Spiderman's breakfast and the physics of spider silk

Electron micrograph of a spider spinneret producing spider dragline silk.
 (photo credit MicroAngela)

I've got the perfect excuse to sit down and watching a load of movies. I need to research a course I'm running called "Science on the screen". We're going to dissect the science in some movies and then reshoot scenes with the science 'fixed'. Its the brain child of some imaginative chaps in Scotland who call the project NonFiSci.

First up on my list is the 2002 version of Spiderman. It struck me that Peter Parker must have an enormously high protein diet to generate all that spider silk (which is made from protein) he goes through. So being the geek that I am I set about working out what his protein consumption must be and putting it in terms of eggs eaten/100m of spider silk.

Here goes (and feel free to pick me up on any mistakes).
I started with the assumption that Spidey produces spider's silk that has the same characteristics as dragline silk produced by the European garden spider Araneus diadematus i.e. it has a tensile strength ,about equal to piano wire, of 1.1 Giga pascals (Gpa) and a density of 1.3 g/cm³.¹

1.1 Gpa = 1.1x10⁹ Newtons per meter squared (N/m²)

Spiderman weighs 75kg (according to Marvel)²  so the force he exerts is 75 x the acceleration due to gravity which is 9.8 m/s/s  = 735 Newtons

Therefore cross sectional area of spider silk required to support Spiderman = 735N/1.1x10⁹N/m² = 6.68x10^-7 m² = 6.68x10^-3 cm². That works out to be a bit of silk with 0.046 cm radius. Pretty cool huh, you could hang from a piece of spider silk about 1mm thick!

A 1m long piece of silk has a volume of 0.668 cm³ which would weigh in (given a density of 1.3g/cm³) at just  0.87g. Therefore 100m weighs about 87g

There’s about 6g of protein in an egg.  So it looks like Spidey only needs about 15 eggs for breakfast if he plans to use 100m of silk. That’s not too bad.

But, spider silk consists predominantly of a protein call fibroin which is about 42% glycine.³ Whilst egg consists of just 10.7% glycine and serine (I’m counting serine because it is easily metabolised to glycine). So lets say Spidey needs 60 eggs for his 100m of silk.

The thing is I’m not quite happy with that,  I think he’d need a reasonable safety margin, I’m sure he doesn’t what to hang about on a thread just barely strong enough to hold his weight. Plus he’s busy swinging from tall buildings whilst carrying Mary-Jane Watson around. So lets go for a thread 5 times thicker than what’s barely adequate. So that gives us 300 eggs/100m of Spidey silk.

But that’s not really the end of it either. After all what happens if he leaps from a building to save a falling MJ and deploys his webslingers to save the day (like at 1hr 6 min into the movie). Hmm...I’m guessing he’s still using his drag silk (which stretches by 27%). He could got for the flag silk, its not as strong (tensile strength = 0.5 Gpa) buts its got amazing elastic properties; it can stretch to 2.7 times its original length! But that might be a bit too bouncy.

In the scene I’m talking about Spidey leaps from the  balcony and falls for 7 seconds before his silk starts to arrest his fall. He’s caught MJ so lets say their combined weight is 125kg. How much silk is he going to need here?

Spidey accelerates at 9.8m/s² for 7 seconds giving him a velocity of  68.6 m/s.  Given that velocity =√2.g.h we can work out Spidey fell for 240m (wow, that’s one high balcony).  So assuming the silk stretches to its maximum that gives us a stopping distance of 64.8m. The impact force on the silk rope as they slow down is F=1/2mv²/d = 4,538 N (where d is the stopping distance, m is mass and v is velocity)

That’s not too bad, just 6.17 times greater force than Spidey just hanging around by himself. But still he’ll need 1288g of silk (minimum) to catch his fall. So I reckon he must have had about 860 eggs from breakfast that morning. I think Aunt May might have noticed.

Anyone what to check my working?

p.s. As for the scene at the end of the movie where he catches a cable car full of kids, I draw the line there.

p.p.s I know, I know in the comic books he has some wrist mounted devices, but in the films he glads the silk out of his wrist, so that's what I'm working with here




1.Lin Römer and Thomas Scheiber, The elaborate structure of spider silk. Structure and function of a natural high performance fiber, 2008 Prion 2:4, 154-161
2. Marvel directory http://www.marveldirectory.com/individuals/s/spiderman.htm (accessed Jan 2012)
3. S.O. Anderson. Amino acid composition of spider silk. 1970. Comparative Biology and Physiology, 35, 705-711
4.http://en.wikipedia.org/wiki/Egg_(food) (accessed Jan 2012)

Number 24: K'nex DNA

60 years ago James Watson and Francis Crick were busy trying to figure out the structure of DNA.  So to celebrate the anniversary I thought I'd come up with a series of DNA related activities.

Watson and Crick eventually worked it out the structure by building models of the molecule and comparing them to X-ray pictures collected by Rosalind Franklin.

Watson and Crick built their model from bits of lab equipment and other odds and ends. You can see just how cobbled together it is if you visit  London's Science Museum. Which just goes to show that even Nobel prize winners use whatever they have to hand.

The model shows that DNA is made from two intertwined helices. Sort of like two corkscrew shapes wound around each other. Each helix is made from a backbone of sugar-like molecules bound to phosphates. Then each helix is zipped up to its partner via something called basepairs.

So what else could we use to build a model of DNA? How about K'Nex?


You'll need:

A load of these three K'nex peices (or similar)

These will represent the 3 main parts of DNA. The green pieces are going to be the sugars, the white bits are the phosphates and the blue ones are the base pairs.

To make 1 complete turn of K'nex DNA you'll need 14 green bits, 14 white and 7 blue bits.

What to do.

1) Connect up the 3 green pieces with the 3 white pieces and then 3 green pieces with 4 white pieces, so you have two sections that look like this.

2) Add the blue pieces onto the green pieces, like so.

3) Here's the tricky bit. Connect up the blue pieces to the other section like this.

You should end up with a twisted bit of k'nex model that is starting to resemble the double helix of DNA. 

4) Now just continue to build more of the same and link them together to form as long a strand of k'nex DNA as you can!

The model you've built isn't quite accurate but its still shows some of the important features of DNA. Most importantly it's a double helix  and you might notice that it's also got grooves running around the double helix. One of these is narrower than the other. These are known as the minor and major grooves , DNA has them as well.

If you want to do more DNA related stuff with K'nex you can buy the official model kit or have a look at this great site from Mount Sinai University.