# Help 1D kinematics problem

A motorist is traveling at 18 m/s^2 when he sees a deer 38 m ahead. If the maximum negative acceleration is -4.5 m/s^2, what is the reaction time (delta t) of the motorist that will allow him to avoid hitting the deer? Answer in units of 's'.

I tried using x(t)=x0+v0t+1/2at^2 and it didn't work out for me. I know the answer is supposed to be 0.1111 s and I can't get this no matter what I try. help!

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mjsd
Homework Helper
trying using v = u +at and s = 0.5( u+v) t
and 0.1111s actually is 1/9 or 2/18 or ....

Mjsd...that makes sense to me except I don't understand why 's' is equal to '1'?? Also, in the s = 0.5 (u+v)t equation I would think that acceleration should be used somewhere in the equation since that is how fast it is decelerating.
My original idea was the use the equation I mentioned above and then use the quadratic equation to get the answer, but that didn't work out so well.

ranger
Gold Member
Please dont double, triple post the same question.

mjsd
Homework Helper
firstly, let's remember that there are many ways to do the same problem. My way is to do it in two steps. By the way the equations I listed are correct, the second one is just: distance travelled = av. speed x total time.

Logic: how long does it take to stop from initial speed assuming the full $$-4.5m/s^2$$, then in that time how far the vehicle will travel, then what's left for reaction time? etc....