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Inclined plane and elapsed time

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data
    A block is given a initial speed of 5.0 m/s up the 20° plane. (a) How far up the plane will it go?
    (b) how much much time elapses before it returns to its starting point?


    2. Relevant equations
    I think

    V= u + at
    s=ut t 1/2at^2
    v=d/t

    3. The attempt at a solution

    (a) V= u + at
    0= 5-9.8(sin20)t
    t = 1.65s

    s=ut+1/2at^2
    =5(sin20)(1.65)+ 1/2(9.8)(1.65^2)
    s= 16.16m

    (b) v=d/t
    t=d/v
    =16.16/5
    t= 3.2s


    I know this is wrong but i can't figure it out can someone help me?
     
    Last edited: Dec 7, 2008
  2. jcsd
  3. Dec 7, 2008 #2

    Doc Al

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    Staff: Mentor

    Recheck your arithmetic. (Tip: Once you've solved for the time the block takes to rise, what's the answer to part b?)

    The initial speed up the ramp is already parallel to the incline, so there's no need to multiply it by sin20. On the other hand, the acceleration along the incline is only a component of g, so you do need the sin20 there. Also, should the acceleration be + or -?
     
  4. Dec 7, 2008 #3
    acceleration should be positive
     
  5. Dec 7, 2008 #4
    So what your saying is that the sin20 is suppose to beby the 9.8?
     
  6. Dec 7, 2008 #5

    Doc Al

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    Staff: Mentor

    No. The ramp slopes up, so acceleration is negative (down).
    Yes, the acceleration of the block is g sin20.
     
  7. Dec 7, 2008 #6
    Oh, Is the first part of (a) correct because I found the time so that i could find the distance, is that what is suppose to be done?
     
  8. Dec 7, 2008 #7

    Doc Al

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    Staff: Mentor

    Yes, that's a perfectly fine way to solve the problem. You just made an error when you did the final arithmetic to solve for the time.
     
  9. Dec 7, 2008 #8
    so its:

    (a) V=u+at
    0=5+9.8(sin20)t
    t = 1.65s

    s= ut+1/2at^2
    = 5(1.65)+1/2(-9.8)(sin20)(1.65^2)
    =3.69m

    (b) v=d/t
    5=3.69t
    t=3.39/5
    t=0.68s
     
  10. Dec 7, 2008 #9

    Doc Al

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    Staff: Mentor

    The final answer is still wrong. Redo the calculation.

    You'll need to redo this with the correct time.

    No. Read my "tip" in post #2.
     
  11. Dec 7, 2008 #10
    v=u+at
    = 5-9.8(sin20)t
    t= 1.64s

    I still get the same answer
     
  12. Dec 7, 2008 #11

    Doc Al

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    Staff: Mentor

    I'm curious how. t = 5/(9.8sin20) = ??
     
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