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Need wrong answers

  1. Mar 5, 2008 #1
    I'm making a question paper with answers having multiple options. I made/got the questions and right answer, but I'm unable to get wrong answers. Please people try to make some basic level mistake to get a wrong answer and tell me (Pardon me if I've posted this in wrong place)

    1) Find the shortest period of rotation of a planet in terms of [tex]\rho[/tex] ([tex]\rho[/tex] represents uniform density of spherical planet)
  2. jcsd
  3. Mar 5, 2008 #2
    2) A meter stick of total length l is pivoted a distance d from one end of a frictionless bearing. The stick is suspended so that it becomes a pendulum. Assume the total mass of stick is constant and distributed uniformly over the body. The acceleration of gravity is g. Find value of d for frequency of small oscillations to be maximum.
  4. Mar 5, 2008 #3


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    Be careful not to make the wrong answers too plausible.
    In my first try at a multiple choice test, nobody picked my right answers.
  5. Mar 6, 2008 #4
    Ya, I will. But I need wrong answers, please help.....
  6. Mar 6, 2008 #5
    I donno abt the rongs but i no the rights..

    I think ans 1 is [tex]\sqrt{\frac{3\pi}{G \rho}}[/tex]

    & ans 2 =>> f= [tex]\frac{1}{2\pi}[/tex][tex]\sqrt{\frac{g (\frac{l}{2}-d)}{(\frac{l}{2}-d)^{2}+\frac{l^{2}}{12}}}[/tex]

    for f to be maximum d=[tex]\frac{l}{2\sqrt{3}}(\sqrt{3}-1)[/tex]
    Last edited: Mar 6, 2008
  7. Mar 6, 2008 #6
    pl tel if they are rite or rong.
  8. Mar 6, 2008 #7

    Andy Resnick

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    One way to generate wrong answers is to explicitly write down the entire solution procedure, and at a few steps, selected randomly or because they represent something you are testing, do something like swtich a multiply to a divide, or invert, or something elementary like that.
  9. Mar 6, 2008 #8
    Negative marking is reprehensible. Deliberately devious choices even more so. I urge you to reconsider your evaluation scheme.
  10. Mar 6, 2008 #9

    Andy Resnick

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    Please. The goal of any exam is for the instructor to evaluate the students' ability.
  11. Mar 6, 2008 #10
    Yeah I've got to agree with Andy. Putting *close to right* answers is a great way to test if people actually know the material to the fullest extent.
  12. Mar 6, 2008 #11


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    Dearly Missed

    Nope. Random crossings are to give no more points than a blank paper.
    Nope, It pinpoints where the flaws lie.
    Here in Norway, when I studied, our exams were in "long hand", where of course the examiner could pinpoint flaws in the reasoning.
    There is nothing wrong in preserving this feature in other examination types as well.
  13. Mar 6, 2008 #12
    The first question isn't very good for multiple option exercise, since the student who can derive frequency independend on any other parameters except density will almost certainly find the true solution. Changing formula without mismatch in units seems almost impossible, so it is probably best to make the wrong answers basicly the same as the right one (the same dependence on the variable ro and gravity constant), but with different numerical constants.

    A "decent" wrong answer for the second question would be the one where a student fails to use parallel axis theorem and simplifies things by putting


    into the equation. The corresponding solution conveniently turns out to be d=l.
    Last edited: Mar 7, 2008
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