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Homework Help: Peg and pendulum help!

  1. Oct 22, 2011 #1
    1. The problem statement, all variables and given/known data
    i need to calculate the d for a 45 degree drop of pendulum that the lenght is .44m. i did the one for 90 degrees but i cant figure out how to get a different outcome.


    2. Relevant equations
    ug initial= KE + Ug final
    mgh=0.5mv^2=mgh
    gl=0.5rg+g2r
    l=0.5r+2r
    l=0.5(l-d)+2(l-d)
    l=5/2l-5/2d
    l=d+r
    r=l-d
    v^2=rg
    m(v^2/r)=mg


    3. The attempt at a solution
    i got 26.4 cm
    L=.44m
    dexp=.28 @90 degrees
    dexp=.41 @ 45 degrees
     
  2. jcsd
  3. Oct 22, 2011 #2
    I would like to lend a hand but I am a little confused about the variables you are using.

    If I interpret correctly,
    L=length of pendulum string (0.44m)
    d=height of pendulum bob at θ=45° (unknown)
    r=difference between L and d (unknown)

    If this is the case you don't need to worry about energy or forces. Drawing a diagram and looking at the geometry of them problem will be enough to find and equation for d in terms of what you already know.
    A good place to start would be to notice that L and r form a right triangle when the bob is held at 45°.
     
  4. Oct 23, 2011 #3
    ok so i drew this and i get that the r is whats left of the lenght of the string after it hits d. r is also the radius of the circle that the bob makes when it makes one revolution. i dont understand how to get either one of them.

    L=1/2r+2r
    L=3/2r
    0.44=3/2r
    r=0.2933333
    i guess this is how i could find that and if i plug this in...

    r=L-D
    -D=r-L
    D=-r+L
    D=L-r
    D=0.44-0.29333333
    D=.129067m
    D=12.9cm
    that doesnt make sense because the Dexp= .41
    thats .28m off...
    the Dexp@90 was only 1.6cm off...
    please help
     
  5. Oct 23, 2011 #4
    Sorry, but I'm still a little confused here...

    I see your experimental d, called dexp, at 45° is larger than your dexp at 90°.
    With the way I've defined d above, this should not be possible, so perhaps I do not understand the experiment correctly.

    Could you start at the beginning and describe the experiment? (and possibly include a picture?) Defining any variables you use would also be very helpful.
     
    Last edited: Oct 23, 2011
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