed (mph) | 11 |
| Method of travel | Cycling |
|---|---|
| Typical speed (m/s) | 7 |
| Typical speed (mph) | 15 |
| Method of travel | Car |
|---|---|
| Typical speed (m/s) | 13 - 30 |
| Typical speed (mph) | 29 - 67 |
| Method of travel | Train |
|---|---|
| Typical speed (m/s) | 56 |
| Typical speed (mph) | 125 |
| Method of travel | Plane |
|---|---|
| Typical speed (m/s) | 250 |
| Typical speed (mph) | 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.
Converting speed units
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 Ă 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 Ă 1,000 = 25,000 m/h. In m/s this is: 25,000 Ă· 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 Ă· 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 Ă· 1000 = 0.33 km/s. 0.33 Ă 3,600 = 1,188 km/h. |
| mph to km/h | |
|---|---|
| Conversion | 1 mile = 1.6 km. Multiply by 1.6 to change mph into km/h. |
| Example | A car travels at 30 mph. In kilometres per hour this is: 30 Ă 1.6 = 48 km/h. |
| km/h to m/s | |
|---|---|
| Conversion | 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. |
| Example | A cyclist travels at 25 km/h. In m/h this is: 25 Ă 1,000 = 25,000 m/h. In m/s this is: 25,000 Ă· 3,600 = 6.9 m/s. |
| km/h to mph | |
|---|---|
| Conversion | 1.6 km = 1 mile. Divide by 1.6 to change km/h into mph. |
| Example | A bus travels at 55 km/h. In miles per hour this is: 55 Ă· 1.6 = 34 mph. |
| m/s to km/h | |
|---|---|
| Conversion | 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. |
| Example | A sound wave travels at 330 m/s. In km/h, this is: 330 Ă· 1000 = 0.33 km/s. 0.33 Ă 3,600 = 1,188 km/h. |
Making calculations
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 = \(\pi \) Ă diameter (in metres, as speed is measured in m/s)
\(\pi \) Ă 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 Ă 1.57 = 6.28 m
Then use the formula for calculating speed, and substitute in the values that are known:
\(speed~(m/s) = \frac{distance~(m)}{time~(s)}\)
\(speed = 6.28 \div 0.8\)
\(speed = 7.9~m/s~(2~sf)\)