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How would you solve a momentum problem including friction and/or springs?

  1. Dec 25, 2008 #1
    how would u implement kinetic friction and springs into a momentum question?

    None of the questions in my textbook shows a momentum+friction/springs question

    lets say

    Car A (350 kg) is traveling 60 m/s east and car B (400 kg) is traveling 30 m/s west. The collision is completely elastic. The coefficient of kinetic friction on the road is 0.45 (assume its not rolling friction). The cars make an eventual stop. What is the total distance traveled by cars A and B after the collision. And lets say part 2 is what if the collision was inelastic, what would the answer then be?

    and lets have another question.

    A 1 kg cart is moving 4 m/s east and then runs into a spring on the side of a wall. The spring's constant is 1000 N/m. the spring gets compressed 0.6 m. The cart gets pushed back west and hits a 0.5 kg cart at rest. What is the velocity of the cart that was still if it was an elastic collision? And what would the velocity be if it was an inelastic collision

    i just make up the problems

    well for elastic collisions since energy is conserved

    i just assumed you first do
    m1v1i + m2v2i = m1v1f + m2v2f and
    Wnc + KEit = KEft
    (non conservative work + total initial kinetic energy) = final kinetic energy

    but i have no clue how to do one that is inelastic
  2. jcsd
  3. Dec 25, 2008 #2
    Are you saying that you made up these problems?
  4. Dec 25, 2008 #3
    yes i did why?
  5. Dec 25, 2008 #4
    WIth your first question, I assume the velocities are right before the colission and friction occours only after colission. FIrst off, it the colission is inelastic, the cars don't stick together. I would

    !. Do a reagular colission problem to find velocities of each car aftger the colission.
    2. Friction is a constant force. Now, you can find distance traveled using kinematics equations.
  6. Dec 26, 2008 #5
    Because the problem might be lacking sufficient information to solve. For example: question 1 part 2 (the inelastic version). By looking at the problem inelastically, you have eliminated one equation, namely conservation of KE. But you will still have two unknowns--the final velocities of the two objects.

    This can be handled in two ways: Assume that the two bodies "stick" and therefore there is only one final velocity of the mass (m1+m2).

    Or if you are given the coefficient of restitution or COR, you can solve for the final velocities. Check the wikipidia definition of COR, it's pretty short and sweet-->Here.

    Then you could find out the final KE of each car and use conservation of energy to find the distance each car skids.

    Or as flatmaster said you could use kinematics. However, if you have learned conservation of energy, I suggest you try that method too as it is a simpler calculation. I would actually suggest that you take the extra five minutes to do it both ways as that will deepen your understanding of the subject.

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