1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

One Dimensional Motion Question

  1. Aug 29, 2012 #1
    1. The problem statement, all variables and given/known data
    Pelicans tuck their wings and free-fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 18.0m and cannot change its path once committed. If it takes a fish 0.25s to perform evasive action, at what minimum height must it spot the pelican to escape? Assume the fish is at the surface of the water. (Answer must be in 2 significant figures.)


    2. Relevant equations
    X=vot+1/2at^2


    3. The attempt at a solution
    I tried to go about solving this but it seems im still getting the wrong answer.

    x=vot+1/2at^2
    18=(1/2)(9.81)t^2
    18=4.9(t)^2
    sqrt(3.7)=t^2
    t=1.9s
    1.9s-.25s=1.65s

    x=1/2(9.81)(1.7)^2
    x=4.9(1.7)^2
    x=14m
    x=18m-14m=4m
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 29, 2012 #2
    It looks ok to me except for significant figures.
     
  4. Aug 30, 2012 #3
    Tried 4.0 tells me I got the wrong answer.
     
  5. Aug 30, 2012 #4
    I'll try giving this problem a shot.

    We know that the initial height is 18m, and the final height for when the pelican swoops down is 0m.
    From the equation Δ y= VoyT + .5gT2, we have to plug Yf as 0 m, Yi as 18m,and Voy as 0m/s.

    -18m= .5gT2

    √[-18/(.5g)]=T=1.9

    (pelican to catch fish)-(time for fish to react)
    1.9s-.25s=1.65
    Yf-Yi= .5gT2

    Yf-18= .5gT2
    Yf=.5g(1.65)2+18=4.6m

    So,it needs to see the pelican at 4.6m.

    Sorry if I didn't write my English well enough.
     
    Last edited: Aug 30, 2012
  6. Aug 30, 2012 #5
    One of my previous answers I entered was 4.7m. A slight discrepancy of about .1m and it still tells me I'm wrong so not exactly sure why it's telling me that. The concept and mathematics is suppose to work.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: One Dimensional Motion Question
  1. One dimensional motion (Replies: 2)

Loading...