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2. motion in straight line — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysics2. motion in straight line1 Mark
MCQ
A man swimming down stream overcome a float at a point $M$. After travelling distance $D$ he turned back and passed the float at a distance of $D/2$ from the point $M$, then the ratio of speed of swimmer with respect to still water to the speed of the river will be
  • A
    $2$
  • ✓
    $3$
  • C
    $4$
  • D
    $2.5$
✓

Answer

Correct option: B.
$3$
b
swimming downstream with distance $D$ with speed $= s+r$

swimming downstream with distance $\frac D2$ with speed $= S-f$

by float $=\frac {Dr} 2$

total time,

$\mathrm{D} / \mathrm{s}+\mathrm{r}+\mathrm{D} / 2(\mathrm{s}-\mathrm{r})=\mathrm{D} \mathrm{r} / 2$

$1 / \mathrm{s}+\mathrm{r}+1 / 2(\mathrm{s}-\mathrm{r})=\mathrm{r} / 2$

by solving above equations we get

$\frac Sr=3$

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