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 So many Unknowns

  1. Jun 27, 2012 #1
    1. The problem statement, all variables and given/known data
    Need Help
    I have been trying to solve this problem for a while now. I know the quadratic formula has to be used in order to solve this problem but I am having trouble getting started.


    A soccer ball is kicked from the ground level at an angle, such that the wall is hit at (point C) 1.9 seconds later. Point C (the wall) is 5.7m above the ground and the wall is 30m from where the ball is kicked. Ignore the effect of air resistance. Determine the magnitude and direction of initial velocity. How high the ball goes (point B, the highest point) the magnitude and direction of the velocity at the point of impact with the wall.

    Voy-not given
    Vox-not given
    Vmag-not given
    Angle -not given
    Distance to point B-not given
    Time to point B-not given
    Time from point B to wall-not given
    Distance from point B to wall not given

    total time 1.9 sec
    Height of wall where soccer ball hits 5.7m
    Displacement -30m


    At Part B:
    Vy = 0

    2. Relevant equations
    DeltaX=V(T)+ .5(at)

    ax2 + bx + c = 0

    3. The attempt at a solution

    I am not sure which formula to use since there are so many unknowns. I was thinking about using the distance formula just to part B and use the quad formula to solve to the two times but the distance to part B is not given.

    Or should I use the third equation and say the final velocity is zero?

    Is this how to get the Angle:
    30m, 5.7m, and X as the hypothenuse.

    X= 30.5
    Angle= arc Tan (5.7/30.5)= 10.75
    Last edited: Jun 27, 2012
  2. jcsd
  3. Jun 27, 2012 #2
    Actually Never mind about the angle. I know that is completely wrong for this kind of problem.
  4. Jun 27, 2012 #3
    For this kind of problem you need to write the equations of motion separately for the horizontal and vertical directions (x and y). Then see what is given for each direction and can be found.
    Your equation are some sort of mixture between the two directions.
    What kind of motion is the horizontal component?
  5. Jun 27, 2012 #4
    I know they need to be broken up into two components. There is just not enough information to go from here
    Vox-?, Vfx-? total distance 30 m
    vx = v0x, x = v0xt,
    Voy-?, Vfy-?, Vy at point B-Zero, Point B-?, Point C- 5.7m
    vy= v0y - gt, y = v0yt - (1/2)gt2.

    Time 1.9sec
  6. Jun 27, 2012 #5


    User Avatar
    Homework Helper
    Gold Member

    There's enough information. :smile:
    Okay, let's start with the x-component before moving to the y.

    What are the forces involved in the x-direction? (Remember, you get to ignore air resistance.) Are there any? What does that tell you about the acceleration in the x-direction? And thusly, what does that tell you about the relationship between v0x and vfx?

    By the way, before we move onto the y-direction, has your coursework introduced the concept of conservation of mechanical energy yet? If so, it will make the rest of this problem a little easier. If not, it's still solvable anyway. I'm just curious is all.
  7. Jun 27, 2012 #6
    deleleting my post
  8. Jun 27, 2012 #7
    Hello orangem,
    Well the problem involves you to understand the following points:
    (i) How does the horizontal component of velocity change with time?And by that nature how can we express the horizontal displacement of the ball w.r.t the time.The horizontal distance corresponding to a given time is given right?
    (ii)How does the vertical component change w.r.t time? How does one express the vertical displacement in given time then?The vertical displacement and the time taken also given.Calculate the maximum height of the projectile.
    Then after calculating the components the magnitude and the direction are asked to be calculated.
    Note that in your fourth post you gave v(y)=0 at collision which may/may not be true.
    Does this help?
    Last edited: Jun 27, 2012
  9. Jun 27, 2012 #8
    Ok x component
    No air resistance. The initial velocity will be the same as the final velocity. And there is no acceleration. So as I previously stated Vfx=Vox. Does this mean the Vox=15.8 because displacement=VoxT. So Vx=15.8 and Vox= 15.8
  10. Jun 27, 2012 #9


    User Avatar
    Homework Helper
    Gold Member

    (Don't forget your units.)

    Okay, that looks good so far. :approve: Now come up with an expression for vyi as a function of θi. (Keep it in terms of θi for now, since you don't know what that is yet. Don't worry, you'll be solving for θi soon enough.)
  11. Jun 27, 2012 #10
    Sounds correct :smile:
    Now for the y component?

  12. Jun 27, 2012 #11
    ok anyways vy = v0sinθ-at
  13. Jun 27, 2012 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Well, the horizontal component of the ball is 30m, & it covers this horizontal distance in 1.9 seconds, so (V0)x = 30/1.9 . Is that how you got 15.8 m/s ?

    If there were no gravity, the ball would travel in a straight line, correct? Let's call that point, D.

    But there is gravity, so the ball hits at point C. The distance from D down to C is how far the ball will drop in 1.9 seconds. (It's just like the "monkey shoot" if you're familiar with that.) That's one way to figure this out without too many equations. --- I don't much like looking through a bunch of equations to randomly pick out one that might work.
  14. Jun 27, 2012 #13
    The equation works no doubt with a=g but will that help you in getting the y component here?Think of an equation of kinematics which correlates the total displacement with initial velocity ,time and acceleration.You have the vertical displacement ,the acceleration and the time taken given so the vertical velocity (initial) is the only unknown.
    Does this help?
  15. Jun 27, 2012 #14
    y=VoyT-.5*gt^2 but I do not know the vertical displacement
  16. Jun 27, 2012 #15
    or Voy
  17. Jun 27, 2012 #16


    User Avatar
    Homework Helper
    Gold Member

    From the problem statement:

    "Point C (the wall) is 5.7m above the ground."​

    So you know what 'y' is, you know what t is, you know what g is, so you should be able to solve for v0y, right? :wink:
    Last edited: Jun 27, 2012
  18. Jun 27, 2012 #17
    is it 12.31m/s

    5.7=Voy (1.9) - .5(9.8)(1.9^2)
    5.7=Voy (1.9) - (17.689)
  19. Jun 27, 2012 #18


    User Avatar
    Homework Helper
    Gold Member

    That looks about right (out to the first three significant figures anyway.) :smile:

    So so now that you know the initial y-component of velcity v0y, what's the final vy when it hits the wall? (Back to kinematics.)*

    *(At around this point, or a little before, you could use conservation of energy to complete the rest of this problem. But since you've been using kinematics so far, you might as well stick with that, since kinematics works just as well.)
  20. Jun 27, 2012 #19

  21. Jun 27, 2012 #20


    User Avatar
    Homework Helper
    Gold Member

    Ummm. :uhh: I think you are confusing two of your kinematics equations, and mixing them [STRIKE]up[/STRIKE] together. (Try a different kinematics formula.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook