1. Not finding help here? Sign up for a free 30min 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!

Projectile motion calculation

  1. Mar 4, 2015 #1

    Femme_physics

    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    A cannon has to fire a ball through a window in the wall. The window is 10 meters at the Y axis, and 8 meters (away from the cannon) at the X axis. The window is just slightly bigger than the ball (let's suppose that the dimensions are negligible). Find the required launching power.

    sketch.jpg


    Ball's mass: 0.6 kilograms
    Angle of launch: 51.34 degrees

    3. The attempt at a solution
    formula.jpg

    I seem to be getting a strange initial velocity, way too high. Is my formula off? Or calculations?
     
  2. jcsd
  3. Mar 4, 2015 #2

    Suraj M

    User Avatar
    Gold Member

    Well,in your formula..,your ##g## should be ##g= -9.8 units ##.
    Could you check your arithmetic again. And I hope you know how to do the rest of the question, though you have not posted it.
     
  4. Mar 4, 2015 #3

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No, the lady is doing just fine. It takes a bit of sleuthing to track what you are doing, but it's correct.

    Cannons can get huge muzzle speeds nowadays. It's not like they are throwing a stone or a ball....

    However, a straight line to the window has slope 10/8 and atan(10/8) = 51.3402##^\circ##

    And then the expression 10 - 8*tan gives zero !
    In other words, muzzle speed has to be infinite.

    That you get a finite result could well be due to rounding in the calculator.


    Then: If the window is only slightly bigger than the cannonball, you'll have to lift the cannon 10 m up, or else the ball won't get through :wink:
     
  5. Mar 4, 2015 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What is the arc-tangent of 10/8 ?
     
  6. Mar 4, 2015 #5

    gneill

    User Avatar

    Staff: Mentor

    There are any number of parabolic trajectories that can originate at the canon and reach the window. Given the tight fit, I suppose the trick is to choose one that happens to be traveling horizontally at the moment it passes though the window...

    Fig2.gif
     
  7. Mar 4, 2015 #6

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So where does the given firing angle come from ? Femme or problem statement ?
     
  8. Mar 4, 2015 #7

    gneill

    User Avatar

    Staff: Mentor

    It looks to be based upon the assumption of a straight-line trajectory from canon to window (not feasible with gravity acting). With a high enough initial velocity one could approach that goal if the window opening allows the ball to pass through at such an angle. Then it would be a matter of checking tolerances for such a scenario, but we don't have specifics.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Projectile motion calculation
Loading...