Speed, Distance and Time
Primary 6 · Mathematics
Speed, Distance and Time
Lesson 4 of 5
15 min
Let's begin
The PE teacher timed Ishimwe and found that he sprints at 5 m/s. His cousin says, "That's nothing. A moto in town drives at 18 km/h." Who is faster? You cannot tell yet, because one speed is in metres per second and the other is in kilometres per hour. You need to change one of them.
Change the kilometres into metres (× 1 000) and the hours into seconds (÷ 3 600).
Worked example
Change 72 km/h into m/s.
72 km in 1 h = 72 000 m in 3 600 s.
In 1 s: 72 000 ÷ 3 600 = 20 m.
So 72 km/h = 20 m/s.
Because × 1 000 ÷ 3 600 is the same as ÷ 3.6, there is a shortcut: divide by 3.6. For example, 90 km/h = 90 ÷ 3.6 = 25 m/s. (You can also multiply by 518: 90 × 5 ÷ 18 = 25.)
Do the opposite: multiply by 3.6.
Worked example
Change 5 m/s into km/h.
In 1 h there are 3 600 s, so 5 m/s means 5 × 3 600 = 18 000 m in one hour.
18 000 m = 18 km, so 5 m/s = 18 km/h. The shortcut gives the same: 5 × 3.6 = 18.
So Ishimwe (5 m/s) and the moto (18 km/h) are moving at exactly the same speed!
| km/h | 18 | 36 | 54 | 72 | 90 | 108 |
|---|---|---|---|---|---|---|
| m/s | 5 | 10 | 15 | 20 | 25 | 30 |
Worked example
A train moves at 72 km/h. How far does it go in 30 seconds?
72 km/h = 20 m/s. Distance = 20 × 30 = 600 m.
Worked example
Which is faster, 20 m/s or 70 km/h?
20 m/s = 20 × 3.6 = 72 km/h, which is more than 70 km/h. So 20 m/s is faster.
Tip
km/h → m/s: divide by 3.6. m/s → km/h: multiply by 3.6. Check yourself with the easy pair: 36 km/h = 10 m/s. The number in m/s is always the smaller one.
Answers
What you learned today
Sources and credits
Written by Ganzaa.
Quick check
Change 54 km/h into m/s.
