Hair care
Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
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.