Work total

  1. 1. The problem statement, all variables and given/known data
    a 2.1*10^3 kg car starts from rest at the top of a driveway that is sloped at an angle of 20 with the horizontal. An average friction force of 4.0*10^3 N impedes the car's motion so that the car's speed at the bottom of the driveway is 3.8m/s., What is the length of the driveway?


    2. Relevant equations



    3. The attempt at a solution

    Ff=4000N
    2100 Kg
    X=20 degrees
    Vf=3.8 m/s
    Vi=0 m/s

    Wnet=1/2MVf^2-1/2MVi^2
    Wnet= 1/2*2000*3.8^2
    Wnet= 15162 N

    Wnet = FfdcosX
    15162=4000cos20D
    D=4.03m

    but the book says 5.1m?
    i cant figure out what i did wrong
    thx for the help.
     
  2. jcsd
  3. Doc Al

    Staff: Mentor

    You forgot about gravity. Friction just slows the car; it's gravity which pulls it down the slope.

    To find the net work you must consider all the forces: gravity and friction. (Don't forget that they act in different directions.)
     
  4. i see so the force applied would be Fgravity*sin20 which would give me 7045.96 and i would subtract the Ff to get the total force. right?
     
    Last edited: Apr 7, 2008
  5. Doc Al

    Staff: Mentor

    Right!
     
  6. thxxx
     
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