1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Projectile motion football question

  1. Jul 31, 2009 #1
    Hi folks,

    Need some help on a projectile motion question.
    1. The problem statement, all variables and given/known data
    - a football kicker can give the ball an initial speed of 25m/s
    - kicker wants to score a field goal 50m in front of goalposts
    - horizontal bar of goalposts is 3.44m above the ground

    What are the a.) least and b.) greatest elevation angles at which he can kick the ball to score a field goal?

    2. Relevant equations

    3. The attempt at a solution
    I have done the following:
    1.) y = y0 + (v0sinθ)t - 0.5(9.81)t2
    so, 3.44 = (25sinθ)t - 4.91t2
    2.) x = x0 + (v0cosθ)t
    so, 50 = (25cosθ)t
    so, t = 2/cosθ

    I then substituted eq. 2 into eq. 1, and got the following:
    3.44 = 50tanθ - 19.62sec2θ

    but I am stuck here.

    Could someone please tell me if I am on the right track? Is there a way to solve the above equation, or am I approaching this in the wrong way?

  2. jcsd
  3. Jul 31, 2009 #2
    Don't have time to check it now. But here is what I would try. There is a trig identity that will turn sec^2 into tan^2. You would end up with a quadratic in tan that could be solved for two roots. That would get you the two angles. In other words, change sec^2 to tan^2 using the identity and them make a substitution like z = tan(theta). Solve for the two values of z and then take the inverse tangent.
  4. Jul 31, 2009 #3
    Thanks Chrisas! I did look at my trig identities, but for some reason I did not see this.

    I got my two angles (31 degrees and 63 degrees), which I know are correct.
  5. Jul 31, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There is another way that might be easier. Let x = cos[tex]\theta[/tex]. Express the tangent as [tex]\frac{\sqrt{1-x^{2}}}{x}[/tex] and you know what to do with the secant. Isolate the radical on one side of the equation and square both sides to get rid of it. You should end up with a fourth degree equation, but one that you can solve because the odd powers in x are missing. This means that you can use the quadratic formula to solve for x2. Throw out any negative roots for x2 as unphysical. Get the angle from the cosine.
    Last edited: Jul 31, 2009
  6. Jul 31, 2009 #5
    Hi kuruman - thanks for this alternative. I had been trying for a while to get a single trig function, but failed. In hindsight I cannot believe I did not see it. More practice required. ;-)
  7. Jul 31, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Think of it this way: What I call the auxiliary trig functions (tan, cot, sec, csc) can all be cast in terms of sin and cos and then sin can be converted to cos if needed. From my experience, this general method will see you through most of the time. If not, the next best thing is to try the half-angle trig identities.
  8. Jul 31, 2009 #7
    That is helpful - I will certainly keep this in mind. Thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook