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Homework Help: Altitude Problem

  1. Feb 6, 2005 #1
    Here is the problem:

    Homer Hickim and his friends shoot off a rocket. A fire is reported 3 miles away from the launch pad (1mile = 5289 feet). Homer and his friends are accused of the fire.

    Homer knows that the rocket took 14 seconds to come down from its highest point. This means that the rocket was in the air for about 28 seconds before it hit the ground. Use the following equation from the movie to find out the initial velocity of the rocket in feet per second.

    s = 1/2at^2 + vt

    a= -32
    t= time in seconds
    s= altitude in feet
    v= initial velocity

    14x5280 = 1/2(-32)(28)^2+v(28) <--- this is what i came up with for my equation... im not sure wether the value of S is right, s=alititude how would i find altitude for this problem?
  2. jcsd
  3. Feb 6, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Didn't you already have a thread going with this same problem? https://www.physicsforums.com/showthread.php?t=62879

    In any case, find the horizontal and vertical components of the initial velocity separately:

    Horizontal velocity is constant; use: speed = (horizontal distance)/time.

    Vertical direction is accelerated motion: use: [itex]v_f = v_i + at[/itex] (Hint: what's the speed at the top of the motion?) To me, that's the easiest way to find the vertical component of the initial velocity, but you could certainly use the equation "s = 1/2at^2 + vt": just realize that when t = 28 seconds, s = 0 (it comes back down to earth).

    Once you find the horizontal and vertical components of the initial velocity, find the total velocity using the Pythagorean theorem.
    Last edited: Feb 7, 2005
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