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Homework Help: Help with seemingly easy problem on energy and momentum

  1. May 19, 2009 #1
    http://www.webassign.net/userimages/barrett.3@osu/prelab-131-07A.gif [Broken]

    If A = 34 cm and the pendulum bob diameter is 2.4 cm, what is the appropriate height difference magnitude to use for evaluating the change in potential energy of the pendulum bob-earth system? I got 29 cm by subtracting 5 cm from A. It was correct.

    Suppose B = 22 cm. The distance traveled by the box, modeled as a point particle, is: I got 80 cm, logical, it was correct.

    Suppose the friction force between the table and the box is 0.16 N and B = 22 cm. What is the work done by friction on the box?

    The last part I have no idea what to do. I thought I could just multiply the friction force by the distance and I would get the distance traveled but that didn't work. Mass isn't given in the problem so what do I do?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 19, 2009 #2


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    Not distance, work. Multiply the force acting along the path traveled by the distance traveled along the path and you get the work done. (I guess that was just a typo in your post?)

    It might help for you to post your calculations on that last part, along with the correct answer.
  4. May 19, 2009 #3
    I don't know the correct answer until I have it since it's an online activity.

    Regardless, so I should multiply the 0.16 N by 0.8 m? This is what I tried earlier and it was not correct. Do you think the ball's acceleration has something to do with the acceleration of the box? In the problem though, no masses or velocities are given, so that concept becomes even more confusing.
  5. May 19, 2009 #4
    I agree with diazona.

    However, as an alternative method, you could try a conservation of energy method assuming all the energy is converted to the block when the pendulum hits it.

    In this case the work done by friction would just be the potential energy of the pendulum.

    PS. You might also want to check the units the online site wants you to give the answer in.
    Last edited: May 19, 2009
  6. May 19, 2009 #5
    How would I take the mass into account when calculating the potential energy?
  7. May 19, 2009 #6


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    If that's really the case, the online site is apparently wrong - the work done by friction on the box is just the force (0.16 N) times the distance traveled (0.8 m). Although Coto has a good suggestion to check the units.

    I suppose you know that potential energy is given by U = mgh . . . (under the approximation of constant gravitational acceleration)
  8. May 19, 2009 #7
    That's exactly what I thought it would be but I guess the site is wrong. Is there a reason the distance traveled is 80 cm and B is 22 cm. Why is it even given?
  9. May 19, 2009 #8


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    I'm not sure, honestly... B doesn't seem to be used for anything. Maybe it's just there to distract you.
  10. May 19, 2009 #9
    Hm...guess I'll leave it blank. Thanks for your help!
  11. May 19, 2009 #10


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    No problem :smile:

    I'd suggest checking with someone in charge (e.g. a professor) about that problem, just to make sure there isn't some little subtlety we missed. And you may get the chance to point out a bug in the website that should be fixed for future students ;-)
  12. May 20, 2009 #11
    Work = cos (Angle) * D * F = -1 * .16N * .8M = -.128 ....... you just forgot the negative sign infront, the work is opposite to the work done by the ball thus it is negative and thats where the cos (180) or the - sign comes from.
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