The original length $($without any external force applied$)$ of a massless spring is $l=65cm,$ hang a $1kg$

mass to the spring, its length becomes $l=85cm.$

Questions:

$(1)$ If we hold the subject by hand and let the spring back to its original length, then suddenly

release the mass without any initial velocity, calculate the mass's vibration amplitude, vibration

period and the maximum force of the spring;

$(2)$ The same spring and mass, if the mass starts vibration at its equilibrium position with an

initial velocity $v_0,$ and the vibration amplitude is found as $4cm,$ what is the value of $v_0?$