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Homework Help: Projectiles problem, help needed

  1. Feb 11, 2015 #1
    Here is the problem (Q3 (A));
    ImageUploadedByPhysics Forums1423676612.394766.jpg

    Here's what I have so far;
    ImageUploadedByPhysics Forums1423676690.906861.jpg

    Any help would be appreciated, thank you!
  2. jcsd
  3. Feb 11, 2015 #2


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    It appears you made an algebra error when you went from ## 21 \cos \alpha = \frac {30}{ t} ## to ## \cos \alpha = \frac {630}{ t} ##.
    Also, I would first find your time of impact based on your horizontal component.
    With that time of impact, you can make an expression that is in terms of alpha only.
  4. Feb 11, 2015 #3
    Yes I realised this after I posted, I'll try that, thanks!
  5. Feb 11, 2015 #4


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    I made a substition of x = cos^2(alpha) and manipulated it into a quadratic equation to get the two solutions.
    part b comes right out of part a if you have already solved for time of impact.
  6. Feb 11, 2015 #5


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    Something else to note: If you have the expression ##\tan \alpha = \frac{A}{B}##, then you cannot conclude that ##\sin \alpha = A## and ##\cos \alpha = B##.
  7. Feb 11, 2015 #6
    Could you elaborate on this a bit? Not quite sure what you mean..
  8. Feb 11, 2015 #7


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    Do you have an expression that can be written as some trig functions of alpha = 0?
    Use sin = sqrt(1-cos^2) to convert everything in terms of cos.
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