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Homework Help: Find where and when the trajectory angle is 10 degrees?

  1. Sep 15, 2012 #1
    1. The problem statement, all variables and given/known data
    Okay, this is a projectile motion problem(No Drag) where a rock is thrown as 35m/s at an angle of 48 degrees. The question that im struggling with is: "Find where and when the trajectory angle is 10 degrees?"

    2. Relevant equations

    The Equation that my teacher was used was that he used the path equation: y = xtan(∅) - gx2/2u2 (tan2(∅)+1)
    He then derived it and let it equal tan 10.

    However my main problem is that i dont understand why he derived it and then let it equal tan 10. Because the path equation depicts motion of projectile in relation to it y and x motion, thus if you derive it you would get the velocity motion which would not be useful for this application right?

    3. The attempt at a solution

    This is my teacher's solution:
    y = xtan(∅) - gx2/2u2 (tan2(∅)+1)
    tan 48 = 1.11 u = 35 g = 9.8

    y = 1.11x - 9.8x2/2450(2.2334)
    = 1.11x -0.004x2*2.2334
    = 1.11x - 0.00893x2
    dy/dx = 1.11 - 0.01763x
    tan 10 = 0.1763 = 1.11 - 0.01787x
    0.01787x = 1.11-0.1763
    x = 52.248m
    Ive tried using a range of equation, but they all rely on the initial angle, thus any attempt to sub 11° kinda wrecked the whole equation.

    Anyways if anyone could help me understand why the derivative of the path equation can work out when and where the trajectory angle is 11° it would be much appreciated,
    Cheers :)
  2. jcsd
  3. Sep 15, 2012 #2


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    Homework Helper

    In the projectile motion, at any instant, dx/dt is the velocity in the horizontal direction and dy/dt is the velocity in the vertical direction. The direction of the velocity at that instant is given by tanθ = vy/ vx = dy/dx. That is what your teacher did.
  4. Sep 15, 2012 #3
    Okay thanks, that makes a lot of sense actually. Haha thanks for your help :)
  5. Sep 15, 2012 #4
    I do it this way, with simple equation applied.

    At 10° trajectory angle,


    x=2.23 x 35.Cos48°=52.22m
    Last edited: Sep 15, 2012
  6. Sep 16, 2012 #5
    Hahaha wow thats even simpler, haha i guess it saves messing around with the path equation then, thanks heaps :)
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