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Homework Help: I have mass and velocity, how do I get accel?

  1. Feb 17, 2009 #1
    1. The problem statement, all variables and given/known data
    The driver of a 1750 kg car traveling on a horizontal road at 110 km/h suddenly applies the brakes. Due to a slippery pavement, the friction of the road on the tires of the car, which is what slows down the car, is 25% of the weight of the car.

    2. Relevant equations

    (a) What is the acceleration of the car?

    (b) How many meters does it travel before
    stopping under these conditions?

    3. The attempt at a solution
    I keep getting -2.7m/s^2 and i cannot figure out the correct way to do it.
  2. jcsd
  3. Feb 17, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    How did you get that value? (Hint: Use Newton's law.)
  4. Feb 17, 2009 #3
    I dont know, I cannot seem to get that value again.
  5. Feb 17, 2009 #4
    For part a, you do not even need the velocity of the truck. If you use Newton's law like Doc Al says, you will have -F(friction)=ma --> F(friction)=-ma, which makes sense because the truck is decelerating. You know the force of friction is (1/4)mg so a should equal (-1/4)g m/s^2. Then to find the distance traveled, use the equation Vf-Vi=2*a*delta x, where Vf is final velocity and Vi is initial velocity. In this case, your final velocity will be zero.
  6. Feb 17, 2009 #5
    Thank you very much. I was trying to figure out using kinematics, and getting very angry. I also was assuming that she wanted the acceleration of the vehicle, not that of the friction force.
  7. Feb 17, 2009 #6
    That is the acceleration of the vehicle. The friction force is used because it is the only force acting on the truck.
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