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I will be FOREVER grateful if someone can tell me the answer to this: involves KE+PE!

  1. Oct 31, 2012 #1
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    c, d, and e I don't know how to solve

    For d I tried doing

    vf2 = vi2 +2ad
    Vf = √(2*9.8*2.3) = 6.71 m/s

    Assuming the inclined plane is frictionless
    Since m2 is roped to m1, it's velocity up the ramp will be 6.71m/s too

    The total increase in KE is then 0.5[8+6]*6.71^2 = 315.16 J

    ^

    "INCORRECT"

    Right now I need a genius to tell me how to get the answer ASAP -.-

    Since (d) is wrong, so is e. -.-
    Story of my life.
     
  2. jcsd
  3. Oct 31, 2012 #2

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    For (c), how did you get that number? Isn't it just the sum of the two ΔU values?
     
  4. Oct 31, 2012 #3
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    m1 falls 2.3m and therefore has -6*9.8*2.3 = -135.24 J difference.

    m2's PE increases 8*9.8*sin 30° = +39.2 J difference.

    The total change then is 39.2 - 135.24 = -96.04 J

    ^ That was my previous answers, but I got that wrong too...
    So I'm pretty much screwed.
     
  5. Oct 31, 2012 #4

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    You left off the 2.3 for m2...
     
  6. Oct 31, 2012 #5
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    (c)

    m1 falls 2.3m and therefore has -6*9.8*2.3 = -135.24 J difference.

    m2's PE increases 8*9.8*sin 30°*2.3 = +90.16 J difference.

    The total change then is 90.16 - 135.24 = -45.08 J

    Ok so how does this relate to part d and e though? How would I solve for those two last ones?

    (d)

    (e)
     
  7. Oct 31, 2012 #6

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    (d) asks for the change in KE. Since the system is frictionless...
     
  8. Oct 31, 2012 #7
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    But I already tried solving for it and i got it wrong... (???????)
     
  9. Oct 31, 2012 #8

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    But you had the wrong answer for (c) before. Now you have that right.

    What is the total energy of a system in terms of the potential and kinetic energies? If the system is lossless (no friction), what can you say about the total energy over time...?
     
  10. Oct 31, 2012 #9
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    KEinitial + PEinitial = KEfinal + PEfinal
    KEinitial + PEinitial = 0 + 0 (No friction)
    KEinitial + PEinitial = 0?
     
  11. Oct 31, 2012 #10

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    Rearrange so you show the relationship between ΔKE and ΔPE...
     
  12. Oct 31, 2012 #11
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    KE = -PE
    KE = -mv???
     
  13. Nov 1, 2012 #12

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    If you put Δs in front of your first equation, you would be on the right track...
     
  14. Nov 1, 2012 #13
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    Consider the total energy over time which is a constant. Hence change in PE must balance out change in KE. The rest is obvious.
     
  15. Nov 1, 2012 #14
    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    As for (e) consider that you have a final change in KE, as well as the masses. You can now plug that in to solve for velocity.
     
  16. Nov 1, 2012 #15

    berkeman

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    Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

    Oops, Looks like riseofphoenix is on a temporary vacation from the PF for a different thread.
     
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