Concepts

Class 9
Chapter 8 Class 9 - Motion

## What is Average Speed?

It is the total distance travelled divided by total time taken.

Average Speed = Total Distance/Total Time Taken

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## Questions

NCERT Question 3 - Abdul, while driving to school, computes the average speed for his trip to be 20 km hβ1. On his return trip along the same route, there is less traffic and the average speed is 30 km hβ1. What is the average speed for Abdulβs trip?

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NCERT Question 10 - An artificial satellite is moving in a circular orbit of radius 42250 km. Calculate its speed if it takes 24 hours to revolve around the earth

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Example 8.1 - An object travels 16 m in 4 s and then another 16 m in 2 s. What is the average speed of the object?

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Example 8.2 - The Odometer of a car reads 2000 km at the start of a trip and 2400 km at the end of the trip. If the trip took 8 h, calculate the average speed of the car in km h β1 and m s -1 .

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### Transcript

Example 1 Suppose a truck travels 60 Kilometer in first hour and 80 Kilometer in Second hour. What is Average Speed of Truck? Total Distance Travelled = 60 + 80 = 140 km Total Time Taken = 1 hour + 1 hour = 2 hour Average Speed = (πππ‘ππ π·ππ π‘ππππ)/(πππ‘ππ ππππ) = 140/2 =70 km/hour Example 2 Suppose a bus travels first 240 km a a speed of 80 km/hour It covers next 360 km at speed of 90 km/hour What is its average speed? For First 240 Km We know that Speed = π·ππ π‘ππππ/ππππ 80 = 240/ππππ 80 Γ Time = 240 Time = 240/80 Time = 3 hours For Next 360 km We know that Speed = π·ππ π‘ππππ/ππππ 90 = 360/ππππ 90 Γ Time = 360 Time = 360/90 Time = 4 hours Calculating Average Speed Total Distance Travelled = 240 + 360 = 600 km Total Time Taken = 3 hour + 4 hours = 7 hours Average Speed = (πππ‘ππ π·ππ π‘ππππ)/(πππ‘ππ ππππ) = 600/7 = 85.71 km/hr β΄ Average Speed = 85.71 km/hr Example 3 Total Journey is of 600 km. A bus travels first 120 km of trip at speed of 40 km per hour. What should be speed for next 480 km so that average speed is 60 km/hour? For First 120 Km We know that Speed = π·ππ π‘ππππ/ππππ 40 = 100/ππππ 40 Γ Time = 120 Time = 120/40 Time = 3 hours For Next 480 km We know that Speed = π·ππ π‘ππππ/ππππ 60 = 480/ππππ 60 Γ Time = 480 Time = 480/60 Time = 8 hours Calculating Average Speed Total Distance Travelled = 120 + 480 = 600 km Total Time Taken = 3 hour + 8 hours = 11 hours Average Speed = (πππ‘ππ π·ππ π‘ππππ)/(πππ‘ππ ππππ) = 600/11 = 54.54 km/hr β΄ Average Speed = 54.54 km/hr