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Homework Help: Projectile Motion with Relative Motion (w/o Solver)

  1. Apr 9, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    vx = 70*cos(α) + 8.333 (30 km/h = 8.333 m/s)
    vy = 70*sin(α)
    vx*t = 350
    vy*t = 60
    -(1/2)*g*t^2 + vy*t + 2.5 = 60

    3. The attempt at a solution
    This problem is conceptually very simple for me, but I can't solve it without using a solver like Wolfram Alpha. First, I simply rearranged vx*t = 350 into t = 350/vx = 350/(70*cos(α) + 8.333). Then, I plugged this t into the -(1/2)*g*t^2 + vy*t + 2.5 = 60 equation and I got an equation with only the angle α as the unknown. After moving the equation around so that there are no denominators, I get:
    600825.5 + 204167*sin(α) + 1715000*sin(α)*cos(α) = 281750*cos2(α) + 67083*cos(α) + 3993

    Is there a way to solve this equation without using a solver? Is there another method entirely which I'm missing? I have the right answers, which are α = 31.32° and 74.28°.
  2. jcsd
  3. Apr 10, 2016 #2


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    Science Advisor
    Homework Helper
    Gold Member

    You could turn it into a quartic in cos(α), but you would not be that much better off.
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