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Homework Help: Basic Mechanics Problem(2D Motion)

  1. Oct 13, 2007 #1
    Yeah, I pretty much spent 5 hours today trying to figure this question out, yet for some reason I keep getting the wrong answer. I told my friends I'd do it and explain it to them, but I still can't do it right. Can some one please show me the steps using only the letters, not just numbers. Thanx.

    A projectile is launched into the air from the top of an incline that makes an angle f =39° with the horizontal as show in the diagram below. The launch speed, v, is known to be 10.0 m/s but the launch angle, q, is unknown. However, the maximum height above the launch level that the projectile reaches is known to be h = 2.51 m. The trajectory is a perfectly parabolic. What is the angle, a (in degrees), with which the projectile strikes the incline (measured relative to the incline as shown)? Use g = 9.80 m/s2.
    (I dunno if the picture works, but give it a shot)

    2. Relevant equations

    Basic Motion Equations
    Vf = Vi + A(D)T
    (D)X = Vi(D)T + (A(D)T^2)/2
    Vf^2 = Vi^2 + 2A(D)X
    (D) is Delta
    I know Calculus, but my friends don't yet.

    (I would right down one of my attempt, but it would take way too long!)
    I understand how you're supposed to do it(break it up horizontally and vertically) and whatever else(ex: hor. vel. doesn't change, but vert. does due to gravity), but for some reason all the things I tried never gave me the right answer. I filled nearly five pages of stuff!
    Basically, I seperated the problem into four points each with their x and y values(position, velocity, acceleration) and the change in time at each one. Using the 3 equations I'm able to find all the other data, but for some reason I keep messing up no matter what way I do it. I'm certain that q = arcsin(((root)2AH)/10), but I mess u psomewhere along the line. So yeah, any help would be great.
  2. jcsd
  3. Oct 13, 2007 #2
    I did this quickly, so you should double check, but I think your equation is close to correct but not quite. Should it be divided by 100 instead of 10? I think you originally had (10sinq)^2, so don't you have to square the 10 as well as the sinq? (Remeber, I could be wrong).
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