Question

What happens to the kinetic energy of a weight, attached to a spring which oscillates when it is pulled down and released?

Answer 1

Its kinetic energy will oscillates between its maximum value and minimum value. When the spring is not stretched then the weight has maximum kinetic energy as at mean position potential energy is zero.
But at extreme positions, when the spring is stretched or compressed then whole of the kinetic energy is converted into potential energy of the spring. Hence, at extreme position the weight is at rest and kinetic energy is equal to zero.