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Homework Help: How can I solve these physics problems involving torque and rotational energy?

  1. Dec 1, 2008 #1
    In Physics I have a C average right now. These two quizes are my last chance at bringing my grade up to a B for this class. I have attached a copy of all of the problems that are on these two quizes totalling seven questions. THis is open book take home quiz where we can work with whoever we want.

    I was hoping that somebody here can look over my work and tell me if I made any mistakes. I have made the attempt, now I am hoping someone could review it.

    Thanks. Enclosed are the quizes and the work I have done on them

    http://i289.photobucket.com/albums/ll229/Cleaver97/th_Quiz2001.jpg [Broken]

    http://i289.photobucket.com/albums/ll229/Cleaver97/th_Quiz1001.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Dec 2, 2008 #2
    2nd paper Q5: The amount of rotational KE an object has depends on the object's moment of inertia, hence that cannot be an answer. (Hint: Think conservation of energy, what is always the same of two objects of the same mass going down the same height?)

    Otherwise, well done!
  4. Dec 2, 2008 #3
    I would havve to say angular speed?
  5. Dec 2, 2008 #4
    No, you see, angular speed is related to the rotational kinetic energy of the object, which are not equal. Now, examine what is similar for these two objects, there's mass and there's the starting height. These two describe a certain physical quantity, what is it?
  6. Dec 2, 2008 #5
    Aha! It is momentum correct? It is the only one.
  7. Dec 2, 2008 #6
    Again, no. Conservation of energy states that initial energy = final energy. What form is the initial energy in? What are the final states of energy? Are the total energy for the two objects similar?
  8. Dec 2, 2008 #7
    I understand now. They both are rolling down an incline with the force of gravity. Therefore it is gravitational PE? I believe it is. THanks for the help. I really appreciate it.
  9. Dec 2, 2008 #8
    That's it. Because initially, both the mass and the heights are the same, the GPE has to be equal. By conservation of energy, GPE = rot. KE + translational KE.

    Since I is different, rot KE. has to be different (and by extension angular speed), and since the total energy for the two is equal, if rot KE differs, trans. KE also differs (and by extension, momentum also differs).
  10. Dec 2, 2008 #9
    Ohh .. I understand now. THanks a loot!!
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