A truck running at 90, km h ^{-1} is brought to rest over a distance of 25, m . Calculate retardatio

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7. MOTION

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A truck running at $90\, km h ^{-1}$ is brought to rest over a distance of $25\, m$. Calculate the retardation and time for which brakes are applied.

Option A

Option B

Option C

Option D

Solution

$u=90 km h ^{-1}=25 m s ^{-1} ; v=0 ; S =25 m ; a=?$

$t=?$

$(i)$ Applying $v^{2}-u^{2}=2 a S$

$(0)^{2}-(25)^{2}=2 \times a \times 25$

$\Rightarrow$ $a=-\frac{625}{50}=-12.5 m s ^{-2}$

Therefore, retardation $=-a=12.5 m s ^{-2}$

$(ii)$ Applying $v=u+a t$

$0=25-12.5 \times t$

$12.5 t=25 \quad$ or $\quad t=2 s$

Standard 9

Science

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