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Kinetic energy review for final! Please help!

  1. Dec 17, 2008 #1
    1. The problem statement, all variables and given/known data
    Fleas have remarkable jumping ability. If a 0.50 mg flea jumps straight up, it will reach a height of 36 cm if there is no air resistance. In reality, air resistance limits the height to 18 cm.


    2. Relevant equations
    K=1/2 m*v^2



    3. The attempt at a solution
    I tried to use kinematics to get a delta T, and the program is still saying im wrong! The final is tomorrow and this is a practice problem, can someone please explain how to do this! thanks!
     
  2. jcsd
  3. Dec 17, 2008 #2
    What is the problem?? Is it asking for how much energy is used up by air resistance?
     
  4. Dec 17, 2008 #3
    I'm sorry! The problem wants to know:
    a) what the flea's kinetic energy is as it leaves the ground?
    b) at its highest point, what fraction is converted to potential energy?
     
  5. Dec 17, 2008 #4
    I'm assuming "a) what the flea's kinetic energy is as it leaves the ground?" is for
    "Fleas have remarkable jumping ability. If a 0.50 mg flea jumps straight up, it will reach a height of 36 cm if there is no air resistance."

    and "b) at its highest point, what fraction is converted to potential energy?" is for "In reality, air resistance limits the height to 18 cm."

    For a) energy is conserved. soo we pick the lowest point that has a gratational potential of zero. so we can use this

    [tex] \frac{1}{2}mv^2=mgh [/tex]
     
  6. Dec 17, 2008 #5
    Here is my problem though....the statement doesn't separate those two things. The problem reads exactly this: "Fleas have remarkable jumping ability. If a 0.50 mg flea jumps straight up, it will each a height of 36 cm if there is no air resistance. In reality, air resistance limits the height to 18 cm."
    a) what is the flea's kinetic energy as it leaves the ground?

    That's my issue, I dont know whether to use the 36 or the 18.
     
  7. Dec 17, 2008 #6

    LowlyPion

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    Homework Helper

    You use 36. That's what the problem is telling you when it says if ...
     
  8. Dec 17, 2008 #7
    Is there even a part b)? it would be pointless if you use point 36 for part b) because it would be 100%, of course.
     
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