Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Ponytail
The numerical values used in this equation were the average density of human hair (1.3 grams per cubic centimeter), a bending modulus ( "A" ) of 8 × 10 Newton meter, a linear mass density of hair ( "λ" ) of 65 micrograms per centimeter and the major diameter of the hair (human hair is elliptical in cross section) of 79 ± 16 micrometers. "N" was taken to be 100,000 - the average numbers of hairs on a human head. The constant "E" was that of nylon which is similar to hair in its bend and twist moduli. The length "L" was taken to be 25 centimeters. These values give a length at which gravity bends a hair of "l" = 5.