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Homework Help: Simple Pressure Problem

  1. Jan 20, 2013 #1
    1. The problem statement, all variables and given/known data
    1. A 0.3 kg apple falls from rest through a height of 40 cm onto a flat surface. Upon impact, the apple comes to rest in 0.1 s, and 4 cm2 of the apple comes into contact with the surface during the impact. What is the average pressure exerted on the apple during the impact? Ignore air resistance.
    (A) 67,000 Pa
    (B) 21,000 Pa
    (C) 6,700 Pa
    (D) 210 Pa
    (E) 67 Pa
    (the answer is b)

    2. Relevant equations
    p = mv
    f = Δp/Δt
    v = √(2gh)
    pressure = force / surface area

    3. The attempt at a solution
    The velocity the apple is traveling at when it hits the ground is
    v = √(2gh) = √(2* 10 * 0.4) = 2.83 m/s
    The average force on the apple is
    Δp/Δt = (0.3 * 2.83) / (0.1) = 8.48 N
    So the average pressure on the apple should be
    force/surface area = 8.48/(0.042) = 5300 Pa

    But that is not an answer. What did I do wrong?
  2. jcsd
  3. Jan 20, 2013 #2


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    Science Advisor
    Homework Helper

    4 cm^2 is not the same as (4 cm)^2.
  4. Jan 20, 2013 #3

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