A driver in a car traveling at a speed of 21.8 m/s sees a car 101 m away on the road. How long will it take for the car to uniformly decelerate to a complete stop in exactly 99 m?

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Answer 1 · 2018-04-05T02:20:52

$9.1s$

Since we know that velocity is uniformly decelerating, we can take the average velocity:

$(V_i + V_f)/2$

Letting $V_f$ equal 0 m/s, we get $1/2*21.8m$ / $s$ or $10.9m$ / $s$ .

Now we know $V = d/t$ and rearranging this gives $t = d/V$ .

Substitute our velocity of 10.9m/s for V and 99m for d:
$t=(99m)/(10.9m*s^-1)$ (Sorry for the $s^-1$ but otherwise I couldn't write m/s)

Finally we get $t=9.1s$ (with significant figure rounding).

Hope this helps!

Cheers