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Generic KE problem with a twist.

  1. Dec 12, 2006 #1
    1. The problem statement, all variables and given/known data
    "Two cars are traveling along a road, and one is going at half the velocity of the other one. What is the distance required to come to a stop for the car going at the slower speed relative to the faster car."

    The possible answers listed were as:
    A:1/2 the distance
    B:1/4 the distance
    C:Not enough info provided

    2. Relevant equations
    I suppose it would be KE=1/2m·v^2

    3. The attempt at a solution
    At first glance I plugged in a mass of 2000kg for each and a speed of 20m/s for the fast one and 10m/s for the slow one. The resulting KE's came out to be 40,000 and 10,000 respectively. However after reading the problem I discovered that it was not stated that the masses were equal, so I answered not enough info. Upon receiving my test back, lo and behold my teacher marked that B was in fact correct. My question is, how is it possible to calculate the braking distance without knowing the masses of the cars, and without also knowing the braking capability of each car. i.e. A Ferrari Enzo can brake from 60-0 in 109 ft, whereas a 2005 Volkswagen Jetta takes 130 ft. to stop 60-0 (both cars weighing 3230 lbs).
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 12, 2006 #2


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    Staff: Mentor

    Great question. Your initial intuitive answer is the same as mine, but maybe increasing mass increases the F=mu*N friction force enough to offest it in the ideal?
    Last edited: Dec 12, 2006
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