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Homework Help: Finding Initial Speed w/ Only One Variable Given

  1. Sep 7, 2011 #1
    1. The problem statement, all variables and given/known data
    If a flea can jump straight up to a height of .440 m, what is its initial speed as it leaves the ground? How long is it in the air?

    2. Relevant equations
    x = (1/2)at^2+Vo+xo
    V + at+Vo
    V^2 = Vo^2+2a(delta x)
    x-xo = ((Vo-V)/2)t

    It has to be one of these, since that's all we've gotten in class so far. Most likely it's some mixture of them, but I haven't figured out just the right mix yet.

    3. The attempt at a solution
    Since all I've been given in this question is the maximum height of the flea jump, I'm assuming that acceleration in any of the equations I use is 9.8 m/(s^2) since the flea is jumping up and going against gravity. Not knowing the amount of time the flea is in the air or how long it takes to reach the maximum height, I couldn't calculate velocity of any kind. To be honest, my attempt doesn't look like much of anything, mostly because I have no idea how to start the problem.

    This is my first physics class ever, and I'm really interested. It's just not clicking yet, so hopefully I can get over this block in my math and really understand what's going on. ^_^;
  2. jcsd
  3. Sep 7, 2011 #2


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    Welcome to PF Miki_Spazz!

    Since you don't have a time value given in the problem, the equation I've highlighted in red
    above is a natural choice. You have the distance travelled (Δx, or maybe Δy would be a more natural choice of symbol in this situation since x is usually used for the horizontal coordinate and y for the vertical). You also have been given the final velocity at the end (it may not seem like it, but think about it for a second). You are correct that the acceleration is indeed g.
  4. Sep 7, 2011 #3
    Thanks, glad I found this site.

    Wait. My final velocity is zero because it's hanging in midair, not going up or down, huh? Yup, my calculator just said so. Wow. I feel silly now. Thank you, though. :) I keep trying to make it harder than it should be. I'll probably be around here for the rest of the school year making goof ups like this.

    Haha, an hour spent thinking and two minutes punching numbers on the calculator. Hope the rest of my work doesn't wind up like this. [:
  5. Sep 7, 2011 #4


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    Yes, exactly. If the ball has reached its maximum height, it must have slowed to a stop at that instant. If it still had non-zero speed in the vertical direction, then it would have gone higher, and that wouldn't have been its maximum height.

    It did??? How?:tongue2: Or do you mean that when you assumed that the final velocity was zero and computed the result, you got the right answer (which has been given to you)?

    You're very welcome, and there's no need to feel silly. :smile: Physics is not hard -- it just requires thinking about things and approaching problems in a very different way from other subjects. The more practice you get, the better you'll get at that. By the way, we need more new members like you. Most people just come here expecting answers from other people who will do their work for them (which is against the rules). It's refreshing to see someone who is willing to learn and to do the work him/herself with guidance from us. That's what the homework help forums are for.
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