calculate the time it takes for a free fall based on
distance traveled by a falling object and the gravity of the free fall
using the equation of motion h = 1/2*gt^2. It also returns
a plot compares the time it takes for the free fall on different planets.
calculating the friction force for static object. The formula is fr = mu * N, where
the mu is the coefficient of friction which incorporating the characteristics of the surface.
For example, if you want to calculate the friction when you put your phone on a table, then the coefficient of friction is
depending on the matirial of the table. N is the normal force which means the perpendicular force of the surface which object
touching on. In our case, it would be m*g since it is on the ground. fr = mu * m * g.