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Finding acceleration of flea using force analysis

  1. Sep 21, 2011 #1
    1. The problem statement, all variables and given/known data

    A flea jumps by exerting a force of 1.20x10-5 N straight down on the ground. A
    breeze blowing on the flea parallel to the ground exerts a force of 0.500x10-6 N on the
    flea. Find the direction and magnitude of the acceleration of the flea if its mass is
    6.00x10-7 kg. Do not neglect the force of gravity.

    2. Relevant equations

    F = ma

    F= sum of Fx + sum of Fy

    weight = mg

    3. The attempt at a solution

    I figured the normal force exerted on the flea is flea's weight plus the force it exerts on the ground when it jumps. So, I used weight = mg to find the flea's weight and added it to 0.500x10-6N which equaled 1.79x10-5N. I then thought the only horizontal force was the force of the wind so that's what the sum of Fx equals. The sum of Fy equals the normal force minus the weight of the flea (which is 5.88X10-6N). I then added Fx and Fy, which equals 1.25x10-5N, to get F and set it equal to ma. I then divided 1.25x10-5N by the mass of the flea to get the acceleration. Is this right? The magnitude of acceleration I got seems high (20.8 m/s^2). Also, how do I determine the direction of acceleration?
     
  2. jcsd
  3. Sep 21, 2011 #2

    PeterO

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    Homework Helper

    You are trying to find the acceleration of the flea, so should not include the force the flea exerts on the ground, but the force the ground exerts on the flea [Newtons 3rd Law: action Reaction pair]. That force is up, the weight force is down, so they do not just arithmetically add together.

    For the Direction you use Trig.
     
  4. Sep 21, 2011 #3

    gneill

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    Staff: Mentor

    Perpendicular vector components don't add as a simple sum. They add in quadrature. That is, you take the square root of the sum of the squares of the components. So where you say you added Fx and Fy, that is an incorrect operation.

    Instead, calculate the vertical and horizontal accelerations separately from their respective net forces. Sum the acceleration component as the square root of the sum of squares to find the magnitude of the net acceleration. The direction of the acceleration is computed from the components, too: Use the arctan() function appropriately.
     
  5. Sep 22, 2011 #4

    PeterO

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    Homework Helper

    BTW, I have seen many a "nature" program that claims that a flee has the greatest acceleration of and animal/insect on the planet, so are you sure that 20.8 ms-2 is out of the question?
    Did you use Pythagorus during your calculations, because you should have.
     
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