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Homework Help: Projectile Motion 2D - Spring Launched

  1. Feb 13, 2014 #1
    Projectile Motion 2D -- Spring Launched

    There is this ball launched from a slingshot. Given the mass is 0.9 kg at the angle 60° to the horizontal, with k=750 N/m and is stretched by length l = 0.6 m. The ball is shot 1 m above the ground. (g=10 m/s2).

    The questions are to derive the equation describing the velocity and trajectory with the aerodynamic drag negligible.

    My approach is first to find the acceleration in x and y direction.
    F = k.x = 750x0.6 = 450 N
    Fx = F cos 60 = 389.71 N, Fy = F sin 60 = 225 N

    Fx = m*ax
    ax = 250 m/s2

    Fy - mg = m*ay
    ay = 423.01 m/s2

    Then I find the Velocity
    Vx = Vox + 250t
    Vy = Voy + (423.01-10)t = Voy + 413.01t

    My problem is now finding the initial Velocity Vo. I was thinking of using Energy principal but then I am stuck with the velocity of the ball at certain point, thus, I was not able to solve it.
  2. jcsd
  3. Feb 13, 2014 #2
    The initial velocity is zero.
  4. Feb 13, 2014 #3


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    Gold Member

    So the initial velocity will be nonzero.
  5. Feb 13, 2014 #4
    Oh yes that's right my bad. Use an energy equation 1/2kx^2 = 1/2 mv(0)^2.
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