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Homework Help: Vectors Hard

  1. Feb 19, 2010 #1
    1. The problem statement, all variables and given/known data
    A projectile os launched with velocity of 200m/s at an angle of 45deg to the horizontal. Calculate:
    a) the maximum height reached
    b) the time of flight
    c) the range


    2. Relevant equations
    v^2=u^2+2as
    s=ut+1/2 at^2


    3. The attempt at a solution
    I know the first 2 but c) I am very confused.
    s=(u cos theta)
    t=(200 cos 45)

    Please give me a hand with c)
    Thanks
     
  2. jcsd
  3. Feb 19, 2010 #2

    berkeman

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    Staff: Mentor

    Your notation is confusing -- it would be easier to call the components of V by something intuitive like Vx and Vy.

    In this type of problem, the usual approach is to use the fact that Vx is constant (no gravity that way), and you only have an acceleration component in the Vy direction. Calculate the time of flight based on the fact that your projectile falls back down to y=0 at some time t, and solve for t. Then since Vx is constant, you can ....
     
    Last edited: Feb 19, 2010
  4. Feb 19, 2010 #3
    That doesn't make sense sorry.
     
  5. Feb 19, 2010 #4

    berkeman

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    Staff: Mentor

    Which part doesn't make sense? Start by writing the kinematic equations of motion using Vx and Vy notation to keep things clear.

    x(t) =
    y(t) =

    Vx(t) =
    Vy(t) =

    Then write the two simultaneous equations for the x and y motion of the projectile:

    x(t) = (has to do with the Vx(t) which is constant, and time)
    y(t) = (has to do with the Vy(t) which depends on Vy(0) and gravity g and time)

    Solve the 2 simultaneous equations for when the projectile hits the ground at the far end (hint -- what does y= when it hits the ground). That gives you the time of impact. The rest is gravy...
     
  6. Feb 19, 2010 #5
    A projectile os launched with velocity of 200m/s at an angle of 45deg to the horizontal. Calculate:
    c) the range

    To figure out range you can just use: s = vt or << answer deleted by berkeman >>
     
  7. Feb 19, 2010 #6

    berkeman

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    Staff: Mentor

    It's okay to help, but it's not okay to give out an answer. Please re-check the Rules link at the top of the page, particularly the part about Homework Help.

    This thread may also be instructive:

    https://www.physicsforums.com/showthread.php?t=373889

    .
     
  8. Feb 19, 2010 #7
    s=vt or what?
     
  9. Feb 19, 2010 #8
    Why was a formula deleted? How's that giving an answer
     
  10. Feb 19, 2010 #9
    Please give me the formula Sirsh.
     
  11. Feb 19, 2010 #10
    I wont give you it directly. but if you have a look on wikipedia under trajectory or trajectory of a projectile. it should be there
     
  12. Feb 19, 2010 #11
    But there are so many equations which one is right :(.
     
  13. Feb 19, 2010 #12
    oh don't worry found it :D
    Thanks
     
  14. Feb 19, 2010 #13
    Is this eqaution good for finding the answer to c),
    s=v^2/g.
     
  15. Feb 19, 2010 #14
    That could work. don't forget the rest of the equation just incase you get a problem, as those equations in comparison to the 2D motion ones are always off by around a tenth place. just stick with s=vt, if you have found the time then you should be fine with that equation.
     
  16. Feb 19, 2010 #15
    Thanks, that makes sense.
     
  17. Feb 19, 2010 #16
    No problem mate.
     
  18. Feb 19, 2010 #17

    berkeman

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    Staff: Mentor

    You did the right thing later in this thread. Instead of handing out a formula, hint at what it might be and mention a resource where they might find it. Making the OP do more of the work helps them learn how to solve problems better on their own in the future.

    Even better yet, don't hint at a canned formula, give tutorial hints at how to solve the problem from the basics. That will generally help the OP's understanding of how to approach these general types of problems better in the future on their own (like on tests).
     
  19. Feb 19, 2010 #18
    Oh okay, I'll do that instead then :) also, what does OP mean?
     
  20. Feb 20, 2010 #19

    berkeman

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    OP is original poster, the person who started the thread with the original question. Thanks.
     
  21. Feb 20, 2010 #20
    oh okay. thank you.
     
  22. Mar 1, 2010 #21
    Thankyou guys.
     
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