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Gravitational force, springs, and mass

  1. Oct 4, 2010 #1
    1. The problem statement, all variables and given/known data
    Suppose that I hang a 100 gram mass from a spring. When I do this, I find that the srping stretches 10 centimeters.

    1)What is the gravitational force on the mass that I hang from the spring? give your answer in newtons

    2)what is the force that the spring is exerting on the mass?


    2. Relevant equations
    F=-ky
    F=mg

    3. The attempt at a solution
    ok so I figured most of the problem out, I know that its a simple problem but im just stuck on the first part and dont really know how to begin the second question. What I did was use the two equations i gave to solve for g=-ky/m but my problem is i dont know what k is or how to get it. As for the second part, i just have no idea how to start it. Thanks for any help in advance.
     
    Last edited: Oct 4, 2010
  2. jcsd
  3. Oct 4, 2010 #2

    jhae2.718

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    The gravitational force is just mg.

    For the second part, if the spring-mass system is at rest when the spring is stretched 10 cm, what does that mean the spring force has to be at 10 cm?
     
  4. Oct 4, 2010 #3
    So, all I have to do is use the 100 grams and the acceleration due to gravity, 9.8. So (100g)(9.8)=980 and then just convert that to N?

    For the second part, doesn't that mean that the spring force has to be at equilibrium so its at zero?
     
  5. Oct 4, 2010 #4
    bump anymore help please have things to do today so I need to get this done early. Thanks for anyhelp in advance
     
  6. Oct 4, 2010 #5

    jhae2.718

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    You're correct on the first part. The gravitational force is 0.1 kg * 9.8m/s^2 = 0.98 N.

    For the second part, we treat the system as if it were in equilibrium, so the sum of the forces must be zero. The spring force is nonzero.
     
  7. Oct 4, 2010 #6
    Well since the sum will be zero then the spring force must cancel out the effect of gravity. So would it be 9.8 to counter the -9.8m/s^2.
     
  8. Oct 4, 2010 #7

    jhae2.718

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    You're talking about acceleration instead of force when you refer to gravity. The force of gravity is W=mg, where g=9.8m/s^2. (Sign is based on coordinate system, but it always points "down".)

    So, the magnitude of the spring force when the displacement is 10 cm is equal to mg in this case.
     
  9. Oct 4, 2010 #8
    Ah ok I understand it now. Thanks for all your help kind sir and have a nice day.
     
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