Results 1 to 4 of 4

Math Help - Projectile Motion #1

  1. #1
    Junior Member
    Joined
    Oct 2008
    Posts
    68

    Projectile Motion #1

    Can someone help me with this problem. Thanks a lot!
    Side is going to attempt a field goal by kicking a football from a point 40 yards out from the goal. The ball must clear the crossbar that is 3m off of the ground. Skid kicks the ball with a velocity of 20m/s at an angle of 45 degrees.
    a)Does the ball clear the crossbar?
    b)By how much does it clear or fall short of the crossbar?
    c)Is the ball rising or falling as it approaches the crossbar?

    What I did so far: y = ynot + vnot t +1/2 at^2
    I then solve for t = squaroot 2(y-ynot)/9.8 and got .78. Then I took 20 x .78 and got 15.62 m but this is wrong. Can you tell me what I did wrong or how to work this problem out. Thanks again!!
    Last edited by mr fantastic; October 5th 2009 at 03:20 AM. Reason: Re-titled thread
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,862
    Thanks
    626
    Quote Originally Posted by jsu03 View Post
    Can someone help me with this problem. Thanks a lot!
    Side is going to attempt a field goal by kicking a football from a point 40 yards out from the goal. The ball must clear the crossbar that is 3m off of the ground. Skid kicks the ball with a velocity of 20m/s at an angle of 45 degrees.
    a)Does the ball clear the crossbar?
    b)By how much does it clear or fall short of the crossbar?
    c)Is the ball rising or falling as it approaches the crossbar?

    What I did so far: y = ynot + vnot t +1/2 at^2
    I then solve for t = squaroot 2(y-ynot)/9.8 and got .78. Then I took 20 x .78 and got 15.62 m but this is wrong. Can you tell me what I did wrong or how to work this problem out. Thanks again!!

    first solve for t ...

    t = \frac{\Delta x}{v_x}

    t = \frac{40}{20\cos(45)}

    use that value of t to find y ...

    y = y_0 + v_{0y} \cdot t - \frac{1}{2}gt^2

    y = 20\sin(45) \cdot t - 4.9t^2

    if y > 3 , it clears the crossbar ... if not, oh well, they lose.


    to answer the last question, find v_f in the y-direction at the time you calculated above.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Oct 2008
    Posts
    68
    Hey, I did exactly what u told me and got 2.8 s, then I plug that into the equation and got 1.18, but the correct answer was .54m so can u tell me what I did wrong again.
    40/20cos45= 2.8s
    20sin45(2.8)-4.9(2.8)^2=1.18.....
    for c) V= vnot +at
    20- 9.8(2.8)
    =-7.44 ?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,862
    Thanks
    626
    Quote Originally Posted by jsu03 View Post
    Hey, I did exactly what u told me and got 2.8 s, then I plug that into the equation and got 1.18, but the correct answer was .54m so can u tell me what I did wrong again.
    40/20cos45= 2.8s
    20sin45(2.8)-4.9(2.8)^2=1.18.....
    for c) V= vnot +at
    20- 9.8(2.8)
    =-7.44 ?
    ok ... forgot to change yards to meters

    40 yds = 36.576 m

    so t = 2.59 sec

    y = 20\sin(45) \cdot t - 4.9t^2 = 3.8 m
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Projectile Motion
    Posted in the Calculus Forum
    Replies: 0
    Last Post: October 18th 2010, 10:06 PM
  2. Projectile motion.
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: July 15th 2010, 07:41 AM
  3. projectile motion
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: June 4th 2009, 11:53 PM
  4. projectile motion
    Posted in the Calculus Forum
    Replies: 2
    Last Post: May 19th 2009, 06:00 PM
  5. Replies: 1
    Last Post: February 19th 2009, 02:30 PM

Search Tags


/mathhelpforum @mathhelpforum