What is the spring stiffness?

  1. what is the spring stiffness?!??!

    In outer space a rock of mass 4 kg is attached to a long spring and swung at constant speed in a circle of radius 6 m. The spring exerts a force of constant magnitude 720 N. What is the speed of the rock?

    i found the speed it is 32.8 m/s

    part 2:
    The relaxed length of the spring is 5.7 m. What is the stiffness of this spring?

    thats what i have no idea about could someone help lead me in the direction of solving it or show me how? please and thanks!

    2. Relevant equations
    i used F=mv^2/r to get speed
  2. jcsd
  3. Re: what is the spring stiffness?!??!

    Hooke's law:

    x is the change in length of the spring due to a force F.
    K is the the spring constant or "stiffness"
  4. Re: what is the spring stiffness?!??!

  5. Re: what is the spring stiffness?!??!

    what are you talking about it changes from 6m to 5.7m, there is no material in the world with no elasticity.
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