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Conservation of mechanical energy, centripetal acceleration, independence question

  1. Sep 28, 2008 #1
    Show that a roller coaster with a circular vertical loop. The difference in your apparent weight at the top of the circular loop and the bottom of the circular loop is 6 g's-that is, six times your weight. Ignore friction. Show also that as long as your speed is above the minimum needed, this answer doesn't depend on the size of the loop or how fast your go through it.

    My working for the first half of the problem, the 6g's part is as follows

    Radius if loop is R
    Height from which it is released is h

    The speed at bottom of the loop is determined by the conservation of mechanical energy
    1/2 mvbottom2=2mgh

    Apparent weight at the bottom of the loop is obtained by the below:

    mvBot2= FNorm. Bot.-mg
    Therefore apparent weight at bottom is
    FNorm. Bot.=mvbot2/R+mg
    FNorm. Bot.=2mgh/R+mg (using result obtained via conservation of energy)

    To find speed at top of the loop we have from Conservation of Energy
    1/2 mvtop2+mg(2R)=mgh
    mvtop2=2mg(h-2R)
    Therefore using the above the apparent weight at the top of the loop is

    mvTop2/R = FNorm. Top.+mg
    Therefore Apparent weight is :
    FNorm. Top. = (2mg(h-2R))/R - mg

    Hence
    FNorm. Bot. - FNorm. Top. =
    2mgh/R + mg - [((2mg(h-2R))/R - mg)]=
    2mgh/R + mg - 2mgh/R + 4mg + mg=
    6mg

    That's how I proved the first part of the problem. Can anyone please tell me how to complete the second part of the problem; namely proving that as long as your speed is above the minimum needed, the answer doesn't depend on the size of the loop or how fast your go through it. This part of the problem is really bugging me and I've tried heaps of ways but can't come up with the answer.
     
  2. jcsd
  3. Sep 29, 2008 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi ctpengage! :smile:

    I'm a little confused …

    you seem to have got the right result with the wrong equations. :confused:

    You have two equations for conservation of energy, one for the top and one for the bottom …

    but conservation means that the energy for top and bottom should be in the same equation, doesn't it? :smile:
     
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