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Homework Help: Kinematics of a thrown rock. Calculate speed

  1. Jan 17, 2004 #1
    A rock is thrown at 25 m/s in a 50 degree angle (horizontal). At what time will the speed reach an angle of + / - 30 degree (horizontal) ??

    answer is 1.01s and 2.91s

    thx u
     
  2. jcsd
  3. Jan 17, 2004 #2
    Pls Post your try
    Hint:Write vector Equation for motion
     
  4. Jan 17, 2004 #3
    lol...
     
  5. Jan 17, 2004 #4
    lol.. posted at wrong place

    my try:

    Vx = Vxcos Theta
    = 25cos50
    = 16.07 m/s

    Vy = Vysin(theta)t
    = (25sin50)t - 4.9t²
    = 19.15t - 4.9t²


    Vy / Vx = tan30

    therefore.. 4.9t² - 19.15t + 9.28 = 0.... doesnt give the right answer :( what Do I do wrong??
     
  6. Jan 17, 2004 #5
    is that equation possible to-do without a Calculator ?
     
  7. Jan 17, 2004 #6
    obvs. not.. look at the answers.. 1.01 & 2.91 s..
    If u can get those numbers without a calculator, good luck
     
  8. Jan 17, 2004 #7
    Gimme a min on that one ;)...
     
  9. Jan 17, 2004 #8
    You have applied wrong formula for vy
     
  10. Jan 17, 2004 #9
    I really appreciate your help.. but would you mind explaining instaed of giving hints?
     
  11. Jan 17, 2004 #10
    dang (Should have payed attention to my math teacher...)LOL


    well i know theres gravity 9.8m/s, air friction,

    so vector is 25m/s at 50(Deg)angle
    how long it would take for 30(Deg)angle hmmm....
     
  12. Jan 17, 2004 #11
    and the formula I applied is actually Vy = Vysin(theta)t - gt²/2

    I just forgot the last part
     
  13. Jan 17, 2004 #12
    didnt your mom tell you not to use Calculators in math ?
     
  14. Jan 17, 2004 #13
    gimme 10mins tops and all figure out the Anwser without a Calculator ;)
     
  15. Jan 17, 2004 #14
    Here u have given your try i appreciate and on this network which is an interactive one.

    we try to inculcate the habit of problem solving by the poster itself
    giving u the answer would defy the process of learning

    Still you are welcomed and Pls wait while i give u complete detail
     
  16. Jan 17, 2004 #15
    k.. anyone help?
     
  17. Jan 17, 2004 #16
    ok thx a bunch
     
  18. Jan 17, 2004 #17
    Sorry DR33 but i think he means your on your own...:(
     
  19. Jan 17, 2004 #18
    [tex]v_x=25\cos{\theta}=25\cos50[/tex]
    [tex]v_y=25\sin50 - 9.8t[/tex]

    Now from gien pro
    [tex]\frac{v_y}{v_x}=\pm tan30[/tex]
     
    Last edited: Jan 17, 2004
  20. Jan 17, 2004 #19
    i got rejected in a Physics class, seriouly i would not comment on it but, its not helping someone who is dedicating his time on this forum for help.... :|
     
  21. Jan 17, 2004 #20
    ok so therefore

    Vx = 16,07 m/s
    Vy = 19.15 - 4.9t

    Vx / Vy = tan30... solve for t..

    doesnt give the right answer :((

    answers are 1.01 and 2.91
     
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