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How do I solve this Work and Energy problem?

  1. Apr 28, 2015 #1
    1. The problem statement, all variables and given/known data

    Tarzan (mass of 80kg) has an 18m vine stretched horizontally from his hut to a pivot point P, as shown. He uses it to swing across the gorge to visit Jane's hut, starting from rest to conserve energy. One day Jane spoiled his plans by fastening a strong, thin, smooth, horizontal branch to intercept the vine at B, 15m directly below P. Tarzan hung on and followed the dotted path 1 to 2 to 3. Calculate the tension in the vine as he passed through point 3. How many times greater than his weight is his tension?

    FullSizeRender.jpg

    The answer is 15.34 (I'm unsure which question this answers)

    2. Relevant equations
    Ug =mgh
    ΣE = 0
    Ek = 0.5mv2
    ΣF = ma

    3. The attempt at a solution
    1:
    Ug = 80(9.81)(18)= 14,126.4 J
    Ek = 0.5mv2 = 0

    2: Ug = 0
    Ek = 0.5mv2
    14 126.4= 0.5mv2
    v = 18.795 m/s

    Ft = mg = 784.8 N
     
    Last edited: Apr 28, 2015
  2. jcsd
  3. Apr 28, 2015 #2
    it answers the tension in the vine as he passed through point 3. because if it would be the weight then the units would have been mentioned.
     
  4. Apr 28, 2015 #3

    haruspex

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    In your step 2 you calculate a velocity. At what point is that his velocity? At which point do you need to determine his velocity?
    As I'm sure you know, Fnet=ma. If the tension at point 3 is T, what is Fnet? What is his acceleration at that point?
     
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