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A car is moving along a straight line, say $OP$ in given figure. It moves from $O$ to $P$ in $18\; s$ and returns from $P$ to $\mathrm{Q}$ in $6.0\; s$. What are the average velocity and average speed of the car in going from $O$ to $P$ and back to $Q?$
$20\; \mathrm{m} \mathrm{s}^{-1}\;,\;20\; \mathrm{m} \mathrm{s}^{-1}$
$10\; \mathrm{m} \mathrm{s}^{-1}\;,\;20\; \mathrm{m} \mathrm{s}^{-1}$
$20\; \mathrm{m} \mathrm{s}^{-1}\;,\;10\; \mathrm{m} \mathrm{s}^{-1}$
$30\; \mathrm{m} \mathrm{s}^{-1}\;,\;10\; \mathrm{m} \mathrm{s}^{-1}$
Solution
Average veloctty $=\frac{\text { Displacement }}{\text { Time interval }}=\frac{+240 \mathrm{m}}{(18+6.0) \mathrm{s}}$
$=+10 \mathrm{ms}^{-1}$
$\text { Average speed }=\frac{\text { Path length }}{\text { Time interval }} =\frac{\mathrm{OP}+\mathrm{PQ}}{\Delta t}$
$=\frac{(360+120) \mathrm{m}}{24 \mathrm{s}}=20 \mathrm{m} \mathrm{s}^{-1}$
