## Hair Maths 1: The curly problem with curly hair

May 6, 2015

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More about Troy’s \$million hair here. Hair statistics including how many hairs a human has on their head here.

Mathspig studied hair chemistry at uni. Tricky stuff. Put simply, hair is made of long strands of protein called keratin held together by sulphur (and some hydrogen) bonds. To curl hair, the keratin strands in the outer curve of each hair has to be stretched with curling tongs or hair curlers, heated and dried. The bonds in each hair reform with one side longer than the other … Hence, the hair curls like gift-wrap ribbon. But high humidity allows hair to reabsorb water and straightened hair just goes psycho curly again!

This excellent hair diagram comes from The Chemistry of Shampoo and Conditioner, in an article by EMMA Dux for the Royal Australian Chemical Institute

Some people are born with hair follicles that produce keratin at different rates across the follicle. They have curly hair. Hair perms chemically break and reform the sulphur bonds while the hair is held in small curlers (curly hair) or a very big curlers(relatively straight hair.) thus permanently curling the hair.

# Here’s the Maths:

Curly hair looks like a 3D Helix.

More on 3D helix maths here

But, in fact, one strand of curled hair looks more like a spiral staircase.

The outer edge of the staircase is longer than the inner edge.

More helix maths here.

# CHALLENGE:

Mathspig doesn’t expect Middle School students to plot a 3D Helix. But if they have started TRIGONOMETRY then they can see that the maths they are studying is used in CGIs for films and computer games in this case to generate realistic curly hair!!!! That’s cool. This maths was needed to model Merida’s curly hair in  BRAVE.

# SMARTY PANTS CHALLENGE:

Some middle school students could calculate some points on the helix.

Now students must be introduced to radians.

Simple EXPLANATION: Angles eg. 300 are not useful in calculations but fractions are very useful.

Eg. The circumference of a circle:

C = 2πr

Now imagine if you scan with a floodlight set at a radius of 1 km. So:

C = 2π

So the circumference is 2π.

You scan ¼ of a circle, the distance the light moves is ¼(2π)

or ½ π or 1.57 km (see below)

This measurement of an angle is in RADIANS.

00     = 0 circle = 0

450   = 1/8 circle = 2π/ 8 = 2 (3.14) /8 = 0. 79

900   = 1/4 circle = ½ π = ½ (3.14) = 1.57

1350   = 3/8 circle = ¾ π = ¾ (3.14) = 2.36

1800= 1/2 circle = π = 3.14

2250   = 5/8 circle = 5/4 π = 5/4(3.14) = 3.93

2700 = 3/4 circle = 3/2 π = 4.71

3150   = 7/8 circle = 7/4 π = 7/4 (3.14) = 5.50

3600 = 1 circle = 2π = 6.28

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You will find Cos  tables at NASA Sine tables at Mathhelp

Advanced students may want to look at what the Uber Geek 3D Helix generating program at the free graph website PLOTLY here.

## Hair Maths 4: The Great SHAMPOO rip off

April 27, 2015

According to Salon.com:

## There are two types of ingredients in shampoo. One type cleans your hair. The other type strokes your emotions.

Shampoo has one main ingredient:

# DETERGENT

The two most common shampoo detergents are ammonium lauryl sulphate and ammonium laureth sulphate. That’s all you need to clean your hair. All those shampoo bottles. All that supermarket shelf space offer THE SAME PRODUCT.

Other ingredients include: perfume, dye, preservative, foaming agent, acid (The problem is hair swells in an alkaline solution eg with soap when rinsed out the hair shrinks making knots impossible to brush out. ARggghhhhh!), thickeners, thinners, sheen additive, pearlesence powder (makes shampoo look pearly) etc

According to Steve Antczak, Co-author of Cosmetics Unmasked: Your family guide to safe cosmetics and allergy-free toiletries,

## A typical shampoo is mostly water, containing between 5 and 20 per cent detergent, with shampoo for dry hair containing less detergent than shampoo for greasy hair.

See New Scientist, Sept , 2009

So the potency of shampoo looks something like this:

Assume you buy a 500ml bottle of shampoo for \$10. Now you can work out how much some shampoo manufacturers rip you off by filling in this table:

How much do you pay for water in shampoo? The Answer is here.

What does Bill Bunn from Salon.com recommend?