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How to convert KE into potential gravitational energy?

  1. Dec 5, 2015 #1
    Here is the question with two parts

    The fastest that a human has run is about 12 m/s.
    a) If a pole vaulter could run this fast and convert all of his or her kinetic energy into gravitational potential energy, how high would he or she go? 7pts
    b) Using the 1990 pole vault world record of 20 ft. Find the initial speed in m/s needed for the pole vaulter to reach this height. 6pts
    (This is Problem 24 of chapter 3.)

    Now I know how to get the KE= 1/2md2. Now I am really confused because the question isn't giving me the mass or anything to work with? How do you even go about starting to solve this problem?

    On top of that I have no idea how to solve for gravitational potential energy.

    Haven't even tried to start the second part b) yet.

    I am not looking to get spoon fed an answer I simply need a little guidance for what I am missing. Doing an online class so it's not easy to sit down with the instructor.

    Thank you for your time.
     
  2. jcsd
  3. Dec 5, 2015 #2

    SammyS

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    Hello Byron Anderson. Welcome to PF !

    Use m for the mass. It will cancel out of the equations.

    Are you sure you haven't covered gravitational potential energy? Maybe it was simply called potential energy.
     
  4. Dec 5, 2015 #3
    If potential energy is the same yes we have. I guess I just didn't get a good hold on it. I will take a look back at potential energy right now and try again.

    When you say use m for mass and it will cancel out what does that mean? How can I solve the equation if I still have m as a variable?
     
  5. Dec 5, 2015 #4
    Maybe I am asking the wrong question. If PE= work done = weight x distance raised how do I get all the needed variables to solve this?
     
  6. Dec 5, 2015 #5

    SammyS

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    For a mass of m, the weight (force due to gravity) is mg. Right?

    So to lift an object to a height h, requires doing an amount of work = mgh against gravity.
     
  7. Dec 5, 2015 #6
    I don't know to make sense out of any of that with just the runner can go 12 m/s? So I know their velocity what about the mass or height or anything else to help me solve it? Seems the question is missing something?
     
  8. Dec 5, 2015 #7

    SammyS

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    The question is missing nothing.

    Have you learned at all about potential energy?
     
  9. Dec 5, 2015 #8
    I guess not

    I was looking for some direction or help I have these formulas to read in my book seeing them again isn't helping. I am confused on how to plug in this 12 m/s to get an answer?
     
  10. Dec 5, 2015 #9

    SammyS

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    Earlier, you mentioned something about Work. What do you know about work?

    Also, for an object of mass, m, and speed, v, what is the kinetic energy?
     
  11. Dec 5, 2015 #10
    work = f/d right? I don't have either of those to use

    So I google it and might have found what I was looking for. I didn't see this anywhere in the text book but would height = v squared / 2g? SO 12x12 / 2x9.8? My height is 7.35?
     
  12. Dec 5, 2015 #11

    SammyS

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    Actually, Work = F⋅d .

    Energy can be though of as a property of an object which can be converted into work.

    Kinetic Energy, KE = (1/2)m⋅v2 , for mass, m, and speed, v .

    It takes an amount of work = m⋅g⋅h, to raise an object a height, h, g being acceleration due to gravity.

    ∴ (1/2)mv2 = mgh .

    The mass m cancels out and you can find height, h, as you did. What are the units for your answer?
     
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