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Homework Help: Football Throw

  1. Feb 26, 2008 #1
    On this problem i have gotten stuck and i am not really sure why.

    1. The problem statement, all variables and given/known data

    A quarterback claims that he can throw the football a horizontal distance of 169.1 m (185 yd). Furthermore, he claims that he can do this by launching the ball at the relatively low angle of 30° above the horizontal. To evaluate his claim, determine the speed with which this quarterback must throw the ball. Assume that the ball is launched and caught at the same vertical level and that air resistance can be ignored.

    2. Relevant equations

    kinematics equations

    3. The attempt at a solution

    First I cut the trajectory in half to find the footballs maximum height

    than on the y coordinate is at its vertex its velocity at that point is 0 so i was able to put that in as well

    So my list came out as is

    X coordinate Y coordinate
    X=84.55 Y=48.814
    Xi=0 Yi=0
    a=0 a=-9.8 m/s/s
    Vi=? Vi=?
    V=? V=0

    than in order to get more information i solved for the ball to hit the ground from its highest altitude i.e. 48.814 meters.

    dist = 1/2 g t^2 (g=9.8 m/s/s) so i used this equation and came up with 3.1562 seconds

    Now I was able to solve for Vi for Y coordinate which i got 30.93m/s

    after this i just used the sin(30)=30.93/x and i got 61.8628m/s

    Was there an easier way to do this problem am i missing something very simple? but this answer is not matching up and cant seem to get it

    thanks i advance
  2. jcsd
  3. Feb 26, 2008 #2
    sorry about the double post!
  4. Feb 26, 2008 #3
    This way may be a little easier, and I got an answer that works.

    Just use the kinematic equation:

    [tex]\Delta s = v_i t + \frac{1}{2} a t^2[/tex]

    for each direction x and y. For each velocity, separate [tex]v_i[/tex] into components. Then you have two equations with two unknowns, so you can then solve for each unknown.
  5. Feb 26, 2008 #4
    would i use the seconds that i solved for t?
  6. Feb 26, 2008 #5
    No. You're basically starting over with t as an unknown.

    I did the problem myself this way. It's not too bad and really shouldn't take too long, I don't think as long as finding the height then the time the way you did.

    To get you started a bit, write down the variables you already know, x, y, a.

    Your unknowns are v and t. But you do know how to split v into [tex]v_1x[/tex] and [tex]v_1y[/tex]. So, then you should be able to solve for both t and v.
  7. Feb 26, 2008 #6
    ohh wow that is alot easier thanks for the help
  8. Feb 26, 2008 #7
    Its a good practice to go the basic way--splitting the motion into horizontal and vertical, but this question can also be solved by using this formula for horizontal range. Isn't it faster to put in the values and get the result ?

    [tex] R=\frac{v^2 sin2\theta}{g}[/tex]
  9. Feb 26, 2008 #8
    I have not encountered this equation yet but when i used it i did not get the correct answer. this is as R as the entire range correct?
  10. Feb 26, 2008 #9
    I am pretty much confident that this equation is correct. and yes, R is the complete horizontal range, which, according to your question is 169.1 m

    I got v=43.74 m/s.

    maybe I'm wrong..:redface:
  11. Feb 26, 2008 #10
    No, that's the answer I got doing it the other way, but I'm not familiar with that equation either.
  12. Feb 26, 2008 #11
    Maybe you'll encounter these equations later. They are all over in books on projectile motion.
    My teacher derived these equation for us, by splitting the motion into horizontal and vertical components.
    Its like this :

    Time of flight=[tex] \frac{2vsin\theta}{g}[/tex]

    Range= [tex] \frac{v^2 sin2\theta}{g}[/tex]

    Max height=[tex]\frac{v^2 sin\theta}{2g}[/tex]
  13. Feb 26, 2008 #12
    thats what i got i did it with only half the distance at first ha but when i did it for the full 169.1 it was perfect!

    Thanks alot googlespider and chococat you helped alot!

    What physics are you in google_spider? general, calc based?
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