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Kinetic and potential energy and speed

  1. Feb 25, 2007 #1
    Could someone plz help me out??!!!:confused: :confused:
    I'm so confused cuz we were never taught any of this and i have to hand in this assignment 2morrow

    1.An object is dropped from a height of 10m at what height will it s kinetic energy and potential energy be equal?

    2.A 100kg roller coaster car starts from rest on the top of a 30m high track.It rolls down to a 10m high deep then back up to a 15m crest. What are the speeds of the cart at (a)the deep and (b)the crest?

    Help me plzzz:frown: :frown:
     
  2. jcsd
  3. Feb 25, 2007 #2
    I know i am supposed to at least attempt the problem but I have no idea where to start.We have never been taught this stuff.If i could at least get an equation that can help i would be very grateful.
     
  4. Feb 25, 2007 #3

    Hootenanny

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    Okay, lets take question one first. If we ignore air resistance we can say that the total energy of the object must remain constant, yes?
     
  5. Feb 25, 2007 #4

    Astronuc

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    Also, in question 1, the potential energy is taken with respect to 0 m. Presumably, the object starts 'at rest'.
     
  6. Feb 25, 2007 #5
    yea.I guess there is constant velocity
     
  7. Feb 25, 2007 #6

    Hootenanny

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    No, the velocity is not constant. I said the total energy must remain constant.
     
  8. Feb 25, 2007 #7
    Oh sry i'm not very good at physics.
     
  9. Feb 25, 2007 #8

    Hootenanny

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    No problem, :smile: so now we've established the total energy of the object is constant. What types of energy does the object posses?
     
  10. Feb 25, 2007 #9
    Kinetic rite?
     
  11. Feb 25, 2007 #10

    Hootenanny

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    Are you sure that it only has kinetic energy?
     
  12. Feb 25, 2007 #11
    and potential cuz it starts at rest
     
  13. Feb 25, 2007 #12

    Hootenanny

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    Yes, it also has potential, but not because it starts at rest. It has potential energy because it is some distance above the ground, does that make sense?

    So, now we can say that;

    kinetic energy + potential energy = constant

    Do you follow?
     
  14. Feb 25, 2007 #13
    yea i get it
     
  15. Feb 25, 2007 #14

    Hootenanny

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    So, if we want to find the point at which kinetic energy = potential energy we can say that;

    potential energy + potential energy = 2*potential energy = constant
     
  16. Feb 25, 2007 #15
    Ummmm........i don't really follow
     
  17. Feb 25, 2007 #16

    Hootenanny

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    Which bit don't you follow?
     
  18. Feb 25, 2007 #17
    This bit:
    potential energy + potential energy = 2*potential energy = constant
     
  19. Feb 25, 2007 #18

    Hootenanny

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    Okay, suppose we have an equation;

    a + b = c

    Now, suppose we say a=b, then we have;

    a + a = 2*a = c

    Is that a little clearer?
     
  20. Feb 25, 2007 #19
    Yea i get it now
     
  21. Feb 25, 2007 #20

    Hootenanny

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    Good :smile:. So now we have;

    2*potential energy = constant

    Now we need to find the constant. How do you suggest we do that?
     
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