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Homework Help: Projectile motion - equations giving different answers?

  1. Oct 20, 2013 #1
    1. The problem statement, all variables and given/known data

    If an object is being launched from the top of a staircase 1.31m high from a pipe 16cm long, at an angle of 60 degrees. Find where the object will land if the initial velocity is 5.442m/s.

    2. Relevant equations
    dx = 2v1^2sinθcosθ / g

    dy = vyΔt+1/2gΔt^2

    dx = vxΔt

    3. The attempt at a solution
    I found the components to be
    vx = 2.721
    vy = 4.713

    then i just used these in the two methods
    the answers should both be the same, but the dx i got from both was very different
    Am i doing something wrong? which one would be the correct answer?

    Also, for the first method, i put 120 degrees as negative as it is below my reference point, or it would give me a neg. distance.

    Attached Files:

    • bl.jpg
      File size:
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  2. jcsd
  3. Oct 20, 2013 #2


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    Homework Helper

    There is an issue with the wording of the problem. Is the 1.3 m the exit point of the pipe or the base of the pipe? If it's the base, then you'd need to take into account the pipe's 16 cm length. If it's the exit point you wouldn't need to know the length of the pipe, so it seems more likely that 1.3 m is the base of the pipe.
  4. Oct 20, 2013 #3
    to the base of the pipe
    so the total height would be the stairs + height of top of stairs to the opening of the pipe

    which is 1.324m

    Attached Files:

  5. Oct 21, 2013 #4


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    Homework Helper

    Assuming 60 degrees is from horizontal as you've shown, it looks OK so far.
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