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Calculate Friction

  1. Sep 21, 2006 #1
    Hey guys! I think I have half of the question figured out

    The question:

    Skid is driving his car at 80 mi/hr. While reading the Springfield Reivedw of Books, he looks up and notices he is about to crash into a wall. When he is 100 m from the wall, he puts on the brakes. If Skid does not lock his brakes, calculate the coefficient of friction he needs so that he stops just before he hits the wall. What type of friction does he use?

    My work:

    I know that this involves kinetic friction. I thought I should calculate acceleration so that I can use the force equation.

    I used the V^2 = V0^2 + 2a*delta X

    I got an acceleration of 0.8. In order to use the SumFx = m*a equation, don't I need a mass so that I can somehow solve for the kinetic friction?
     
  2. jcsd
  3. Sep 21, 2006 #2

    Pyrrhus

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    Homework Helper

    Just write down the equation with Newton's 2nd Law. You'll notice the mass will cancel out.
     
  4. Sep 22, 2006 #3
    Hey!

    I got my masses to cancel out, but I didn't get the right answer. I can't figure out what I did wrong. Here's my work it. :smile:
     

    Attached Files:

  5. Sep 22, 2006 #4

    Doc Al

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    Staff: Mentor

    Convert 80 miles/hour to m/s!

    Also: If he doesn't lock the brakes, the tires won't slip. What kind of friction is that? (Locking the brakes will cause you to skid.)
     
  6. Sep 23, 2006 #5
    omg, I can't believe I didn't catch that. Thanks!

    Wouldn't it be kinetic friction if he didn't lock his breaks?
     
  7. Sep 24, 2006 #6

    Doc Al

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    Staff: Mentor

    No, just the opposite. Not locking the brakes means that the tires continue to roll without slipping--which means static friction. When you jam on the brakes, locking them (at least in the pre-ABS days) the wheels scrape along the ground instead of roll--that's when you get those cool skid marks, and kinetic friction applies. (And since under most conditions kinetic friction is less than static friction--giving you less control of the car--locking the brakes is a bad thing.)
     
  8. Sep 24, 2006 #7
    OH! I see waht you mean, thanks!
     
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