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Homework Help: Newton's fundamental laws

  1. Mar 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A parachutist of mass 70kg, in free fall at 136 km/h, opens a parachute. if the chute creates a 895 N air resistance force and the parachutist is being pulled down by a force of gravity at 686 N
    a) find hte acceleration of the parachutist
    b) how far does she fall in 5.0 s?

    2. Relevant equations
    Fnet = ma, d = v1t + 1/2at^2

    3. The attempt at a solution
    First i determined part A and i got the answer 3.0 m/s^2 [up]. which is correct in the answer key.
    then i used the equation d = v1t + 1/2at^2, converting 136 km/h to m/s
    d = v1t + 1/2at^2
    d = 37.7 m/s(5s) + 1/2(3 m/s^2)[(5s)^2]

    and i got 226.4 m. the correct answer is 150m. i dont know what i did wrong. i then tried using the acceleration due to gravity, 9.81 m/s^2 because it is a freefall but the answer was still wrong.
    please help!!
     
  2. jcsd
  3. Mar 17, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The person's initial speed is downward, thus negative.
     
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