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AP Exam Help Oscillations/Hooke's law

  1. Apr 27, 2015 #1
    1. The problem statement, all variables and given/known data

    The exam example is in these links ; front page and back page

    https://cdn.fbsbx.com/hphotos-xpt1/...6d9fdc9617310cae5c7a6e8b4a60&oe=554068FD&dl=1

    https://cdn.fbsbx.com/hphotos-xtf1/...588cc004f584328f0c3769f779d0&oe=5540AA5A&dl=1

    Data is mostly from graphs so I had to put it in a pdf. Other given data : m=0.125kg

    Variables : m(mass), v(velocity), j(energy), t(time), a(acceleration)



    2. Relevant equations
    F=kx
    PE=mgh
    a=v/t


    3. The attempt at a solution

    My work and solutions are all in these links, it's hard to type it out and stuff due to graphs

    http://imgur.com/Q1MKKuB
    http://imgur.com/WpjOnU9
    http://imgur.com/j2PwDAo
    http://imgur.com/7XtxiZl
    http://imgur.com/wJ7gtVy
     
    Last edited by a moderator: Apr 27, 2015
  2. jcsd
  3. Apr 27, 2015 #2

    BvU

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    Hello SHC, welcome to PF :smile: !

    You posted on a homework forum. If you have any questions, please state them.
    This looks more like an exam; I suppose you handed it in and someone is going to try and sort it out, then grade it. So that's being done anyway.

    it's hard to type it out and stuff due to graphs
    Yeah, it's also hard to read and sort it out...

    PS we also have some guidelines on PF, you seem to be aware of some of them already.

    But still: any questions, you're welcome !
     
  4. Apr 27, 2015 #3
    I am sorry about the picture rule, but yeah, could someone help check my work and correct any errors?

    PS I have changed some stuff,
    Part (A) changed minimum to 1.08 and maximum to 1.225 and the wave is from the middle line to the top line

    Part (B) changed acceleration to 3pi, so maximum is 3pi and minimum is -3pi. Got this using derivatives and my teacher wants me to do it the basic "conceptual physics" way using algebra, but I am not sure how
     
  5. Apr 27, 2015 #4
    ( THIS IS THE BACK PAGE PART ) a.The student could use some known masses, a ruler, and a stand. Hang the rubber band on the stand and measure its length. Then hang the various masses and measure the amounts they stretch the rubber band (which is the displacement from equilibrium). Since and , the student can graph the force (which is the weight of the mass) vs. the displacement from equilibrium. If it is a linear relationship, then the rubber band behaves like and ideal spring.
    b. The student would need the unknown mass, the rubber band, a stand, and a timer. The student should attach the rubber band to the stand and the mass to the rubber band. Then pull the mass down to set it into SHM and time 10 cycles. Divide that time by 10 to calculate the period. Now the student can use the equation to find the unknown mass.
     
  6. Apr 27, 2015 #5

    BvU

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    Sure; which of the seven links should I follow ? Is there a particular place where you have such problems that you are even prepared to sort and type them out ?

    Link 1 (/Q1..) is only a half page, so is link 5 (/wJ..)
     
  7. Apr 27, 2015 #6

    BvU

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    Is that the back page part of the back page ? Because I didn't see that before. Or did I miss a link ?
     
  8. Apr 27, 2015 #7
    Hi, it's the first link and it should lead you to the graph.

    Part (A) I got as far as finding the KEmax. I equaled it to 1/2mv^2 and plugged in data from the given graphs above (A), : v=1.5m/s and m=.125 and I ended up with a KE of .14. It then asked to sketch a graph of "the potential energy of the object-spring-Earth system as a function of time" I am not sure if I did it right, but my graph is shown in the imgur links.

    Part (B) I used derivatives to find the acceleration of 3pi, but I believe my teacher wants us to use the conceptual algebraic way of Physics. Not sure how to do that though.

    As of the back page, the question and answer is written above, I just need double checking.

    Thank you! It's due at 1PM
     
  9. Apr 27, 2015 #8
    Yes. The question is just reworded, I am sorry about the misunderstanding.
     
  10. Apr 27, 2015 #9

    BvU

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    I see
    upload_2015-4-27_20-24-23.png
     
  11. Apr 27, 2015 #10
    For part B, I changed the acceleration to 3pi, it's not 6 anymore. I found 3pi using derivatives which the answer should be a little less than 10 according to my teacher, which calculus offered me 3pi. However, my teacher wants me to do it using the algebraic way and I am not sure how, using concepts of physics too.
     
  12. Apr 27, 2015 #11

    BvU

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    @haruspex : what do you think of this thread ? Supa's first posting is rather messy and I tried to nudge towards posing a workable problem statement and an orderly solution. To the tune of: if we want to help you, we need some cooperation from you, too. But it looks as if I bent over backwards too much, because he/she is just going on. Poster is smart enough, I should think, but chaotic and not very empathic with potential helpers. Just my impression, would gladly trade it for a better one.

    (bedtime for me -- I'm in western europe)
     
  13. Apr 27, 2015 #12

    haruspex

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    Here's one way.
    Forget the given problem for a moment. Write a generic SHM equation for position as a function of time, putting in symbols for the amplitude and frequency parameters. The max value of the position is the amplitude. Differentiate it to get the velocity as a function of time. What is the maximum value of this? Differentiate again to get acceleration, and again extract an algebraic expression for the max value.
    Compare the three algebraic formulae you have for the three maxima. What relationship do you notice?
     
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