Bob N Posted May 26, 2008 Posted May 26, 2008 I'm doing this question for physics, and I'm a bit stuck. The question is: A hotel elevator ascends 200m with maximum speed of 5 m/s. Its acceleration and deceleration both have a magnitude of 1.0 m/s^2 a) How far does the elevator move while accelerating to full speed from rest? b) How long does it take to make the complete trip from bottom to top? Part a I did Vf^2 = Vi^2 +2aΔx 25 = 0 + 2(1)Δx 25/2 = Δx = 12.5 m (and I got the answer right) now part b I tried a couple of ways, I tried separating the problem after 12.5 m, so x(t) = (.5 a t^2) + Vit + Xi 200 = .5t^2 + 5t + 12.5 So quadratic is .5t^2 +5t -187.5 = 0 t = 15 s I added 5 s on top of that, since that is the time to the 12.5 m, from Vf = Vi + a t 5 = 0 + 1(t) but 20 s is apparently the wrong answer. I tried treating the whole problem as one part, like 200 = .5at^2 + Vit + Xi 200 = .5t^2 + 0 + 0 400 = t^2 t=20 s what am I doing wrong?
Klaynos Posted May 26, 2008 Posted May 26, 2008 Seperating is the way to go. You need to split it into 3 parts. How long does it take to accelerate? How long does it take to travel Xm at the top speed? How long does it take to decelerate? And add all 3 together. I've not given you X because I guess that's a a place you're likely to make a mistake and therefore something you should think about.
swansont Posted May 26, 2008 Posted May 26, 2008 And pay attention to the sign of the acceleration term for part 3.
Bob N Posted May 26, 2008 Author Posted May 26, 2008 I was thinking the halfway point, but that leads me to another wrong answer. I can't see any other interesting points where that would happen
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