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Projectile motion, I cannot figure out how to do this

  1. Apr 25, 2010 #1
    1. The problem statement, all variables and given/known data

    The problem is, "A footballer kicked a ball up at 50 degrees and it landed 20m away horizontaly", assuming vertical acceleration is -9.81ms-2 how would you find the time it was in the air and the max height reached.

    How would you solve this?

    2. Relevant equations



    3. The attempt at a solution

    Various trigonometric functions, if you need more detail please tell me.
     
    Last edited: Apr 25, 2010
  2. jcsd
  3. Apr 25, 2010 #2
    well you know that when it lands s_j =0
    s0 s_i=20=v*cos(50) and S_j=0=vsin(50)t-9.81*.5*t^2
    you now have a system of simultaneous equations... solve it
    i get t=2.2s and v_init=14.11m/s
    For max height.
    at max height v_j=0
    v_j=0^2=u^2+2as where u= vertical cmpt of v_init(14.11sin(50)) and a=-9.81,/s^2
    0=(14.11sin(50))^2-2*9.81s solve
    i get s=5.95m
     
  4. Apr 25, 2010 #3
    can you let me know if this is right or useful please.
     
  5. Apr 25, 2010 #4
    Well, first you have to have velocity, to even begin this. We know when it lands, it is going 0 m/s. So vf= 0 m/s. Now, once you find Vi, you will need to divide that up into vertical and horizontal components of velocity. (cos * theta, sin * theta)

    Once you do that, you can find the time in air with
    d=vft + 1/2at2

    You have your horizontal distance, now solve for t.
    I'm not sure you could use vf= vi + at could work for t, because of the lack of distance. But if you did use that, t would NOT BE THE TOTAL TIME IN THE AIR. It would be the time it takes to get to the max height of the projectile. So you would have to multiply it by 2.
     
  6. Apr 25, 2010 #5
    No GreatEscapist that is not correct. Any high school physics text will tell you that all projectiles have a final velocity that is equal to the initial horizontal component + a vertical component relative to the acceleration(assuming no air drag). use the information i gave you it is correct.
     
  7. Apr 25, 2010 #6
    Dang it. :P
    Well, I tried. Sorry about that. At least now I know I need to brush up on my projectile motion. :S
     
  8. Apr 26, 2010 #7

    ehild

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    Gold Member

    Apart from the missing t here "s_i=20=v*cos(50)" , your solution is correct. Good job !!

    ehild
     
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