The physics behind 2017’s biggest superhero movies

This blockbuster movie season has already delivered high-octane superhero films like "Wonder Woman"and "Spider-Man: Homecoming," with others like "Thor: Ragnarok" due out later this year.

The films are packed with action sequences and superhero deeds, but could any of the fantastic-seeming stunts actually happen in real life? Rhett Allain, an associate professor at Southeastern Louisiana University and the author of "Geek Physics: Surprising Answers to the Planet's Most Interesting Questions," turned a laser beam of physics on a few to find out.

Could Wonder Woman actually block bullets with her bracelets?

“So this is a great example of your classic introductory physics problem,” says Allain, who wrote about it in detail for Wired’s Dot Physics blog. Suppose, he says, a bullet is fired at Wonder Woman, and it bounces straight back off of her bracelets. (Catch the bracelets in action in this video, 28 seconds in.)

“You can, from that, say the momentum has to be conserved, and Wonder Woman would have to be pushed back also,” he explains. “But if you look at the mass, momentum is mass times velocity. And so the mass of the bullet times its velocity, compared to the mass of Wonder Woman — bullets actually don’t have that much momentum. They don’t really knock you back that much, even if you’re wearing a bulletproof vest. So she can deflect these bullets and not get knocked back.”

A bigger feat, he says, is being able to move fast enough to block a bullet. (Here, Wonder Woman’s abilities veer into the truly superhuman.) Suppose she sees the bullet coming at her from 30 meters away. By Allain’s calculation, she would have just 43 milliseconds to lift her hand to meet the bullet.

“And so during that time, she would have to take her arm, she would have to move it up, increase the speed and then decrease the speed over a distance of — I’m just going approximate — about one meter,” he says. “When you’re increasing the speed of this hand, that’s increasing the kinetic energy, so that takes energy,” he explains. “And if you do that in a short amount of time, then we can calculate the power. Power is change in energy over change in time.”

All told, he reckons it would take about 76,000 watts for Wonder Woman to raise her bracelets in time to stop a bullet. “And you can convert that to horsepower like a car, and that’s about 100 horsepower,” he says. “So, just to block, the power she would need to move her hands up in order to get that bullet in time, we’re talking like a car.”

Is swinging from building to building actually faster for Spider-Man than running on the ground?

When Spider-Man swings from his webs, a pendulum is created where the superhero is “just like a mass on the end of the string,” Allain explains. As he swings down, “he increases in kinetic energy until he gets to the lowest point, and then he slows down going back up.”

By measuring the time it takes Spider-Man to swing between two points, you can calculate his average speed — but according to Allain, Spider-Man’s swing is a bit trickier than a Tarzan swing, for instance. Spider-Man isn’t just swinging from vine to vine, “He swings, and then lets go and flies through the air and then shoots another web,” Allain says. “So, it’s a little bit more complicated.” (See it at 0:20.)

So, the speed of Spider-Man’s swing depends on variables like the length of his web, and the angle that he starts with, Allain says. He estimates that with a 30-meter web and an angle of around 45 degrees, Spider-Man could move about 8 meters per second, or 18 miles per hour — which is within human sprinting speed, especially if you're Usain Bolt.

But as Spider-Man, there are other factors to consider in the decision to swing or run. As Allain points out, “I would not want to be watching a movie with just a running Spider-Man. I want Spider-Man to swing. So, even if it’s not necessarily that big of an advantage, it just looks so cool.”

Could Thor actually punch the Hulk without knocking himself over?

We’re talking in mid-air here.

“I think the most important physics idea here is that if Thor punches the Hulk, then he’s exerting a force on the Hulk,” Allain says. “But forces come in pairs, they always come in pairs. So no matter what, [if] Thor pushes on the Hulk, the Hulk pushes back. It’s not that he has a choice. It’s just the way forces work.”

Forces change momentum, he explains — and again, momentum is mass times velocity. “The Hulk’s much bigger, so he most likely has a larger mass. So, they jump up in the air, Thor punches, and those forces do different things. Because now that Thor has a smaller mass, in order to have the same change in momentum, his velocity is going to have to change more,” he says.

So, according to physics, “Thor’s going to jump up, he’s going to hit [the Hulk], and he’s the one that’s going to get pushed back way more than the Hulk,” Allain says.

Even if it doesn’t end up that way in the movie.

This article is based on an interview that aired on PRI's Science Friday with Ira Flatow.