Slow or Fast: Speed

Class 7 Science · Chapter 8: Measurement of Time and Motion · NCERT Curiosity section 8.2–8.3

Learn

In a 100 m race, Ishaan, Kabir and Neel start together. After a few seconds Kabir is ahead. In the same time he has covered more distance, so he is faster. But how do we compare a 100 m dash with a 400 m run? We find the distance covered in unit time: the speed.

Speed = total distance ÷ total time. Its SI unit is metre per second (m/s); on roads we use kilometre per hour (km/h). Zoya cycles 3.6 km to school in 15 min: 3600 m ÷ 900 s = 4 m/s. Since 1 km/h = 1000 m ÷ 3600 s, multiply km/h by 5/18 to get m/s, and m/s by 18/5 to get km/h.

speed = distance ÷ time

Rearrange to find the others: distance = speed × time and time = distance ÷ speed. A bus at 50 km/h for 2 h goes 100 km; a train at 90 km/h needs 4 h for 360 km. A vehicle's speedometer shows speed in km/h and its odometer shows the distance travelled.

distance = speed × time; time = distance ÷ speed

Remember

• Speed = distance ÷ time; units m/s or km/h (× 5/18 to change km/h to m/s).
• Distance = speed × time; time = distance ÷ speed.

Quick check

A car travels 180 km in 3 h. What is its speed?

1. 60 km/h
2. 540 km/h
3. 183 km/h
4. 6 km/h

Think about it

A horse gallops at 18 m/s and a train runs at 72 km/h. Which is faster?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.