A person goes from point P to point Q covering 1 / 3 of distance with speed 10 ,km / h , next 1 / 3

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2.Motion in Straight Line

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A person goes from point $P$ to point $Q$ covering $1 / 3$ of the distance with speed $10 \,km / h$, the next $1 / 3$ of the distance at $20 \,km / h$ and the last $1 / 3$ of the distance at $60 \,km / h$. The average speed of the person is ............ $km / h$

A

$30$

B

$24$

C

$18$

D

$12$

(KVPY-2017)

Solution

(c)

Let total distance is $3 x \,km$.

For first part, time taken is $t_1=\frac{x}{10} \,h$

For second part, time taken is $t_2=\frac{x}{20} \,h$

For third part, time taken is $t_3=\frac{x}{60} \,h$

Average speed for $P Q$ distance

$=\frac{3 x}{\left(\frac{6 x+3 x+x}{60}\right)}=18 \,km / h$

Standard 11

Physics

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