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Some mixed questions

  1. Jan 28, 2008 #1
    1. The speed of a body of mass, m, moving horizontally, increases from u to v over a distance, d, under the influence of a constant resultant force, F, in the direction of motion. The energy transferred to the body is:

    1. Fd
    2. m/2*(v^2 - u^2)
    3. 1/2*(mv)^2

    The options are: 1 only, 2 only, 1 & 2 only or 1 & 3 only.

    I'd say 1 & 3 only seeing as I recognize they are equations dealing with energy and I don't know of the equation in 2.


    2. A steel balling is falling with constant velocity through oil. The gravitational force acting on the ball is:
    a) zero
    b) reduced by the upward forces
    c) less than it would be in air
    d) exactly equal to the sum of the upward forces

    I chose D because the other don't make sense to my knowledge.


    3. [​IMG]

    The figure above shows a lamina of uniform thickness. Its centre of gravity is most likely to be at:
    a) P
    b) Q
    c) R
    d) S

    Dunno why also... Q just seems right.


    4. In which of the following situations is the strong force associated with the contraction of a previously heated object made use of?

    1) Joining sections of a railway line
    2) Fitting metal rims onto wheels
    3) Rivets
    4) Clinical Thermometer

    I'd say 1, 2, 3 and 4. Though I'm a bit skeptical about 4.


    5. There are four containers: A B C & D. They contain water of masses m, 2m, 3m and 4m at temperatures 20 °C, 40 °C, 60 °C and 80 °C respectively.

    Which of the above must lose the most energy to cool down to 10 °C? I'd say D because it has the highest mass and would undergo the greatest temp. change and hence the value of E would be high from the equation: E=mc(t2-t1)

    In which of the cases above is the average kinetic energy of the molecules the greatest?
    I again say D.


    6. Which of the following would cause the hot air above a fire to rise?
    1. Increase in speed of the air molecules
    2. Expansion of the air
    3. Decrease in the density of the air

    I'd say all three because increase in speed = expansion = lower density = rise


    7. Which of the following components may be found in both d.c. motors and a.c. generators?
    1) soft iron armature
    2) brushes
    3) slip rings

    I say 2 only



    I hope I am correct. If I am not please feel free to correct me. Thank you guys :)
     
  2. jcsd
  3. Jan 28, 2008 #2

    Hootenanny

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    Write down two expressions, one for the energy that the mass has before the acceleration and the energy the mass has after the acceleration.
    Correct :smile:
    I would agree with you :smile:
    I would agree with you on all of them except (1). Why do you think gaps are left between sections of rail (these gaps are especially visible in winter).
    Correct :smile:
    Correct :smile:. As an aside if all the cups were at 10oC, what would be your answer then?
    Indeed, but which one is the root cause?
     
  4. Jan 28, 2008 #3
    If they were all at 10oC then the answer would still be D [the one with the most mass?]

    The root cause is the decrease density; the more dense air displaces the less dense air resulting the more dense air 'sinking' and the less dense air 'rising.

    The gaps are there because when the trains move over the rail sections their temperature increases- with this comes thermal expansion. The gap therefore facilitates this expansion. These questions are multiple choice and for this specific question:

    4. In which of the following situations is the strong force associated with the contraction of a previously heated object made use of?

    1) Joining sections of a railway line
    2) Fitting metal rims onto wheels
    3) Rivets
    4) Clinical Thermometer

    The options are:
    a) 2 & 3 only
    b) 2 & 4 only
    c) 1, 2 & 3 only
    d) All of the above (1, 2, 3 & 4)


    The one with the energy:

    First expression:

    Kinetic energy = 1/2*(mu)^2

    Second expression:

    Kinetic energy = 1/2*(mv)^2

    Therefore energy transferred:

    1/2*(mv)^2 - 1/2*(mu)^2

    By factorizing:

    1/2m(v^2 - u^2)

    1. Fd
    2. m/2*(v^2 - u^2)
    3. 1/2*(mv)^2

    The options are: 1 only, 2 only, 1 & 2 only or 1 & 3 only.

    So I guess it's 2 only?


    Also, can anyone confirm this:


    7. Which of the following components may be found in both d.c. motors and a.c. generators?
    1) soft iron armature
    2) brushes
    3) slip rings

    I say 2 only
     
    Last edited: Jan 28, 2008
  5. Jan 28, 2008 #4

    Hootenanny

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    Correct, although the average kinetic energy per molecule will remain unchanged, D has the greatest mass and therefore the greatest energy :smile:
    Ahh, but what causes the density to decrease?
    Okay, I'm not sure whether thermal contraction is useful in railway lines but if I had to chose an option it would be (d).
    Sound good to me :approve:
    Not my area of expertise, if it's still unanswered tomorrow I shall raise it in the Homework Helper's forum.
     
  6. Jan 28, 2008 #5

    An increase in temperature of the air causes density to decrease in that the molecules move faster thus taking up more space [mass] per unit volume.

    And thanks alot for your help :) !
     
  7. Jan 28, 2008 #6

    Hootenanny

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    Correct, so it's the molecular speed that's important.
    No worries, it's a pleasure. :smile:
     
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