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Homework Help: Calculate time traveled from launch to land

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data
    So, this is pretty easy and simple, I am just missing something obvious here I am pretty sure. This stems from a question I asked earlier tonight.
    Calculate the time it took from launch to land, given a velocity of 30.197m/s, and a distance traveled of 85m. (other irrelevant data: Object was initially launched at 33 degrees. Refer to https://www.physicsforums.com/showthread.php?p=2890328#post2890328 for more info)

    2. Relevant equations
    Perhaps, X=Xo+Vot+.5at^2 ???

    3. The attempt at a solution
    So, I would use the formula X=Xo+Vot+.5at^2, correct? But then, I get a bit confused...
    divide by t
    but that is incorrect, I think... SO I dunno... Help, please... Thank you

    Oh, wait... Am I maybe just way over-analyzing this? Would it be as simple as Distance/Velocity=Time? Therefore, V=30.197m/s , and D=85m, so 85/30.197=2.815s. But that seems a bit short. The empirical data is 4s (we must find the percent error), that is quite a large percent error...
  2. jcsd
  3. Sep 19, 2010 #2


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    Homework Helper

    In the horizontal direction there is no acceleration.

    s = vo*cos(theta)*t.
  4. Sep 19, 2010 #3
    Which is why I excluded 'a' when simplifying the equation, because a=0. Let me try that equation. Hmm... that seemed to yield a much more reasonable answer, 3.356s. That seems to be correct. Yeah, I get it now. It makes sense. Funny how things can just click and then you get it. Thank's for the help, I knew that it was really simple, I was just making it complicated.

    Do you think you could help me really quickly with one more? Sorry.
    Q: Find the initial velocity, when the object is shot at 70 degrees and took 6.5s to land. (hint: look at the y direction first this time)

    Now, I know that there is the method of dividing it up into components of x and y, however I did not quite understand that. When calculating the initial velocity, given distance traveled and degree, I used Range = v02sin2θ/g which worked great (refer to https://www.physicsforums.com/showthread.php?p=2890328#post2890328 ), however, I don't know if that can apply to this too... Sorry, but physics is really confusing me at the minute
  5. Sep 20, 2010 #4


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    Q: Find the initial velocity, when the object is shot at 70 degrees and took 6.5s to land. (hint: look at the y direction first this time)

    If it lands on the ground, vertical displacement y is zero. Vertical component of the velocity is vo*sinθ.
    Use the equation
    y = vo*sinθ* - 1/2*g*t^2

    Substitute the values and find vo.
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