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Homework Help: A question about Change in Momentum

  1. Aug 5, 2004 #1
    I have an answer to this question and then a question:

    A car with a mass of 1,000kg moves at 20 m/s. What is the braking force needed to bring the car to a halt in 10s.

    I used m2v2-m1v1 and came up with -20,000 kg(m/s)
    then -20,000kg(m/s)/10s

    I came up with -2,000 N

    Is this right? Does the negative mean slowing down?
    Am I even on the right path?
     
    Last edited: Aug 5, 2004
  2. jcsd
  3. Aug 6, 2004 #2
    I agree with you answer. I believe the negative sign is correctly in place because say that car is travelling 20 m/s to the right taken as the positive direction, and we know acceleration is going to be negative. From newton 2nd law F=ma, F will be in the same direction of a.
     
  4. Aug 6, 2004 #3
    thanks for the response. I hope I am right too, it's for a grade.

    thanks again
     
  5. Aug 6, 2004 #4
    u = 20, v = 0 , t=10, a =?

    v = u + at
    0 = 20 + 10a
    -20=10a
    a=-20/10
    a=-2m/s/s.

    F=ma
    F=1000x2
    F=2000N, opposing the motion of the car.
     
  6. Aug 8, 2004 #5
    yeah...knowledge of power...you calculations look fine to me...as force is a vector, so a negative value should simply mean that it is opposite to the motion of the car.
     
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