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Subjects
Science CBSE
Class 9
Motion
Textbook questions

3. Exercises III

Exercise condition:

5 m.

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?

The formula to calculate the average speed,

$=\ \frac{Total\ distance\ travelled}{Total\ time\ taken}$
$=\ \frac{Total\ displacement}{Total\ time\ taken}$
$=\ {Total\ displacement} \times {Total\ time\ taken}$
$=\ {Total\ distance\ travelled} \times {Total\ time\ taken}$

Consider,

Distance travelled to reach the school

$=$ Distance travelled to reach home

$=$

$d$

Time taken to reach school from home

$=\ t_1$

Time taken to reach home from school

$=\ t_2$

Average speed of Abdul while going to school,

$=\ {d} \times {t_1}$
$=\ \frac{d}{t_2}$
$=\ \frac{d}{t_1}$
$=\ {d} \times {t_2}$

Average speed of Abdul while returning to home,

$=\ {d} \times {t_2}$
$=\ \frac{d}{t_1}$
$=\ {d} \times {t_1}$
$=\ \frac{d}{t_2}$

Abdul’s average speed for the entire trip

$=$

$kmh^{-1}$

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