Speed
Units of speed
There are many different units of speed. Different units are sometimes used in different countries and for different things. For example, the speed of ships is often measured in knots, whereas aeroplanes often use Mach. Some countries have road speed limits in kilometres per hour, whereas some countries use miles per hour.
So, to avoid confusion, scientists use standard units for measurement in all countries.
The standard unit for speed is metres per second.
The word per means ‘in each’. Therefore, metres per second means the number of metres travelled in each second. For example, a horse running with a speed of 15 metres per second means the horse travels a distance of 15 metres in each second.
Metres per second is written as m/s.
Calculating speed
The way you calculate speed is linked to the unit metres per second, m/s.
For example, think of a bus that travels a distance of 100 m in a time of 20 s.
The bus has travelled 100 m in 20 s, so how many metres does it travel in each 1 s?
number of metres travelled in each second =
total distance travelled ÷ total time
number of metres travelled in each second = speed
speed = total distance travelled ÷ total time
= 100 m ÷ 20 s
= 5 m/s
You can summarise this equation for speed as:
speed = distance ÷ time
The highest possible speed
Since the 1930s, the highest possible speed is thought to be the speed of light, which is 1 000 000 000 km/h. This was predicted by calculations made by Albert Einstein and confirmed by other scientists doing experiments. No scientist, so far, has observed anything moving faster.
This is how science advances: through collaboration (scientists working in groups) and peer-review (scientists checking each other’s work).
Check your understanding
1. If a rocket travels at 18,000 km/h, how long would it take to cover a distance of 36,000 km?
Show Answer
Answer: time = distance ÷ speed = 36,000 ÷ 18,000 = 2 hours
Questions
1a. Write an equation for speed, when you know the distance travelled and the time taken.
1b. Write down the standard scientific unit of speed.
1c. Write an equation for distance travelled, when you know the speed and the time taken.
1d. Write an equation for time taken, when you know the speed and the distance travelled.
Show Answers
1a: speed = distance ÷ time
1b: metres per second (m/s)
1c: distance = speed × time
1d: time = distance ÷ speed
2a. An Olympic sprinter completes the 100 m race in a time of 10 s. Calculate the average speed of the sprinter.
2b. Explain why this value is an average speed.
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2a: speed = 100 m ÷ 10 s = 10 m/s
2b: It is an average speed because the sprinter may not have run at the exact same speed throughout the race.
3a. A car travels a distance of 210 m in a time of 6 s. Calculate the speed of the car in m/s.
3b. Calculate the distance, in m, travelled by the car in 14 s.
3c. Calculate the time taken, in s, for the car to travel a distance of 1925 m.
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3a: speed = 210 ÷ 6 = 35 m/s
3b: distance = 35 × 14 = 490 m
3c: time = 1925 ÷ 35 = 55 s
4a. An aeroplane flies between two cities that are 8100 km apart. The aeroplane takes 9 hours to complete the journey. Calculate the average speed of the aeroplane, in km/h.
4b. A different aeroplane can fly at 800 km/h. Calculate the distance, in km, that this aeroplane could fly in 6 hours.
4c. Another aeroplane can fly at 950 km/h. Calculate the time taken, in hours, for this aeroplane to travel a distance of 7125 km.
Show Answers
4a: speed = 8100 ÷ 9 = 900 km/h
4b: distance = 800 × 6 = 4800 km
4c: time = 7125 ÷ 950 = 7.5 hours
5. Anna sees a worm on the grass. Anna sees the same worm 2 hours later. The worm has moved a distance of 3 m in that time. Calculate the average speed of the worm, in metres per hour.
Show Answer
5: speed = 3 ÷ 2 = 1.5 m/h
Think Like a Scientist
Calculating speed
In this investigation, you will make measurements to calculate the speed of a tennis ball.
- ramp (such as a plank of wood or thick card)
- tennis ball
- metre rule
- coloured tape
- books
- smooth level surface (such as a desk or the floor)
- stopwatch
- Use coloured tape to fix the bottom of the ramp to the desk or floor.
- Fix some coloured tape 1 m from the end of the ramp.
- Fix some coloured tape near the top of the ramp to mark where you will release the ball.
- Measure the height from the desk or floor up to the position where you will release the ball.
- Release the ball and measure the time it takes to move between the two pieces of coloured tape on the desk or floor.
- Repeat this two more times and calculate the average time to travel between the two pieces of tape.
- Do this for a range of different heights.
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Create a table with columns for height (cm), time (s), and speed (m/s).
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Use the formula: speed = distance ÷ time. For example, if distance = 1 m and time = 0.5 s, then speed = 2 m/s.
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Graph height (cm) on the x-axis and speed (m/s) on the y-axis. Use a clear scale and label the axes with units.
a. the independent variable in this investigation?
b. the dependent variable in this investigation?
Show Answer
a. Height of ramp (independent variable)
b. Speed of the ball (dependent variable)
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(1) The ball used
(2) The distance between the two tape markers (1 m)
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To improve reliability and reduce the effect of any timing errors. Repeats help calculate a more accurate average.
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As the height increases, the speed of the ball increases. There is a direct relationship between height and speed.
- Decide how well you:
- made measurements
- recorded results in a table
- drew the graph of the results
- Choose one thing that you could do better next time.
- How will you do this better next time? What will you change?