I was driving along at 20 m/s, trying to change a CD and not watching where I wa

Question

I was driving along at 20 m/s, trying to change a CD and not watching where I was going. When I looked up, I found myself 48 m from a railroad crossing. And wouldn't you know it, a train moving at 30 m/s was only 60 m from the crossing. In a split second, I realized that the train was going to beat me to the crossing and that I didn't have enough distance to stop. My only hope was to accelerate enough to cross the tracks before the train arrived. If my reaction time before starting to accelerate was 0.50 s, what minimum acceleration did my car need for me to be here today writing these words? Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

Time taken by train to cross the crossing

T=60/30 s=2 s

So including reaction time driver must have to cross the crossing before 2 s

tr =0.5 s

t=1.5 s

So

S= utr + ut +1/2 at2

48= 20*(0.5) + 20*(1.5) +0.5*a*(1.5)^2

48=40+1.125 a

a= 8/1.125= 7.11m/s2

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