Page 130 - ICSE Math 7

Average Speed

Usually a moving object does not cover a given distance with the same speed. Sometimes it picks up
speed and sometimes it slows down. So, average speed is calculated by dividing the total distance
covered by the total time taken.
Total distance covered
⇒ Average speed =
Total time taken
–1
Example 4: A car covers the first 75 km at the speed of 45 km h and the next 25 km at the speed of
–1
60 km h . Calculate its average speed for the whole journey.
75 5
Solution: Time taken to cover 75 km = = h
45 3
25 5
Time taken to cover 25 km = = h
60 12
5 5 20 + 5 25
Total time taken = + h = h = h
3 12 12 12

Total distance travelled = (75 + 25) km = 100 km
Total distance covered 100 100 × 12
\ Average speed = = = = 48 km h –1
Total time taken 25/12 25

Example 5: A cyclist covers a distance of 16.8 km between points A and B in 1 hour 20 minutes.
The road from point A is in level for 11.2 km and then it is uphill to B. The average
–1
speed of the cyclist on the level road is 12 km h . Find his average uphill speed.
Solution: On the level road: Distance = 11.2 km, speed = 12 km h –1
Distance 11.2 14
Time taken = = = h
Speed 12 15
Distance for the uphill road = (16.8 – 11.2) km = 5.6 km

4 14 6 2 1 4
Time taken = – h = h = h 1 h 20 min = 1 + h = h
3 15 15 5 3 3
Distance covered
\ Uphill speed of the cyclist = Time taken
5.6
= km h –1
2/5

–1
= 5.6 × 5 km h = 14 km h –1
2
EXERCISE 11.1

–1
1. A truck covers a distance of 40 km in 30 min. Find the speed of the truck in km h .
–1
2. In how much time can a distance of 57.6 km be covered at a speed of 8 m s ?
3. A train travels a distance of 165 km between two stations in 10 min. Determine the:
–1
(a) speed of the train in km h . (b) time taken to cover a distance of 495 km.
4. A person travels a total distance of 360 km by a car. He covers the first half at a speed of
–1
–1
60 km h and the other half at 45 km h . Find:
(a) the total time taken to complete the entire journey.
(b) average speed of the car.

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