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Homework Help: Diving is a Drag

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data
    You dive straight down into a pool of water. You hit the water with a speed of 7.0m/s, and your mass is 65kg.
    Assuming a drag force of the form Fd=(-1.40x10^4)v, how long does it take you to reach 2% of your original speed? (Ignore any effects of buoyancy.)


    2. Relevant equations
    Fd=(-1.40x10^4)v
    Fg=mg
    Fnet=ma
    a=(vf-vi)/t


    3. The attempt at a solution
    Fnet=mg-(-1.40x10^4)v
    a =(mg-(-1.40x10^4)v)/m
    a = -1500= vf-vi/t=0.14-7/t
    t= -6.86/-1500
    t= o.oo46s

    When I enter this number or 4.6x10^-3, says it is wrong?? could someone help me, thisi s really frustrating me. The physics all seems to make sense to me.
     
  2. jcsd
  3. Oct 16, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Careful with signs. The drag force acts up; gravity acts down.

    Since the force varies with speed, the acceleration is not constant. You have to set up and solve a differential equation to get the time. Hint: a = dv/dt.
     
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