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Homework Help: KInetics sum please help

  1. Sep 8, 2008 #1
    Pelicans tuck their wings and free-fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 20.0 m and cannot change its path once committed.If it takes a fish 0.15 s 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.
  2. jcsd
  3. Sep 8, 2008 #2
    bump please help
  4. Sep 8, 2008 #3
    I am a bit rusty with these types of problems, but i believe that i have it.

    Where you would start, is to find out how long it would take the pelican to get to the water, regardless of fish.

    change in X=20m
    acceleration=freefall, or 10m/s2 (its 9.8 to be exact, but im' just using 10)
    starting Velocity=0

    so change in X=Vot+.5At2

    so, you get the time it takes the pelican to reach the water by solving that. Now since the fish can evade in .15 seconds, take the time yu found in the previous problem and subtract .15 from it. This will give you the time that it takes for the pelican to get to .15 seconds away.

    Now that you have the time it takes for the pelican to get within evading distance, plug THIS time back into the change in position equation to get the distance from the fish the pelican can be with the fish still being safe.

    I would like you to go back through and do the steps, but to check your answer, the answer that i ended up with is 2.8875m.

    Good luck in physics!
  5. Sep 8, 2008 #4
    Hey thanks a million! i perfectly understood it....i worked it out the way u said and it came upto 2.86 (since i used 9.8m/s2)...so i checked it and it was the right answer..thanks alot..
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