Calculate the tension in the string with spring?

Question

an ideal spring of unstretched length .2 m is on a frictionless table. One end is fixed on wall and other is attached to mass M=8kg. M is attached to string over pulley and block of mass m=4kg hangs from other end of string. In equilibrium, length of spring is .25m and m is .7m above the floor.
Calculate tension in string
Calculate force constant of spring

Answer 1

You've stretched the spring .05 m. The force required is

F = 1/2 k x^2

This force is equal to the mg of the hanging mass. So

the tension in the spring is mg
the spring constant is found by solving

mg = 1/2 k x^2

for k