pendulum...?

Question

What is the effective force constant for a simple pendulum of mass 1.2 kg and length 1.67 meters?

Answer 1

7.0467 n/m.
If by "effective force constant" you mean the stiffness of an "equivalent spring", i.e., one connected to the end-mass of a pendulum that produces the same restoring force as gravity (for small deflections), you can derive this a couple of ways.
You can solve for the natural frequency omega of the pendulum, which is sqrt(g/l) (where L is pendulum length) = 2.4233 rad/s. Then k=omega^2*m = 7.0467 n/m.
Or you can say the restoring force f=m*g*theta (small-angle approximation), and the spring constant is f/x=m*g*theta/(theta*L) = m*g/L = 7.0467 n/m. (Basically the same equation, so same result is no surprise.)