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A 2.0 kg mass is attached to one end of a spring

  1. Jun 30, 2003 #1
    I am currently taking physics right now and i had the following question on a homework assignment. I have the right answer but i don't believe its correct. Can anyone help me prove him wrong or give me a better explanation of why his answer is correct?

    -------here is the problem-----------
    A 2.0 kg mass is attached to one end of a spring with a spring constant of 100 N/m and a 4.0 kg mass is attached to the other end. The masses are placed on a horizontal frictionless surface and set into motion. At one instant the 2.0 kg mass is observed to be traveling to the right with a speed of 0.50 m/s and the 4.0 kg mass is observed to be traveling to the left with a speed of 0.30 m/s. Since the only forces acting are the force of gravity, the normal force of the surface, and the force of the spring, we conclude that:
    A) the spring is compressed at the time of the observation
    B) the spring is not compressed at the time of observation
    C) the motion was started with the masses at rest
    D) the motion was started with at least one of masses moving
    E) the motion was started by compressing the spring \

    he said the answer was D can anyone help?
    a picture is attached to help see both ways

    https://webspace.utexas.edu/pourkc/PHYS/spring.jpg [Broken]

    Attached Files:

    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Jun 30, 2003 #2
    No, he is correct.

    Compare the ratios of the speeds and masses of the two weights.
  4. Jul 1, 2003 #3
    A)As we don't know the natural length of the spring and the length at the observation, we can't say spring is compressed or extended.
    B) same as above.
    C)If so, masses stay still.
    E)Starting by compressing and stretching both can make the same motion. We can't say so.
    Remaining D) is correct.
  5. Jul 1, 2003 #4
    I think it has to do with the conservation of momentum. The momentum of the system is
    m1*v1 - m2*v2 (the signs can be changed if you change the reference system) = 2*0.5 - 4*0.3 = -0.2 N/s
    So the initial momentum of the system is not 0 => something was initially moving....
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