Science notes, unit 7

Speed

Textbook Science Year 6 (DLP, Semakan 2017), pages 113–126

What this chapter covers

You can say what speed is and name its units: cm/s, m/s and km/h (7.1.1)
You can explain that a faster object takes a shorter time over a fixed distance and goes farther in a fixed time (7.1.2)
You can calculate speed, distance and time with Speed = Distance ÷ Time (7.1.3)
You can write an operational definition of speed from an activity (7.1.4)
You can solve problems about speed, including changing minutes to hours or seconds first (7.1.5)

Race cars, boats and drones all try to win by being fast. Being fast is about speed. In this unit you learn what speed is, how to measure it and how to calculate it.

7.1Speed, distance and time

What is speed?

Speed is a measurement of how fast an object moves from one place to another. All moving objects have speed, and different objects move with different speeds.

A lorry, a van, a car, a motorcycle and a bicycle in a row — Which of these moves the fastest? We cannot tell by looking. We must measure and calculate.

Picture: Science Year 6 (DLP) textbook, p. 114.

To find a speed we need two measurements: the distance the object travels and the time it takes.

What we measure	Units
Distance	centimetre (cm), metre (m), kilometre (km)
Time	second (s), minute (min), hour (h)
Speed	centimetre per second (cm/s), metre per second (m/s), kilometre per hour (km/h)

"Per" means "in each". A speed of 25 km/h means the cyclist travels 25 kilometres in each hour. A speed of 100 metre/minute means 100 metres in each minute.

Moving thing	Sensible unit	Why
A snail or a tortoise	cm/s	It moves only a few centimetres in a second.
A runner	m/s	It covers some metres in a second.
A car or an aeroplane	km/h	It covers many kilometres in an hour. cm/s would give a huge number.

Did you know? The cheetah is the fastest animal on land, with a speed of more than 100 km/h.

Always write the unit. "25" is not a speed, but "25 km/h" is. Pick a unit that matches the distance and time you have.

What does a speed of 5 m/s mean?

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The object travels 5 metres in each second.

Speed, distance and time are linked

Two toy cars race over the same distance. The faster one gets to the finishing line first. Two cars also run for the same time. The faster one goes farther.

Two toy cars on a track: the red car finishes in 17 seconds and the blue car in 20 seconds; after 10 seconds the red car has gone 5 m and the blue car 4 m — Fixed distance: red car 17 s, blue car 20 s. Fixed time (10 s): red car 5 m, blue car 4 m. The red car is faster.

Picture: Science Year 6 (DLP) textbook, p. 116.

The object moves faster, so it...	when we fix
takes a shorter time	the distance
travels a longer distance	the time

Two fair tests

The textbook uses a ramp, made higher by adding books, to change the speed of a marble or a toy car. A higher ramp gives a faster speed.

	Speed and time	Speed and distance
Manipulated variable	Number of books (speed)	Number of books (speed)
Responding variable	Time taken to reach the cardboard	Distance the toy car travels
Constant variables	Length of ramp, same marble	Same toy car, same time, same ramp
Conclusion	The faster the speed, the shorter the time	The faster the speed, the farther the distance

To measure the distance more accurately, read the metre ruler from straight above, repeat the test and take an average.

Calculating speed

Speed = Distance ÷ Time. Speed is the distance an object travels within a fixed time.

An electric scooter at positions A, B and C after 1 s, 2 s and 3 s, having gone 2 m, 4 m and 6 m — The scooter goes 2 m, 4 m and 6 m in 1 s, 2 s and 3 s. Each time, 2 ÷ 1 = 4 ÷ 2 = 6 ÷ 3 = 2 m/s.

Picture: Science Year 6 (DLP) textbook, p. 119.

The speed is the same at every position, so the scooter moves at a steady 2 m/s: in every second it covers the same 2 m.

Using the formulae

To find	Formula	Check the unit
Speed	Speed = Distance ÷ Time	Unit of distance per unit of time
Distance	Distance = Speed × Time	Time must be in the unit used in the speed
Time	Time = Distance ÷ Speed	Distance must be in the unit used in the speed

Example 1: find the speed

A train travels 250 km in 2 hours. Speed = 250 km ÷ 2 h = 125 km/h.

Example 2: find the distance

A hoverboard goes at 5 m/s for 1 minute. Change the time first: 1 minute = 60 s. Distance = 5 m/s × 60 s = 300 m.

Example 3: find the time

A car goes 10 km at 50 km/h. Time = 10 km ÷ 50 km/h = 0.2 h. Then 0.2 × 60 = 12 minutes.

Make the units match before you calculate. If the speed is in m/s, the time must be in seconds and the distance in metres. To change hours to minutes multiply by 60. To change minutes to seconds multiply by 60 again. To change km to m multiply by 1000.

Remember it as D over S and T: D is on top, S and T sit side by side below. Distance = S × T. Speed = D ÷ T. Time = D ÷ S.

A snail moves at 5 cm/s. How long does it take to move 100 cm?

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Time = 100 cm ÷ 5 cm/s = 20 s.

A lorry has a speed of 0 km/h. Is it moving?

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No. It is stationary. It covers no distance as time passes.

Operational definition of speed

In the "My speed" activity, a runner runs 200 m and stopwatches are set at 50 m, 100 m, 150 m and 200 m. In a steady run the time at each station doubles when the distance doubles, so the speed stays the same. For example, 50 m in 10 s and 100 m in 20 s both give 5 m/s (sample numbers, not measured).

Operational definition

Speed is how many metres the runner travels in one second, found by dividing the distance by the stopwatch time at each station.

If the runner keeps the same pace for 400 m, the speed is predicted to stay the same, because the runner's pace is steady. In real life, a runner may tire and slow down.

Key points

Speed is how fast an object moves from one place to another.
Units of speed: cm/s, m/s, km/h.
Faster means a shorter time for a fixed distance, or a longer distance in a fixed time.
Speed = Distance ÷ Time; Distance = Speed × Time; Time = Distance ÷ Speed.