The displacement of a body at any time $t$ after starting is given by $s = 15 t - 0.4 t^{2}$ . Find the time when the velocity of the body will be $7 m s^{- 1}$ .

t = 20 s

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t = 10 s

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t = 30 s

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t = 40 s

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Solution

The correct option is B $t = 10 s$
$S = 15 t - 0.4 t^{2}$

$\frac{d s}{d t} = 15 - 0.8 t$

$ϑ = \frac{d s}{d t}$ so,

$ϑ = 15 - 0.8 t$

Given, $ϑ = 7$

$7 = 15 - 0.8 t$

$t = 10$ sec

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