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Physics homework help needed

  1. Apr 6, 2007 #1
    I am having trouble understanding how to work this 3 part problem.

    A 9 kg mass slides to the right on a surface having a coefficient of friction 0.34. The mass has a speed of 4 m/s when contact is made with a spring that has a spring constant 159 N/m. the mass comes to rest after the spring has been compressed a distance d. The mass is then forced toward the left by the spring and continues to move in that direction beyond the outstretched position. Finally the mass comes to rest a distance D to the left of the unstretched spring. The acceleration of gravity is 9.8 m/s^2.

    Part 1) Find the compressed distance d. Answer in units of m

    Part 2) Find the speed vfinal at the spring’s unstretched position when the mass is moving to the left. Answer in units of m/s

    Part 3) Find the distance D where the mass comes to rest. Answer in units of m.
  2. jcsd
  3. Apr 6, 2007 #2

    Doc Al

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    Staff: Mentor

    In order to get help you MUST show your work!
  4. Apr 6, 2007 #3
    I don't have a clue, I've been so lost in this class from the beginning. I cannot even figure out where to start and that's the problem. I just need to pass this class. I'm having a baby in a few weeks and just have had so much trouble grasping this stuff.
  5. Apr 6, 2007 #4


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    Gold Member

    Under the circumstances, I'll try and show you how to approach this, but you must make some effort yourself.

    This one of those energy and work problems with a before and after state.

    At the begining the energy in the system is the KE of the mass. There's a simple formula for that. After the block stops moving the first time, the energy it used to have has gone into frictional loss and the spring.

    If the block moved distance d before stopping, the work ( energy) lost to friction is mgdc where c is the coefficient of friction. The energy in the spring is Kd^2 where K is the spring constant.

    If you equate the initial and final energies you get a quadratic equation in d which is a piece of cake to solve.
  6. Apr 7, 2007 #5
    I am very confused about a question about sound waves. I am confused about the given. If the given is not in the question then how do you figure it out.
  7. Apr 7, 2007 #6

    Doc Al

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    Staff: Mentor

  8. Apr 7, 2007 #7
    Yes that's the problem thank you
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