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: Delivering a package by air

  1. Sep 30, 2008 #1
    1. The problem statement, all variables and given/known data

    A relief airplane is delivering a food package to a group of people stranded on a very small island. The island is too small for the plane to land on, and the only way to deliver the package is by dropping it. The airplane flies horizontally with constant speed of 290 mph at an altitude of 850 m. For all parts, assume that the "island" refers to the point at a distance D from the point at which the package is released, as shown in the figure. Ignore the height of this point above sea level. Assume that the acceleration due to gravity is g = 9.80 m/s^2.

    a) After a package is ejected from the plane, how long will it take for it to reach sea level from the time it is ejected? Assume that the package, like the plane, has an initial velocity of 290 mph in the horizontal direction.

    b)If the package is to land right on the island, at what horizontal distance D from the plane to the island should the package be released?

    c)What is the speed v_f of the package when it hits the ground in mph

    x1=x0 + v0x(t1-t0)
    y1= y0 + v0y(t1-t0) -0.5(g)(t1-t0)^2

    3. The attempt at a solution

    c) I am not sure how to do c. At first i thought speed would be 0 when it hits the ground, but i am told that is incorrect
  2. jcsd
  3. Sep 30, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi 05holtel! :smile:

    Hint: vf² = vhorizontal² + vvertical²
  4. Sep 30, 2008 #3
    vf² = vhorizontal² + vvertical²

    Does that mean
    Vf^2 = 290mph^2 +0mph^2
    Vf = Square root of (290^2)
  5. Sep 30, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    erm … vvertical = 0 when it leaves the plane.

    use the usual formula to find vvertical 13.1s later. :smile:
  6. Sep 30, 2008 #5
    So does that mean:
    Vfy=Viy-g(Delta t)
    Vfy=0-4.38(13.1) ***9.8m/s = 4.38mph


    Therefore, Vf = 751.65
    Is that right?
  7. Sep 30, 2008 #6


    User Avatar
    Science Advisor
    Homework Helper

    No, Vfx= 290mph.

    Vfy=Viy-g(∆t) is right.

    But your g is wrong.

    g is not 9.8m/s … it's 9.8m/s² … you need to convert it to miles/s².
  8. Sep 30, 2008 #7
    So does that mean:

    9.8m/s^2 = 0.006 089 mile/s^2

  9. Sep 30, 2008 #8


    User Avatar
    Science Advisor
    Homework Helper

    Yes. :smile:
    i] 290 is Vfx.

    ii] it's 0.07976m/s

    iii] where did 1.178 come from? :confused:
  10. Sep 30, 2008 #9


    That doesnt make any sense. Am I converting it wrong
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook