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Projectile Motion Issue

  1. Feb 4, 2014 #1
    1. The problem statement, all variables and given/known data
    If I know a launched objects initial velocity and angle of launch, how would I find a specific x value given a certain y value?

    2. Relevant equations

    3. The attempt at a solution
    I tried setting the equation y=Vo*sin(angle)*t-.5gt^2 to equal the y value given which is 45. Then I should be able to solve for t and plug that into the x equation for kinematics which is x=Vo*cos(angle)*t in turn finding the desired x value. The equation as result isn't even solvable in my case so I'm confused as to why it doesn't work.
  2. jcsd
  3. Feb 4, 2014 #2


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    Why is it not solvable? Can you post the exact question and your effort?
  4. Feb 4, 2014 #3


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    I'm not sure either. Can you post your work? There may be a hidden error.
  5. Feb 4, 2014 #4


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    Sort out the things that are given.
    Sort out what is being asked
    Collect the relevant equations
    work out a way to solve them.

    What you've written down so far is not sorted out and does not constitute a solvable problem

    1. The problem statement, all variables and given/known data -- what is being asked ?
    -------- given data ? I only see a number 45 for the y value. 45 furlongs ?

    2. Relevant equations. I'm not allowed to shout. In 3 you come up with y=Vo*sin(angle)*t-.5gt^2 and x=Vo*cos(angle)*t All they tell me now is that you are well equipped to deal with the problem.

    3. The attempt at a solution. Not bad -- you are smart enough to try something sensible, count the number of unknowns and conclude: unsolvable.

    My conclusion: either you overlooked something in the original problem formulation, or there is a genuine error in there. The odds are in favor of the former.

    Use the template. You need it.

    I see you're already in the hands of several heavyweights in the time I need to type this. Do your thing!
  6. Feb 4, 2014 #5
    A golf ball is hit with initial speed 116 ft/sec at 45 deg towards a green elevated 45 ft above the tee where the ball was launched. The hole is 369 ft away from the tee. Where will the ball land in relation to the pin?

    After converting ft to meters in all cases: y=(35.3572 m/s) * sin45 * t - 4.905 m/s^2 * t^2

    let y = 45

    0=(35.3572 m/s) * sin45 * t - 4.905t^2 - 45

    Using a computer algebra system to solve for t it came back false. Also, the graph of that function does not even cross the x-axis (tried finding the zeros).
  7. Feb 4, 2014 #6
    . . . I mean 13.7162 m not 45 ft.
  8. Feb 4, 2014 #7
    I think my error was converting all this metric stuff and I confused myself between typing here and typing into my calculator.

    It is solvable the second time around.
  9. Feb 4, 2014 #8


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    You're doing fine. Glad my hunch was right. Not too hasty, not too sloppy and you'll save time and score higher.
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