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Homework Help: Kinetic energy to thermal energy

  1. Jan 15, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    Hey guys, I need help with the following question.
    A lead ball of specific heat capacity 130J/KgK travels with a speed of 100m/s before it hits a wooden cube fixed to floor and gets stuck inside.What will be the rise in temperature of this lead ball by the time it comes to

    I attempted the question as follows,
    Kinetic energy Thermal energy of
    Of the lead ball = the lead ball after hitting
    Before hitting the cube on the cube.
    1/2×m×v^2 = mc(t1-t2)
    1/2 × m×100^2 = m× 130×(t1-t2)
    t1-t2 = 38 Celsius.

    But apparently the answer I was given is 3 Celsius.
    Can someone please tell me where I went wrong and how to answer the question correctly?

  2. jcsd
  3. Jan 15, 2017 #2

    Is there any additional information in the problem that you may have omitted? e.g. Did any of the energy go towards heating up the wood? If all of the kinetic energy of the lead ball was used to heat up the lead, then I can't find any problem with your calculation.

    Edit: I think I figured out the key to reading your statement (which I quoted above). It was written in 2 columns with the equal sign in between. So it reads:
    "Kinetic energy of the lead ball before hitting the cube = Thermal energy of the lead ball after hitting on the cube."
    Last edited: Jan 15, 2017
  4. Jan 15, 2017 #3
    I think your answer of 38 is correct .
  5. Jan 15, 2017 #4
    It doesn't say whether any energy goes to heating up of the wood. But if it does, how do I do the calculation?
  6. Jan 15, 2017 #5
    You can't ...books/sources often have errors

    You say ..."But apparently the answer I was given is 3 Celsius. " ... apparently ? was the answer given 3 or 3.8... and who gave it to you?
  7. Jan 15, 2017 #6
    My physics lecturer gave the answer as 3 Celsius .
  8. Jan 15, 2017 #7
    Tell him he's wrong
  9. Jan 15, 2017 #8
    Yeah,I guess that's what I'll do. Anyway thanks for helping.
  10. Jan 15, 2017 #9
    Well, if it doesn't say, then I think it makes sense to assume all of the kinetic energy goes to heat up the lead. If it did tell you that some of the energy was used to heat up the wood, the problem would have to indicate (somehow) how much energy went to the wood and how much to the lead.
    And like oz93666 indicated, it is not uncommon that incorrect answers are given.
    Regarding telling your lecturer that he is wrong, it may be wise to find a kinder way to address the apparent discrepancy. :)
  11. Jan 15, 2017 #10
    Yes, that seems wise.Thank you.
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