A car starts moving rectilinearly, first with acceleration $α = 5 m s^{- 2}$ (the initial velocity is equal to zero) , then uniformly, and finally, decelerating at the same rate $α$ comes to a stop. The total time of motion equals t=25s. the average velocity during this time is equal to $< v > = 72 k m h^{- 1}$ . How long does the car move uniformly in sec.

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answer is 15.

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Detailed Solution

$V_{a v g} = \frac{72 \times 5}{18} = 20 m / s^{2}$

$20 = \frac{\frac{1}{2} (50 - 2 t) 5 t}{25}$

$t = 5 \sec o r 20 \sec$

$∴ t = 5$

The long us to which car moves $= 25 - 2 \times 5 = 15 \sec$

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