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Homework Help: Static friction question with no mass

  1. Jan 3, 2008 #1
    1. if a car is speeding off on a surface of asphault, no slipping tires, estimate static friction for a drag racer that covers the quarter mile in 6 seconds



    My attempt:

    I made a Free Body Diagram: Weight (W) down wards, Normal force (fn) upwards, and friction static friction to the right (Ffs). ou rteacher set that up for us.
    I then tried calculating acceleration.

    1320=1/2a6^2
    a=18.33 m/s/s i think that answer is completely wrong and then i tried figguring out fn

    Fn-W= ma
    Fn-mg=18.33m
    fn-9.8=18.3
    Fn=28.1

    But i think you have to divide the Fn by the mass times gravity

    but i don't have the mass..... so i don'/t know phelp pleasee
     
  2. jcsd
  3. Jan 3, 2008 #2

    mgb_phys

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    Science Advisor
    Homework Helper

    You don't have enough information in the question
    I could have a tractor pulling a sledge do 1/4mi in 6 seconds (with a big enough engine) or a I could have an air hockey puck do the same.

    You need at least the mass and power.
     
  4. Jan 3, 2008 #3

    Doc Al

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    Staff: Mentor

    coefficient of static friction

    As mgb_phys points out, you don't have enough information to find the friction force. But you can estimate the coefficient of static friction, which might be what your teacher meant.

    Hints: When finding the acceleration, convert .25 miles to meters, not feet. How does the friction force relate to the normal force?
     
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