Section 1 of 7Typical speeds
| Situation | Typical speed (m/s) | Comment |
|---|---|---|
| Walking | 1.5 | About 5 km/h; a steady pace |
| Running | 3 | Jogging speed; sprinters reach about 10 |
| Cycling | 6 | Casual cycling; racing is much faster |
| Car in a town | 13 | Roughly 30 mph |
| Car on a motorway | 30 | Roughly 70 mph |
| Train | 50 | Fast intercity services |
| Passenger aeroplane | 250 | Cruising at height |
| Speed of sound in air | 330 | Used for echo and thunder estimates |
Section 2 of 7Why values vary
People
- People Walking, running and cycling speeds depend on age, fitness, the terrain, the length of the journey and whether the person is carrying anything.
- A child, an athlete and an elderly walker will all have different speeds, so a typical value is only an estimate.
Vehicles
- Vehicles Speeds of vehicles depend on their design, shape, purpose and cost, and also on speed limits and traffic.
Take care with units
- Take care with units Speeds are usually quoted in m/s in calculations.
- To convert, multiply km/h by 1000 and divide by 3600, or simply divide by 3.6.
Section 3 of 7Typical accelerations
What acceleration is
- What acceleration is Acceleration is the rate at which velocity changes, found from a = Δv ÷ t and measured in metres per second squared (m/s2).
A family car
- A family car A car going from rest to about 27 m/s (roughly 60 mph) in 10 s has an acceleration of about 2.7 m/s2.
Other examples
- Other examples A person starting to walk accelerates at less than 1 m/s2.
- A sprinter at the start of a race is nearer 3 to 5 m/s2.
- An object falling freely near Earth's surface accelerates at about 10 m/s2 because of gravity, ignoring air resistance.
Section 4 of 7Checking a claim
1
Read the claim carefully and note the quantity and its units.
2
Recall the typical value for the situation, for example walking at about 1.5 m/s.
3
Compare the two. A claim that people walk at 6 m/s is four times too large; that is about 21 km/h, which is a fast run, not a walk.
4
State whether you agree and justify it with the typical value.
5
If asked for a factor, give one such as age, fitness, terrain or distance of the journey.
Section 5 of 7Worked estimate
Cyclist example
- Cyclist example A cyclist accelerates from rest to 6 m/s in 4 s.
- a = Δv ÷ t = 6 ÷ 4 = 1.5 m/s2,
- which is a sensible value for a cyclist pulling away from traffic lights.What the symbols mean: a = acceleration (m s−2) · v = final velocity · u = initial velocity (m s−1) · Δv = change in velocity · t = time taken (s)
- which is a sensible value for a cyclist pulling away from traffic lights.
Time for a journey
- Time for a journey A person walking 3 km at 1.5 m/s needs t = s ÷ v = 3000 ÷ 1.5 = 2000 s, which is a little over half an hour.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Speed | How fast an object is moving, measured in metres per second (m/s). |
| Acceleration | The rate of change of velocity, measured in metres per second squared (m/s2). |
| Typical value | A sensible, approximate figure for a quantity in an everyday situation. |
| Estimate | An approximate calculation or value found using reasonable assumptions. |
| Terrain | The type of ground, such as flat, steep, wet or rocky, that a person or vehicle travels over. |
Section 7 of 7Exam technique
1
Using miles per hour or km/h in a formula that needs m/s without converting.
2
Giving the unit of acceleration as m/s rather than m/s2.
3
Giving one fixed 'correct' speed for a person; typical values are approximate and vary with age and fitness.
4
Judging a claim without comparing it with a typical value, which loses the marks for reasoning.
5
Forgetting that a slowing object has a negative acceleration (deceleration).