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: Dynamics f=ma

  1. Feb 3, 2012 #1
    1. The problem statement, all variables and given/known data

    A baseball slugger connects with a pitch 4 ft above the ground. The ball heads toward the 10-ft-high center-field fence, 455 ft away. The ball leaves the bat with a velocity of 125 ft/s and a slope of 3 vertical to 4 horizontal. Neglecting air resistance, determine whether the ball hits the fence (if it does, how high above the ground?) or whether it is a home run (if it is, by how much does it clear the fence?).

    2. Relevant equations

    F=ma (Euler form)

    3. The attempt at a solution
    The book gives the answer as home run, clears fence by 2 ft.

    -mgj = m[ x2i+y2j+z2k) Note that i,j,k are unit vectors, and x2, y2, and z2 means double dot or acceleration.

    -g = y2
    0 = x2

    y1 = -gt + C [C=125*(3/5)] => y1 = -gt + 75 (y1 is y dot or velocity)
    y = (-gt^2)/2 + 75t + C [C=4] => y = -(gt^2)/2+75t+4

    x1 = C [C=125*(4/5)] => x1 = 100
    x = 100t + C [C=0 because no movement in x direction at t = 0] = > x=100t

    Got these equations, then plugged in 455 ft into x=100t to get t=4.55s. So at 4.55s the ball would reach the fence. As to how high it will be, plug t=4.55 into the y equation, and my answer is 243 ft. The fence is 10 ft, so it clears it by 233 ft. But the book's answer is 2 ft. I know I screwed up somewhere, but I can't see it. Any help would be appreciated.
  2. jcsd
  3. Feb 3, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    hi xzibition8612! :smile:

    (btw, pleeease don't write x1 x2 etc, write x' x'' :wink:)

    did you use 9.81 for g, or 32 ?
  4. Feb 3, 2012 #3
    wow :cry:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook