The driver of a car slams on the brakes when he sees a treeblocking the road. Th

Question

The driver of a car slams on the brakes when he sees a treeblocking the road. The car slows uniformly with acceleration-5.80 m/s2 for 4.10 s, making straight skid marks 63.5 m long ending at the tree. With what speed doesthe car then strike the tree?
I understand how to do this problem. What I do no understandis that the formula they use, acceleration is positive, when it'sstated as negative in the problem. It is P25 of the text I've listed. My question is why is the acceleration positive in theequation, and not negative. Thanks!
The driver of a car slams on the brakes when he sees a treeblocking the road. The car slows uniformly with acceleration-5.80 m/s2 for 4.10 s, making straight skid marks 63.5 m long ending at the tree. With what speed doesthe car then strike the tree?
I understand how to do this problem. What I do no understandis that the formula they use, acceleration is positive, when it'sstated as negative in the problem. It is P25 of the text I've listed. My question is why is the acceleration positive in theequation, and not negative. Thanks!

Explanation / Answer

If the acceleration is positive, maybe the velocity in the answerwas negative ? At the end it's all convention, if you take onequantity as positive, you have to take any quantity with theopposite direction as negative. let's say the initial velocity is u. a=-5.8m/s2 S=ut+1/2at2 63.5=u*4.1-0.5*5.8*4.12solving for u, u=27.38 m/s v=u+at =27.38 -5.8*4.1 =3.6 m/s

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