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A car speeds through an intersection at 41.0 mph (18.3 m/s) and starts skidding (sliding) to a stop 23.8 m from a pedestrian. What is the minimum value of k that will prevent the car from reaching the pedestrian if he/she does not move? (Assume g = 10 m/s2).

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    The idea is that the car is packing a lot of kinetic energy at teh start of hte problem (1/2mv^2). This energy must be bled to zero by the friction force. Energy is bled by a force acting over a distance (the technical term is that the friction force is doing negative work on the car), so you know how much energy needs to be drained and you have a distance over which the force can act (23.8), so mathematically, energy = (1/2mv^2) = Friction force * 23.8m. Notice that on the left and right side of the equation there will be an m term for hte mass of the car, which you don't know. The neat part is that you have to notice that the m's cancel out so you don't need to deal with it.

    Source(s): I tutored physics in college.
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