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Homework Help: Calculating Velocity

  1. Apr 28, 2008 #1
    [SOLVED] Calculating Velocity

    A shell is fired at an elevation of 60degrees from horizontal, it needs to reach a target
    Sx=15000m, Sy = 400m. What is the initial velocity of the shell?

    So far I have done this...
    Ax=0 Ay=-g
    Vx=V cos 60 Vy=-gt+V sin 30
    Sx=V cos 60 t Sy=((-gt^2)/2)+sin 30

    I've tried to work from here (using Sx to find t=15000/Vcos60, then substitute that into Sy formula) but I've been getting a negative number which can't be right.
    Could someone show me the working or give me any help at all?
  2. jcsd
  3. Apr 28, 2008 #2
    Your equations are correct (except for a missing V in Sy=.

    Your methodology should work as-well, you must be getting a negative from taking the wrong square root, or something about the trig functions.
  4. Apr 28, 2008 #3
    Thanks Izkelley
    So I've done this...
    400= (-9.8/2)(15000/Vcos60)+Vsin30

    Is that the correct?
  5. Apr 28, 2008 #4
    The initial vertical velocity component is not Vsin30.

    Sy=1/2at^2+ut. So the second term on the right hand side of your equation should be multiplied by t and you need to square t in the first term.
  6. Apr 28, 2008 #5
    damn yeah, sorry.
    the sin30's should be sin60 also (same angle opposite trig function) or cos30's (same trig function opposite angle --- sin30 = cos60, sin60=cos30 ).
    and not only is Sy missing a V, its missing a t i.e.
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