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2 theory related multiple choice questions

  1. Sep 20, 2005 #1
    hey, I'm currently in grade 12, and so far we just started a review over the conservation laws unit, which includes (energy related theorm, work, power, momentum, collisions, mechanical energy ect.)

    Anyways I cant seem to figure out these two multiple choice theory questions, was hoping someone can explain them to me, thanks.

    Which statement describes a conclusion that can be made about a closed system where there has been an increase in its enery?
    a)Work has been done on the system
    b)Mass has been converted to energy
    c)Kinetic energy of the system is increased
    d)Potential energy of the system has decreased

    And: "Ruth" is diving from a tower 10.0m above the water. When she is 5.0m above the surface of the water her:
    a)momentum and kinetic energy are about equal
    b)velocity is half of the velocity she will have when she touches the water
    c)potential energy, with respect to the water, and her momentum are about equal
    d)potential energy, with respect to the water, and her kinetic energy about equal
    was hoping someone can explain these theory questions to me, thanks alot.
  2. jcsd
  3. Sep 20, 2005 #2
    Conservation of energy

    for the second question:
    find the potential energy when she was 5 m above the water.
    use the law of conservation of energy to find her kinetic energy 5 m above the water : potential energy at 10 m = potential energy at 5 m + kinetic energy when she was 5 m above the water.

    You should find that they are equal.
  4. Sep 20, 2005 #3
    thanks, that makes sense, any ideas for the 1st one tho? i just know no matter can enter or leave, so i'm gettin confused how their can be an increase in energy
  5. Sep 20, 2005 #4


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    Well, there's only one choice there that could increase the energy of a closed system - emphasis on the word "closed". One of them could increase the energy, but only if the system were open. The other two don't say anything about the total energy of the system.

    Is this enough?
  6. Sep 20, 2005 #5
    hm my guess is Work has been done on the system but then again i'm notsure.
  7. Sep 20, 2005 #6


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    If work is done on the system, then it isn't closed, is it?
  8. Sep 20, 2005 #7
    aw nuts, my next choice would of been kintetic, because i wouddt even know how mass would change into energy
  9. Sep 21, 2005 #8


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    Fission? Fusion? Antimatter?
  10. Sep 21, 2005 #9


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    It used to be the case that, in a closed system, energy was conserved! But now, post relativity, it's "the total mass-energy is conserved".

    As for #2, without doing any calculation, I would say the whole idea of "momentum and kinetic energy are the same" or "momentum and potential energy are the same" is non-sense. They have different units and can't be compared! (Are "one acre" and "one pound" "the same"?) That kind of reduces the possibilities doesn't it! Since acceleration due to gravity is a constant, the correct answer is kind of obvious anyway.
    Last edited by a moderator: Sep 21, 2005
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