...of the child's motion? A child swings on a playground swing with a 1.4 m long chain.
a) What is the period of the child's motion?
b) What is the frequency of vibration?


and if it's not too hard...

A balsa wood block of mass 63 g floats on a lake, bobbing up and down at a frequency of 2.5 Hz.
What is the value of the effective spring constant of the water?