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2002 ap physics c free response #1 question

  1. Apr 24, 2004 #1
    hello, in lieu of the upcoming ap exam, I have been studying old free response questions from ap exams. I have a question about this one

    A crash test car of mass 1000kg moving at constant speed of 12m/s collides completely inelastically with an object of mass M at time t=0. the object was initially at rest. the speed v in m/s of the car-object system after the collision is given as a function of time t in seconds by the expression

    v=8/(1+5t)

    (a)calculate the mass of the object
    (b) assuming the initial position of x=0, determine an expression for the car object system after the collision as a function of time t
    (c)determine an expression for the resisting force on the car-object system after the collision as a function of time t.
    (d) determine the impulse delivered to the car-object system from t=0 to t=2s

    I am currently doing A, and I came up with:

    m1v1=(m1+m2)v2 <= (law of conservation)
    (1,000)(12)=(1000+M)8

    I got the 8 by plugging t=0 into v=8/(1+5t)

    so M=1400KG? For some reason it does not seem right ...
     
  2. jcsd
  3. Apr 25, 2004 #2

    Doc Al

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    Staff: Mentor

    It's not. Your method is correct, but you must have made an arithmetic error.
     
  4. Apr 25, 2004 #3

    Doc Al

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    Staff: Mentor

    Right.

    Yep.

    Think F = ma. You have v(t). Find a.
     
  5. Apr 25, 2004 #4
    thanks! I solved it!
     
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