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Velocity of spring being made to go in a circle

  1. Sep 12, 2009 #1
    1. The problem statement, all variables and given/known data
    A 5 kg block is attached to the end of a spring with relaxed length 0.5 m and spring constant 250 N/m. The free end of the spring is held stationary and the block is made to go in a circle of radius 0.75 m, thus stretching the spring. What is the speed of the block (neglecting gravity and air resistance)?

    2. Relevant equations
    F*[tex]\Delta[/tex]t= m*v

    3. The attempt at a solution
    F=250 (.5-.75)

    I don't know what to do from here... this is on a practice exam, so I have the answer: 3.1 m/s, but I have no idea how to get to it. I tried F=mv, but got v to be 2.0, which isn't even an option. Please help!
  2. jcsd
  3. Sep 12, 2009 #2
    You should get 62.5N with this, check your calculation.

    Note that This is not linear motion.
    You should be using the Centripetal force of Circular motion to find the answer,
    in which F=(mv^2)/r.
  4. Sep 12, 2009 #3
    Oops! I did a hasty calculation in my head... thanks for catching that! It makes perfect sense, too- thank you very much.
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