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CBSE Class 9 Science Question 3 of 30

Motion — Question 3

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Question 3

Abdul, while driving to school, computes the average speed for his trip to be 20 kmh-1. On his return trip along the same route, there is less traffic and the average speed is 30 kmh-1. What is the average speed for Abdul's trip?

Answer

Distance travelled to reach the school = distance travelled to reach home = d

Time taken to reach school = t1

Time taken to reach home = t2

Average speed = total distance travelledtotal time taken\dfrac{\text{total distance travelled}}{\text{total time taken}}

On going to school :

20 = dt1\dfrac{\text{d}}{\text{t}_1} or

t1 = d20\dfrac{\text{d}}{20}

On return trip :

30 = dt2\dfrac{\text{d}}{\text{t}_2} or

t2 = d30\dfrac{\text{d}}{30}

Now, the average speed for the entire trip

= d+dt1+t2\dfrac{\text{d+d}}{\text{t}_1 + \text{t}_2}

= 2dd20+d30\dfrac{\text{2d}}{\dfrac{\text{d}}{20} + \dfrac{\text{d}}{30}}

= 2d3d + 2d60\dfrac{\text{2d}}{\dfrac{\text{3d + 2d}}{60}}

= 2d×605d\dfrac{\text{2d} \times {60}}{5\text{d}}

= 24 kmh-1

∴ Abdul's average speed for the entire trip is 24 kmh-1.