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Help with simple problem

  1. May 28, 2008 #1
    I was trying to do this physics problem which I thought is really damn
    easy but it won't ever count my answer right on webassign i just wanna
    see what other people get.

    A flower pot is knocked off a balcony 18.4 m above the sidewalk and
    falls toward an unsuspecting 1.76 m-tall man who is standing below. How

    close to the sidewalk can the flower pot fall before it is too late for

    a shouted warning from the balcony to reach the man in time? Assume
    that the man below requires 0.300 s to respond to the warning.
  2. jcsd
  3. May 28, 2008 #2
    This answer only provided as a reference. I'm no physics ace...
    Assuming speed of sound at sea level = 340.3 m/s

    There are 17.4m between the balcony and the man, so it takes D/T=R -> 17.4/340.3 = .05s for the sound to reach the (top of the) man. So you need to shout about 0.3s+.05s=0.35s before the pot reaches his head for him to be able to get out of the way in time (time for sound to reach his head and time for him to react.)

    The pot will be traveling at Vf^2=Vo^2+2*a*x -> ?^2 = 0+2*9.81*17.14 ->sqrt(336.29) = 18.33m/s. 0.35s before that, the pot will be traveling at 9.81m/s/s*0.35s=3.433m/s slower.

    So at the last possible second the pot will be traveling at 14.897m/s. Plugging that into Vf=Vi+A*T we get 14.897=0+9.81*t ->14.897/9.81 = 1.52 seconds. This is the amount of time you can let pass before the pot will inevitable strike the poor man.

    So after 1.52 seconds how far will the pot be above the pavement? (14.897^2-0^2)/(2*9.81) = distance from balcony = 11.32 m
    Which is 18.9-11.32 = 7.59m above the pavement.

    This problem was much harder than it looks. What did you get?
  4. May 28, 2008 #3
    Yea, close enough to what I got...7.51m. KBL-8, I think you used the wrong balcony height. It's 16.64m between the balcony and the mans head.
    Last edited: May 28, 2008
  5. May 28, 2008 #4
    Whops. Can't subtract.
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