If you've thought long and hard about the implausibility of Spider-Man stopping a runaway NYC subway train with just a few strands of spider silk, then you aren't alone. Three University of Leicester physics students have recently published a paper attempting to quantify the possibility of this very thing.
Before we take you through the paper, entitled "Doing whatever a spider can," watch the scene from Spider-Man 2:
Now, here's the data that was used to figure out whether this is possible:
- The train consists of four R160 New York City Subway cars, each containing their maximum occupancy of 246 people per car, for a total mass of 200,000 kilos.
- It's a runaway train, traveling at top speed of about 55 mph. This also means that the train is attempting to accelerate the whole time that Spidey is trying to stop it.
- It takes about 50 seconds for Spidey to bring the train to a complete stop.
If you crunch all of these numbers, it works out that Spider-Man's webbing can handle a force of 300,000 newtons. Great, let's keep going.
- To stop the train, Spidey actually shoots out eight separate strands of silk, each with a width of about five millimeters, between 15 meters and 18 meters to either side of the train.
- Since we know how fast the train was going as well as the time it takes to bring it to a complete stop, it's easy to calculate the distance that it travels after the webbing starts to decelerate it: 615 meters.
- After the train is brought to a stop, the webbing doesn't yank it backwards along the track, implying that the webbing is at its elastic limit at that point.
Crunching more numbers, you end up with a Young's modulus (a measure of elasticity) of Spider-Man's spider silk of 3.12GPa, and a material toughness of about 500 MJ per cubic meter. The elasticity is middling for spider silk, which has typical Young's moduli values of 1.1GPa 10 1.5GPa, although it's worth noting that orb-weaver spiders can produce silk with a Young's modulus of up to 12GPa.
The material toughness, on the other hand, is something else. 500 MJ per cubic meter is a lot; Kevlar (which stops bullets, remember) tops out at just 33 MJ/m^3. However, the silk of the Darwin's bark spider, a tiny little thing that lives in Madagascar, has been measured at up to 520 MJ/m^3 (!), making it by far the toughest spider silk in the world.
The upshot of all this is that the scene is a realistic portrayal of the capabilities of a type of spider silk with average elasticity and top-notch toughness. This isn't to say that you'd be able to make this happen in real life: firstly, you'd have the same issue that Spider-Man has in the movie, which is that you'll have to come up with some way of anchoring the stuff. And secondly, real spiders cannot deploy strands of silk with a thickness of five millimeters. Not even radioactive ones.
But none of that matters. There's no diminishing the awesomeness here, which is that it's possible to use science to show that science fiction is often a lot more science that you might think.