How the Jaegers in Pacific Rim Uprising Violate Physics

March 20, 2019 Off By HotelSalesCareers

It's time for another giant robot-like machine that battles other huge stuff—yes, it's the release of Pacific Rim Uprising. The movie comes out on Friday, March 23, and while I don't know much about the plot, I do know that humans pilot super large Jaegers that are almost 300 feet tall. So now is a good time to bring up the physics of scale—or how big things are not like small things.

Let me start with some human-sized action. Suppose I have a human that either jumps or gets hit and then goes flying through the air. Once an object is off the ground, there is essentially only one force that influences the motion—the downward gravitational force. With negligible air resistance, this gravitational force makes objects accelerate (not move) downward, changing their vertical velocity at a rate of -9.8 m/s². The horizontal velocity during this time remains constant since there are no horizontal forces. We call this stuff projectile motion (so that you can look it up in your physics book).

I'm going to skip over most of the physics, but here is an animation of a human going through the air—it's a typical motion for some type of superhero or action movie. If you want the code, here it is.

Yes, I am modeling the human as a sphere—there's nothing wrong with that. But let's ramp it up. How about something closer to a Jaeger with a height of 86 meters? I still want it to be similar to the human—just bigger—so I made a sphere with a radius of 43 meters instead of 1 meter (oh, the ground is bigger too). Since the human had a projectile range of about 10 times the human radius, I want the Jaeger to fly the same proportional distance. This means that it has to be launched at a higher velocity. Note: If you double the size of the object, you would have to increase the launch velocity by a factor of the square root of two in order to get double the range—don't worry, it's just physics stuff (but here are all the details).

Now here is a second animation of the jumping (or falling) Jaeger. The code is exactly the same as the human jump but with a different scale factor.

Notice the difference? The two trajectories look the same. In fact, if you just took a screen shot at the end of the motion, they are identical. The difference is the time. It takes the human 1.17 seconds to complete the motion but it takes the Jaeger 7.63 seconds. Like I said, big things are not the same as little things. Although the velocity is scaled up for the Jaeger, the gravitational acceleration is the same for both cases. That means with a larger starting vertical velocity, the big thing is going to be in the air for a longer time. Oh, you can go small too. Try putting in some super tiny scale value like 0.01 (this would be like Ant-Man). Again, size matters.

But why? Why do the Jaegers move in an unphysical way? The answer is simple—time is boring. Just imagine the scene where a Jaeger makes an epic jump towards a giant monster. While the machine is in the air, you start counting. One-Mississippi, two-Mississippi, three-Mississippi—oh wait, I'm already tired of this movie. It's sad but true, sometimes real physics isn't nearly as exciting as pretend physics. In this case, I'm sure the movie director is trying to make a connect between giant fighting robots (OK, not actual robots) and a classic martial arts movie. It seems to work. I like it the way it is.

Remember, the number one job of a movie is to tell a story. If they have to bend the rules of physics to get a particular effect—I'm totally OK with that.

How the Jaegers in Pacific Rim Uprising Violate Physics

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