A spring is characterized by a spring constant k (in N/m) which gives the stiffness of the spring, or how hard you have to pull to stretch it. If you connect a mass m on one end, and connect the other end to a fixed wall or ceiling, then the resulting system will vibrate. This vibration has period T given by
The frequency of the vibration is defined as
f = 1/T
If the period increases by 50%, how does the frequency change?