Question
A girl sits and stands repeatedly for 5 minutes. Draw a graph to show the variation of potential energy of her body with time.
Solution
Consider the sitting position of the girl as the position of zero potential energy.
Let,
h = position of centre of gravity while standing above the sitting position.
m = mass of the girl.
Potential energy while standing = + mgh
P.E while sitting = zero.
There is no acceleration or deceleration while standing and sitting.
Given that, sitting and standing is repeated after every minute.
The graph for the repeated motion is shown below: 
