Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dropping an object from a moving plane.

  1. Sep 20, 2006 #1
    I have to solve the following problem:

    "A rescue plane is flying at a constant elevation of 1200m with a speed of 430 km/h toward a point directly over a person in the water. At what angle of sight [tex]\theta[/tex] should the pilot release a rescue capsule if it is land near the person in the water?"

    The answer we are given is 57 degrees.

    Here is the setup I think we have:
    (we are not given a diagram)

    http://img137.imageshack.us/img137/5049/setupsq1.jpg [Broken]

    Wouldn't the angle needed depend on the "horizontal distance" as labeled in the graph? In other words, if the angle is a set 57 degrees then the horizontal distance must be some fixed value.

    We are not given the "horizontal distance" in the problem.

    I imagine I could figure out that distance since I know the answer of 57 degrees, but if I wasn't given the answer there is no way to have solved this problem without knowing the horizontal distance - correct?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 20, 2006 #2
    I think I figured out the solution. I am posting my solution.

    The object falls at a constant rate so we can determine the time the object takes to fall to the ground:

    y_f = - \frac{1}{2}gt^2 \\
    y_f = - 4.9t^2 \\
    - 1200 = - 4.9t^2 \\
    t = 15.642 \\

    We can now find the distance the object moves horizontally using the time we found earlier and knowing that

    x_f = v_i \cos \left( \alpha \right)t

    Alpha is zero since the object is dropped and falls straight down.

    x_f = 430\cos \left( 0 \right)t

    In the next step I also have to convert 430km/h to m/s

    x_f = \left( {\frac{{430{\rm{km}}}}{{1{\rm{h}}}}\frac{{1{\rm{h}}}}{{3600{\rm{s}}}}\frac{{{\rm{1000m}}}}{{{\rm{1km}}}}} \right)\cos \left( 0 \right)t

    x_f = 119.44t

    x_f = 119.44\left( {15.642} \right)

    x_f = 1868.3{\rm{ meters}}

    Here are my final answers:

    http://img164.imageshack.us/img164/7245/diagram0001np1.jpg [Broken]

    So I guess the correct angle the problem was looking for was [tex]\gamma[/tex]. Does this solution look correct?

    Except in my drawing [tex]\gamma[/tex] should be measured counterclockwise from the -90[tex]^{\circ}[/tex] axis (the arrow indicating the direction of measurement for [tex]\gamma[/tex] should be reversed.)
    Last edited by a moderator: May 2, 2017
  4. Sep 20, 2006 #3


    User Avatar
    Science Advisor
    Homework Helper

    Actually, calculating that horizontal distance is precisely the problem. Once you know that, figuring out theta is trivial.

    You can calculate how long it will take before the rescue capsule knocks the guy underwater. Since you also know the speed of the plane you can figure out at what horizontal distance you should release the capsule.

    And by the way, I think the angle theta should be measured from the vertical.
  5. Sep 20, 2006 #4
    I dont get how the object falls straight down if it has a velocity equal to that of the plane..
  6. Sep 21, 2006 #5


    User Avatar
    Science Advisor
    Homework Helper

    That's the only step that looks wrong. The horizontal velocity of the capsule is constant and equal to that of the plane. So that's why you use x=vt, where v is the velocity of the plane.
  7. Sep 21, 2006 #6
    here is the solution . i am not sure its right though because I didnt get the answer you mentioned.


    s = 1/2gt^2
    t=4*(15)^1/2 = 15.49

    x = Vx(t)
    x = 131(t)
    x = 131*15.49= 2029.19

    tan(0)= y/x
    = 1200/2029.19= 0.59

    therefore theta = 30.5

    (edit : i might be a little off. I have used g=10m/s^2)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook