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A bus moving along a straight highway with speed of $72 \mathrm{~km} / \mathrm{h}$ is brought to halt within $4 s$ after applying the brakes. The distance travelled by the bus during this time (Assume the retardation is uniform) is _____m.
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40
$\begin{aligned} & \text { Initial velocity }=u=72 \mathrm{~km} / \mathrm{h}=20 \mathrm{~m} / \mathrm{s} \\ & \mathrm{v}=\mathrm{u}+\text { at } \\ & \Rightarrow 0=20+\mathrm{a} \times 4 \\ & \mathrm{a}=-5 \mathrm{~m} / \mathrm{s}^2 \\ & \mathrm{v}^2-\mathrm{u}^2=2 \text { as } \\ & \Rightarrow 0^2-20^2=2(-5) . \mathrm{s} \\ & \mathrm{s}=40 \mathrm{~m}\end{aligned}$
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