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Force of a parachute on parachutist

  1. Feb 20, 2012 #1
    1. The problem statement, all variables and given/known data
    A parachutist of mass 65 kg jumps out of a plane deploys a chute.
    When the chute opens, she slows down from 80km/h to 10km/h in 0.8s. What is the foce of the chute on her shoulders at that time?

    m = 65 kg
    v1 = 80km/h → 22 m/s
    v2 = 10 km/h → 2.8 m/s
    change in time = 0.8 s

    2. Relevant equations

    a = v/t
    Fnet = ma

    3. The attempt at a solution
    We're assuming that up is positive.
    I used to sig figs for all my values.

    First we need to find the acceleration

    a = v/t
    a = (2.8 - 22)/0.8
    a = -24 m/s^2

    Let Fp = force of the parachute

    Fnet = ma
    Fg + Fp = ma
    -640 + Fp = (65)(-24)
    -640 + Fp = -1560
    Fp = -1560 + 640
    Fp = -920 N

    I'm confused as to what I'm doing wrong. My teacher told us the answer should be about 2200 N and that would be positive while my answer is turning out completely off and negative.
     
  2. jcsd
  3. Feb 20, 2012 #2

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    by using positive values for v, you have assumed down is positive and up is negative
    since the acceleration is up, the net force must be up; thus, Fp > Fg. Correct your plus and minus signs.
    Net force is always in the direction of the acceleration.
     
  4. Feb 20, 2012 #3
    Okay, so then I changed the velocity values to negatives. That in turn made the acceleration a positive and then following through with

    Fnet = ma
    Fg + Fp = ma
    -640 + Fp = (65)(24)
    Fp = 2200 N

    Thanks :)
     
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