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Homework Help: Car skidding to a stop

  1. Nov 5, 2012 #1
    1. The problem statement, all variables and given/known data
    The driver of a car is travelling on a wet, level road, and slams on the brakes. The car slides 88M to a stop, with a coefficient of friction of 0.42. What was the initial speed of the car?

    2. Relevant equations
    KE = .5mv2
    Δd = v1Δt + .5aΔt2
    Any other kinematics, dynamics, and work/energy/power equations.

    3. The attempt at a solution
    I have tried subbing equations together but I always end up with 3 variables, v1, a, and Δt. I feel like mass is required for any solution, because then friction could be calculated and the rest would be simple.
  2. jcsd
  3. Nov 5, 2012 #2
    well so there's only one force acting on the car during this time, and that's the force due to friction. Try and get the acceleration of the car from that.
  4. Nov 5, 2012 #3
    But you can't find the force of friction without the normal force (requiring mass), and if you wanted to find acceleration you would need time, correct?
  5. Nov 5, 2012 #4
    Use Newton's second law of motion and the definition of the force of friction.
  6. Nov 5, 2012 #5
    How would I use that, Fnet=ma doesn't really help as I have none of those values.
  7. Nov 5, 2012 #6
    Well you know that the only force acting on the car is the force due to friction, this is

    Ffric = μFN = μmg

    and you know that the sum of all the forces acting on something is

    Fnet = ma

    what can you get from that?
  8. Nov 5, 2012 #7
    My brain is telling me:
    μg = a
    But I have the strangest feeling that's wrong.
    Thanks for the help by the way.
  9. Nov 5, 2012 #8
    your brain is correct :)

    Since we know that Fnet = ma and that Ffric is the only force acting on the car and we also know that Fnet = Ffric

    we can therefore say that ma = μmg

    and therefore a = μg

    from that you can use one of the basic equations of constant acceleration.
  10. Nov 5, 2012 #9
    Thanks, I understand how to do the problem now, and that should help me with other ones.
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