In the 2017 Wonder Woman movie, the superhero crosses through a raging battle during World War I. But whenever a bullet zips toward her, she deflects it with one of her bracelets. Even though that bullet is flying through the air at about 700 meters (2,300 feet) per second, its impact doesn’t cause the superhero’s arm to move an inch.
“What you have to think about is momentum,” says Nicole Gugliucci, a physicist at Saint Anselm College in New Hampshire. Momentum is the amount of motion in a moving object and is equal to its mass times its velocity. “If I throw a bullet at you,” says Gugliucci, “you could block it with no problem.” The tossed bullet has a small amount of mass and a low velocity, resulting in low momentum. “But a bullet fired from a machine gun is going much faster, so its momentum is much greater.” It’s probably safe to assume that Wonder Woman’s enhanced strength is what allows her to swat away the high-speed projectiles without even feeling them.
As enemy fire intensifies, Wonder Woman raises her shield to hold off the barrage. This time, the force of so many bullets striking her shield causes it to bounce back slightly. That shows a scientific principle at work: When objects collide, their momentum is conserved. The bullets’ momentum can’t just disappear. Instead, some of it is transferred to the shield, pushing it in the direction of the bullets’ force.
What’s truly impressive is Wonder Woman’s ability to move fast enough to block bullets in the first place, says Gugliucci. Bullets travel faster than a normal person’s reaction time—how quickly an organism responds to a stimulus. “Wonder Woman needs superhuman reaction time to deflect those bullets,” says Gugliucci. She’d have just milliseconds to lift her arm into the line of fire.