On seeing a speed limit road sign, you apply brakes that bring your car from the speed of $30 m/s$ to a speed of $20 m/s$ , covering a distance of $100 m$ at a constant deceleration. How much time is required for the given decrease in speed?

3 s

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4 s

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2 s

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5 s

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Solution

The correct option is B $4 s$
Initial speed, $u = 30 m/s$
Final speed, $v = 20 m/s$
Using third equation of motion, $v^{2} = u^{2} + 2 a s$
$a = \frac{(v^{2} - u^{2})}{2 s} = \frac{20^{2} - 30^{2}}{2 \times 100}$
$a = - 2.5 {m/s}^{2}$
Using first equation of motion, $v = u + a t$
$t = \frac{v - u}{a}$
$t = \frac{20 - 30}{- 2.5} = 4 sec$

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A truck on a straight road starts from rest, accelerating at $2 m s^{- 2}$ , until it reaches a speed of $20 m s^{- 1}$ . Then the truck travels at a constant speed for $20 s$ until the brakes are applied causing the truck to decelerate uniformly to stop it in additional $5 s$ .

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On seeing a speed limit road sign, you apply brakes that bring your car from the speed of 30 m/s to a speed of 20 m/s, covering a distance of 100 m at a constant deceleration. How much time is required for the given decrease in speed?

The correct option is B 4 sInitial speed, u=30 m/s Final speed, v=20 m/s Using third equation of motion, v2=u2+2as a=(v2−u2)2s=202−3022×100 a=−2.5 m/s2 Using first equation of motion, v=u+at t=v−ua t=20−30−2.5=4 sec

On seeing a speed limit road sign, you apply brakes that bring your car from the speed of $30 m/s$ to a speed of $20 m/s$ , covering a distance of $100 m$ at a constant deceleration. How much time is required for the given decrease in speed?