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Spring Constants/Work And Energy

  1. Feb 17, 2008 #1
    I'm having a bit of a problem with these homework questions, if anyone could help out that would be great!!

    1. A 15 kg mass is attached to a massless spring by a light string that passes over a frictionless pulley. The spring has a force constant k= 500 N/m and is unstretched when the mass is relased. What is the velocity when it has fallen a distance of .30 m?

    2. A 0.75 kg sphere is dropped through a tall column of liquid. When the sphere has fallen a distance of 2.0 m, it is observed to have a velocity of 2.5 m/s
    a. how much work was done by the frictional force exerted on the sphere by the liquid?
    b. what is the average force of friction during the displacement of 2.0 m?

    I am completely stumped, =( please help!!
     
  2. jcsd
  3. Feb 17, 2008 #2
    PF usually asks that you show an attempt first.
     
  4. Feb 17, 2008 #3

    Doc Al

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    Staff: Mentor

    In both problems, analyze what happens to the mechanical energy.
     
  5. Feb 18, 2008 #4
    I'm struggling with the same questions as eann595, but I attempted to work them out so please help!:

    1.) a 0.75 kg sphere is dropped through a tall column of liquid. When the sphere has fallen a distance of 2.0 m, it is observed to have a velocity of 2.5 m/s.
    a.) How much work was done by the frictional force exerted on the sphere by the
    liquid?

    * I need the net work of friction, so I started with Wnet(Fr) = change in KE = ma.
    * Fr x cos(180) x d = 1/2mv^2 = ma.
    * Fr x -1 x 2.0m = 1/2(0.75kg)(2.5m/s)^2 = (0.75kg)(a)
    * -2.0m x Fr = 2.34 kg x m squared/sec squared = (0.75)(a)

    I have a feeling that b/c acceleration is not given, it equals g aka 9.8 m/s^2 (??) Am I on the right track with this problem?

    b.) What is the average force of friction during the displacement of 2.0m?

    This one I think I need to know the answer to letter a.) to solve.
     
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