Thus, the force of the spring pulling up must have equalled the force of gravity pulling down!

k(0.3m)=(10kg)(9.8m/s^2)

This gives, k=(98/0.3)kg/s^2

putting this into the equation from before gives:

mv^2 = (98/0.3)(0.45)^2 which can be solved for v:

v = {[(98/0.3)(0.45)^2]/(10)}^(1/2)

v = 2.6 m/s