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Homework Help: Newtons laws, dynamics questions

  1. May 27, 2013 #1
    1. The problem statement, all variables and given/known data
    1) A 1000-kg Car is parked on a 10 degree slope. Determine the magnitude of the frictional force acting on the car

    2) A 2000-kg car travelling at an initial speed of 10m/s skids to a stop with a constant acceleration. The coefficient of friction between the tired and the road is 0.4. determine the length of the skid marks

    2. Relevant equations
    Newtons 2nd law f=ma

    (f_s) = static friction
    f_k= kinetic friction
    w= weight
    n= normal force

    3. The attempt at a solution

    For the first part (1) . The free body diagram I drew was on a slant to show that the normal force is perpendicular to the slanted road so it is in the positive y direction, the force of static friction is to the negative x axis, and the weight force is downwards (with two components x and y)

    F_x = -f_s + wsin∅ = 0
    F_y= n-wcos∅=0

    Essentially, I just solved the x component equation, given that wsin∅ is 9800sin10.

    This was then equal to the static friction force.

    This feels wrong, it was too simple, but maybe I could just be overthinking/underthinking.

    Is this correct?

    2)The normal force is pointing up in the positive y axis, the weight force is pointing down, and the kinetic friction force is poiting to the negative x axis(left) for the free body diagram.


    F_x= -f_k=ma_x
    F_y= n-w=0

    I esentially solved for the normal force in the y direction, then plugged that into f_k=μkn , to solve for the kinetic friction force. after solving the friction, I plugged that into F_x and determined the acceleration (which is negative).

    After knowing the acceleration, initial and final velocities I used the equation


    Solved for x, and think i found the answer.

    Is this right as well? (the mah is entirely tough to type out so I simply gave my steps for solving)

    Thanks so much
  2. jcsd
  3. May 27, 2013 #2


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    Science Advisor
    Homework Helper

    The method seems ok on both of those.
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