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Finding the average magnitude of the drag force

  1. May 14, 2017 #1
    1. The problem statement, all variables and given/known data

    A saucer of mass 1.75x10^9 enters
    1.the gravitational field of PlanetX and enters its atmosphere; the gravitational field of the planet is 10.6 m/s2 and is considered uniform throughout the 80-km thick atmosphere.
    If the saucer enters the atmosphere at 460 m/s and slows down to 110 m/s before its crash-landing, what is the average magnitude of the drag force (gravity cannot be neglected)?



    2. Relevant equations

    drag for disk, D = (1/4) AV^2
    W = Total Mechanical Energy

    3. The attempt at a solution

    E1 = (1/2)m1Vi^2 + mgd
    E2 = 0 + 0

    So W = (1/2)m1V1+mgd

    Because W = Fd

    W/d = F

    2.55 x10^10

    However the correct answer is listed as 2.07 x 10^10 N

    I am not sure if my method of using mechanical energy is correct., Or wether I should be using the drag force equation to solve the problem
     
  2. jcsd
  3. May 14, 2017 #2

    ehild

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    The speed is not zero before the crash.
     
  4. May 15, 2017 #3

    haruspex

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    The use of this to find an average force is not valid, though I realise it is what you are forced to use in this question.
    Average force is defined as (change in momentum)/(elapsed time). If the force is not constant then (change in energy)/(distance) will not usually produce the same value.
     
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