what is the spring stiffness?!??!

ohheytai

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

pat666

Hooke's law:
F=-Kx

x is the change in length of the spring due to a force F.
K is the the spring constant or "stiffness"

ohheytai

thanks!

pat666

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