We are asked to produce a model showing the vertical position of a pedal on a child's bike.
For a child's bike the amplitude will be smaller, and the period will probably be smaller also. The midline will also be lower.
The adult model was `y=20cos(pit)+30` where t is time in seconds and y is height in cm.
Let the child model be:
This gives an amplitude of 10 (corresponding to a pedal arm length of 10 cm), a period of `1/2` seconds, and a midline of 15cm (this is the height of the center of the pedals.)
The graph of the adult in black and child in red:
You can change the assumptions made -- measure the distance from the ground to the center of the pedals, time the number of cycles per second, measure the length of the pedal arm to get the amplitude, etc...