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Homework Help: Finding Velocity with Air Drag

  1. Nov 25, 2012 #1
    1. The problem statement, all variables and given/known data
    I am trying to find the velocity of a car as it is going down hill. I have the final velocity which is 11.2 m/s and the initial velocity which is 2.23m/s. I have to include Air drag to get my velocity. I also know that there was 21,456 J of energy lost from up the hill to down the hill. I also know that the hill is 18 m long...Here are more variables that i was given
    Coffiecient of drag: 0.38
    Density of air: 1.293 kg/m^3
    Cross-sectional are of car: 1.4 m by 1.6 m
    Angle of hill: 10 degrees
    Coefficient of rolling friction: 0.020

    2. Relevant equations
    The only thing I have to work with is the drag force equation.
    p=Density of air
    C(D)=coefficient of drag
    A=cross-sectional area of the car
    v=velocity of car

    3. The attempt at a solution
    I am contemplating on using that but velocity is involved in the equation and velocity is what Im looking for.
  2. jcsd
  3. Nov 25, 2012 #2


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    hi michelle2011! :smile:

    total force = ma = mdv/dt

    so what are the forces? :wink:
  4. Nov 25, 2012 #3
    What you want to do is find the net sum of forces acting on the car and then convert that to a velocity by using Tim's equation.
  5. Nov 25, 2012 #4
    Are my net forces rolling force and air drag??
  6. Nov 25, 2012 #5


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    net force is the (vector) sum of all the forces

    (ie there is only one net force)

    your three forces are the friction, the drag, and gravity
  7. Nov 25, 2012 #6
    In the equation you gave me can I just find my (t) from a simple equation??
  8. Nov 26, 2012 #7


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    (just got up :zzz:)
    i'm confused … you're looking for v, not t :confused:

    anyway, what equation do you get? :smile:
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