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Homework Help: Find velocity given y and angle

  1. Jun 22, 2010 #1
    1. The problem statement, all variables and given/known data
    Tarzan swings on a 34.0 m long vine initially inclined at an angle of 38.0° with the vertical.
    (a) What is his speed at the bottom of the swing if he starts from rest?
    (b) What is his speed at the bottom of the swing if he pushes off with a speed of 2.00 m/s?

    3. The attempt at a solution
    I honestly do not even know where to begin.
  2. jcsd
  3. Jun 22, 2010 #2
    Use conservation of energy, you know the height at which he starts, you know the height at the bottom of the swing. You know the initial speed for both cases. This should get you started. KEi + PEi = KEf + PEf
  4. Jun 22, 2010 #3
    (1)Firstly draw freebody diagram. Consider how many forces acting on Tarzan.
    (2) Use Newton's Second law then find acceleration.
    (3) Find the length between tarzan initial and final condition.
    (4) Find the velocity at the bottom.
  5. Jun 22, 2010 #4
    KE + PE = KE + PE
    mgh = 1/2 mv2
    v2 = gh/2
    v2 = (9.8 m/s2)(34m) / 2
    v= 12.91 m/s

    where does the angle come into play?
    when i entered this answer, it told me that I was within 10% of the correct answer
  6. Jun 22, 2010 #5
    See figure attached.

    Attached Files:

  7. Jun 22, 2010 #6
    basically find the change in height using trig... the length of the vine * cos(angle)

    then you take the length and subtract what you found..
    next find (mass)(9.81)(change in height)

    equate that to 1/2mv^2

    so take the answer, multiply by 2, divide by mass and then get the squareroot..

    thats the velocity..

    all this is is mgh=1/2mv^2 (Eg=Ek)
  8. Jun 22, 2010 #7
    I don't know the mass though
  9. Jun 22, 2010 #8
    mandy, if you write out KEi + PEi = KEf + PEf, you will notice a common term which can be canceled.
  10. Jun 22, 2010 #9
    i just did the question.. ill post the solution.. scanning it atm..

    mass cancels out.. are the answers 16 and 16.1m/s?
  11. Jun 22, 2010 #10
    I did cancel out the mass when I solved this equation. Is this not the correct equation?

  12. Jun 22, 2010 #11
    its not bc you're dividing by 2 when you should be multiplying.. you're also ignoring the angle and delta heighthttp://yfrog.com/6bscan0001yj
  13. Jun 22, 2010 #12
    http://img227.imageshack.us/img227/5659/scan0001y.jpg [Broken]

    this is my solution..
    Last edited by a moderator: May 4, 2017
  14. Jun 22, 2010 #13
    The equation is correct, and you canceled out the mass, but the height is not correct. Instead use delta h in the figure which i attached. Tarzan did not release from 0 degrees, therefore take this into account.
  15. Jun 22, 2010 #14
    joshmdmd, you must be rather careful here. If the 38 degrees are measured from the x-axis like I have drawn then in order to obtain the y-component you must take the sine of the angle.
  16. Jun 22, 2010 #15
    your eq'n should be v^2=2gh
  17. Jun 22, 2010 #16


    cos(90-38) = sin38
  18. Jun 22, 2010 #17
    oops.. i took theta in your drawing as 38.. =] you messed me up
  19. Jun 22, 2010 #18
    Sorry for the confusion, just making sure you know whats going on.
  20. Jun 22, 2010 #19
    here we go.. corrections to answer..


  21. Jun 22, 2010 #20
    Josh, the answer in your scan is wrong. When i did it and got 12.91 m, it said that I was within 10% of the correct answer.
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