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Homework Help: Help w/ Problem

  1. Sep 12, 2007 #1
    1. The problem statement, all variables and given/known data

    When jumping, a flea reaches a takeoff speed of 1.1 over a distance of 0.49 .

    How long does the acceleration phase last?
    What is the flea's acceleration during the jump phase?
    If the flea jumps straight up, how high will it go? (Ignore air resistance for this problem; in reality, air resistance plays a large role, and the flea will not reach this height.)

    2. Relevant equations

    How do i get started

    3. The attempt at a solution

  2. jcsd
  3. Sep 12, 2007 #2
    What are the units on your given numbers?
  4. Sep 12, 2007 #3
    My bad .49mm and 1.1m/s
  5. Sep 12, 2007 #4
    The flea cannot accelerate once he's off the ground, because there are no forces being applied. All you will be able to calculate is the average acceleration for the distance given. That's a simple d = vt, v = at, situation. Then the second part, I'm not too sure right now. There is something that's making me think that the acceleration will actually be negative. But I know that the flea is accelerating at more than 9.8, and the resultant is what the average acceleration you calculated is.
  6. Sep 15, 2007 #5


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    Homework Helper

    Actually, it's easier from the information given to solve the second question first and then do the first one. (In textbook or exam questions with multiple parts, keep in mind that you are never *required* to answer the parts in the order given -- occasionally, it may even be impossible to do so...)

    For the second question, you will want to use the "velocity-squared" equation: vf^2 = vi^2 + (2 a L) to solve for a ; we're assuming for the jump that the flea started from rest. [Don't forget to put your values in compatible units!] Once you know the (assumed) constant acceleration of the jumping flea, you can use the equation for velocity to find out how long it takes the flea to reach a speed of 1.1 m/sec. (The result will make it clear why it's so hard to catch fleas...)

    Once the flea is in flight, gravity is taken to be the only force acting on it. You can again use the "velocity-squared" equation with the initial speed being vertically upward and the acceleration g due to gravity being vertically downward to find the vertical distance over which the flea's jump will take it. (What is the final velocity, that is, the velocity at the top of the jump?).

    The only part I'm unclear on is whether they want you to take the vertical distance over which the flea decelerates during the jump and tack on the 0.49 mm of the acceleration phase (though that will turn out to be small in comparison).
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