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Homework Help: Mutiple Choice Question

  1. Jun 5, 2010 #1
    Hi can somebody tell me the answer they think is correct for the following multiple choice questions:
    1. The problem statement, all variables and given/known data
    1) A uniform rod is attached to a wall by hinge at one end. Initially horizontal, the rod is released from rest to swing downwards. Consider the torque about the hinge produced by the rod's weight. As the rod swings downwards, this torque is:

    A. always zero
    B. a non-zero constant
    C. increasing
    D. decreasing

    not sure about this

    2) 10kg of ice at 0degree C absorbs heat so that it starts to melt. Finally there is a mixture of 5kgt of ice and 5kg of water in thermal equilibrium. Which of the following statements is true?

    A. The remaining ice is a at 0 degrees C and the water has a temperature above 0 degree C.
    B. The remaining ice has a temperature less then 0 degree C and the water has a temperature of 0 degree C.
    C. Both The remaining ice and the water have a temperature of 0 degree C.
    D. Both The remaining ice and the water have a temperature above 0 degree C.

    i chose A but not sure it is correct.

    3) You are provided with two identical springs, each with spring constant 120N/m. The two springs are now joined end to end to make one long spring: the spring constant of the combination is k.

    Which of the following is correct?
    A. 60 N/m
    B. 120N/m
    C. 180N/m
    D. 40N/m

    I got 240 but it is not on the list.

    P.S
     
  2. jcsd
  3. Jun 5, 2010 #2

    Doc Al

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    Staff: Mentor

    For each question, show how you arrived at your answer.
     
  4. Jun 5, 2010 #3
    1) not sure how about this it may be downwards?

    2) i drew a diagram and said that because at final equilibrium 5kg of water and 5kg of ice remain, then the 5kg of ice is at 0 degree and the 5kg of water is at a higher temperature than the ice at 0 degree because it has melted into water.

    3) i went 120 + 120 to find the final spring constant of two springs.
     
  5. Jun 5, 2010 #4

    Doc Al

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    What's the definition of torque? What force is involved and how does it change as the rod moves down? How does the moment arm change?

    What does thermal equilibrium mean?

    Well, they don't add like that. To figure out how they do add, imagine applying a force to the ends of the two-spring system. How would each stretch? Compare k = F/x of the two springs together with the F/x of each spring separately.
     
  6. Jun 5, 2010 #5
    1) would it be decreasing because it is moving downwards.

    2) that all values are equal, so would that mean they are both at 0 but how can that be possible.
     
  7. Jun 5, 2010 #6

    Doc Al

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    The torque decreases because the moment arm decreases as the rod moves downward.

    Its perfectly possible that ice and water can be at 0 degrees. To melt ice you must add energy. That energy first goes into changing the state from solid to liquid; if there is any left over, the temperature will increase. In this case, there wasn't enough to melt all the ice.
     
  8. Jun 6, 2010 #7
    i am not given any values besides 120N/m so how am i meant to calculate it?
     
  9. Jun 6, 2010 #8
    Assume the force remains constant in the equation k=F/x.

    What change will adding another spring in series have to the extension?
     
  10. Jun 6, 2010 #9
    won't it halve the value?
     
  11. Jun 6, 2010 #10

    Doc Al

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    That's right. Two springs connected in series has half the spring constant than the individual springs.

    This should make sense from your every day experience. Which is harder to stretch: 10 rubber bands at once? Or one long string of 10 rubber bands hooked together?
     
  12. Jun 6, 2010 #11
    so the answer would be 60N/m?
     
  13. Jun 6, 2010 #12
    Just another quick multiple choice question:

    In a system undergoing SHM such as a mass on a spring, if the amplitude of the motion is doubled what happens to the frequency and energy of the system?

    A. Frequency and energy both remain unchanged
    B. Frequency is unchanged, energy increases
    C. Energy is unchanged, frequency increases
    D. Frequency and energy both increase

    i chose B
     
  14. Jun 6, 2010 #13

    Doc Al

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    Yes.
     
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