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Homework Help: Finding angle of projection

  1. Jul 17, 2016 #1
    1. The problem statement, all variables and given/known data
    A projectile is fired with a velocity of 60 m per second at an angle, theta, from a horizontal ground towards the roof of a tunnel of height 5m. If the projectile just barely touches the roof, determine the
    a) angle of projection
    b) time of the flight
    c) range of the projectile

    2. Relevant equations
    2-Dimensional Equations

    3. The attempt at a solutions
    I had tried to find it by finding the velocity Y componet first, using v square=u square +2as, after i found it, I use tan theta= y/x, and the answer came out were wrong. And now i have no clue how to do it, please help me
  2. jcsd
  3. Jul 17, 2016 #2


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    Homework Helper
    Gold Member

    Your method sounds right. Perhaps you could show the detail so that we can check your calculations?

    Edit: though now I do it myself, your tan(θ)=y/x looks suspicious.
    After you've found Y, the vertical speed, what do you do next?
    Last edited: Jul 17, 2016
  4. Jul 17, 2016 #3


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    Science Advisor
    Gold Member

    The projectile will follow a curved path - not a straight line .

    Have a look at the way this problem was solved . You can use similar methods to solve your problem .
  5. Jul 17, 2016 #4
    yes, you're correct. After you point it out, I look over my drawing, and found out i put the initial velocity at wrong axis(x axis), which should be at the z axis. Now i redraw it, I'm believe this is right. And now i redo it with sin (θ)=y/z, I'm still getting the wrong answer.
    Edit: it's seem my picture is not displaying, I'm looking into it.
    Last edited: Jul 17, 2016
  6. Jul 17, 2016 #5
    Consider the y axis first

    It says that the object barely touches the roof, that means that the maximum height is 5 m
    Also at the maximum height the y component of the velocity is 0

    Set the two equations of kinematics on the y axis with those information, you should get a system of two equations with variables the angle theta and the time
  7. Jul 18, 2016 #6
    thx! I got the answer
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