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-Baseball

  1. Sep 18, 2014 #1
    1. The problem statement, all variables and given/known data

    At bat a baseball player hits a ball at a height of .889 m. The ball leaves the bat at 51 m/s at an angle of 81 degrees from the vertical. The ball skims the top of a 2.74 m wall as it leaves the field.
    a.) Draw position, velocity and acceleration graphs describing the ball between the hit and when it leaves the field. Assume t=0 when the ball is hit, up is positive, the ball moves in positive horizontal direction, and that the pitcher's mound is the origin.
    b) How long is the ball in the air?
    c.) How far away from home plate is the wall?
    d.) What angle relative to the x-axis is the ball moving when it passes the wall?
    e.)If an infield player can jump to get his mitt 2.74 m off the ground, how close to home plate was he to still catch the ball.

    2. Relevant equations

    For the vector components:vy=v0sin(θ)-gt

    3. The attempt at a solution
    I've drawn the graphs for part a and I am sure I have done that correctly.

    I used y=y0+v0sin(θ)t-1/2gt2 to find t which I calculated to be 10.326 s

    I am confused on part C, ;the only reasonable equation seems to be Δx=v0cos(θ)t. But if I use this equation I will need an x(final) and x(initial). I know I am looking for x(final) and I think x(initial) is -18.29. But then I will have to add this to the answer for (v0cos(θ)t). But this does not make sense to me because I think I've already assumed 18.29 to be my origin and I've used this to calculate the time already. SO adding the 18.29 seems, to me, to be redundant.

    For part e: I got an angle of 80.9 deg. And I know that should be slightly less than the launch angle. But I thought it would be a larger difference.

    For part f: I am not sure what to do at all because I am not sure what the question is asking.

    Thank you for your help.
    Last edited: Sep 18, 2014
  2. jcsd
  3. Sep 18, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF;
    Very good - getting these right will make everything else much easier.

    What did you use for the final height?
    i.e. if you used the height of the wall - did the ball skim the top of the wall while it was on it;s way up or on it;s way down? Or was that right at the apex of the flight?

    Do you know how far the pitcher's mound (the origin) is from the batsman's position (home plate)?
    Note: you should be deriving the equations from the graphs rather than trying to pick them from a list. You said you drew these correctly - so you must have put the initial value of x on one of them.

    x=18.29 (units?) cannot be the origin. By definition, the origin is at x=0.

    The wall is somewhat higher than the height the ball got hit - but the parabola is a very steep one.
    Consider - if the ball was hit straight up, then it would have the same angle all the way down as all the way up.

    It's asking where the player has to be to catch the ball.
  4. Sep 18, 2014 #3
    Thank you for the quick reply.
    I used -2.473 as the final height since the ball hit the wall onthe way down. But I'm wondering if I should subtract the original height from this (so 2.473-.889) because the ball y(i) is not 0 but 0.889.

    I know the pitcher's mound is 18.29 m away from the launch point. So since the origin is 0( which is the pitcher's mound), the position of the launch point is -18.29m. So I should add this to the distance I've found, because I've only calculated the distance from 0 to the final height and not from the point of launch. Right?

    I still don't understand part e. If it's a steep parabola, then the difference between two angles in the parabola is not very large. Is that what you mean?

    For part f: So I should calculate the distance to the point where the ball is 2.743m above the ground as it is still going up (before reaching max height)?
  5. Sep 18, 2014 #4


    User Avatar
    Gold Member

    as far as c goes, your x velocity will be constant throughout the whole range of motion. so if you find your x component of velocity, you can just multiply that by time to get your distance.
  6. Sep 18, 2014 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Certainly the vertical displacement is ##\Delta y = y_f-y_i##

    Try just redoing the calculation from scratch - using the correct initial horizontal position.

    For an inverted parabola, the angle of approach does not change very much near the roots (the endpoints of the trajectory). Sketch a bunch and see.
    If the parabola is tall and narrow like this one, then the height can change a lot without changing the angle or horizontal position much ...so a big difference in height does not make a big difference to the angle. Since the angle is the inverse-tan of the slope - it is easier to see in terms of gradients.
    Again - sketch a bunch of them and see.

    ... that's what you have to decide. What makes sense? What happens if you do it the other way (while the ball is coming down) - where does the player have to stand?
  7. Sep 18, 2014 #6


    User Avatar
    Gold Member

    wasn't the angle 81 degrees from the vertical? so it would be 9 degrees from horizontal, very long and short?
  8. Sep 18, 2014 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Oh did I misread that - in which case, switch "vertical" for "horizontal" etc and the statement still stands.
    The ends near the roots don't change their slope by much.

    Note: in the formulas in post #1, ##\theta## is the angle from the horizontal ... another source of possible mistakes for brita.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted