We’ve been promised flying cars for decades. We finally have one. Well, sort of. Last week, Elon Musk revealed Tesla’s long-delayed Roadster 2 with a few bells and whistles. The car flies—but not for very long—and the prototype also did a few other things normal cars can’t, like driving up a vertical wall and jumping over Tesla’s entire lineup of vehicles.

This raises two questions: Should we let humans drive up walls and into the air? Given their trouble navigating two-dimensional roads, this seems like an obvious no. But that’s one for lawyers and the Federal Aviation Administration to figure out. The other question is, how did the Roadster 2 prototype do this stuff? We can answer that ourselves, with physics.

Driving Up a Wall

Let’s start with how the car drove up a circular ramp and then stayed glued to it Spider-Man–style, completely upside down. I don't think you should ever do this, but this feature of the Roadster 2 can actually improve performance on normal roads. It has to do with friction. When two surfaces interact—like rubber tires and a road—there can be a force that prevents the surfaces from sliding relative to one another. This interaction is called static friction. We have a simple model for this static friction force where the magnitude (Ff) depends on how hard the two surfaces are pushed together (FN) and the two types of surfaces (μ).

The force pushing the surfaces together is often called the “normal” force, since it's perpendicular to both surfaces. The coefficient of static friction (μ) describes the types of materials interacting. The practical maximum coefficient of friction is 1, and for rubber on asphalt, the value is somewhere around 0.6. What about that ≤ sign? This is because static friction is a “force of constraint.” It applies the force needed to prevent sliding—up to some maximum value.

So how can a car drive up a vertical wall? The answer is to increase the normal force—and, by extension, the frictional force. For a vertical wall, this friction force must be equal to the gravitational force to prevent the car from slipping. Here's where the Roadster 2 does its first trick. It uses fans to blow air out from under the car. With some side flaps, this lowers the air pressure under the car, akin to how a vacuum cleaner works.

The upside-down Roadster 2 makes for a compelling sales pitch, but the suction actually has a strong use case when it’s driving right side up. Frictional force is the thing that makes a car accelerate. More frictional force, better performance. Pushing the car down means you can accelerate faster and brake faster and make sharper turns. (Performance automakers have harnessed this principle for decades, though not to the degree of Tesla’s new prototype. )

0–60 in 1 second

The Roadster 2 also boasts an acceleration of 0 to 60 mph in less than a second. For a traditional car, this is impossible. Since a normal car uses the frictional force between the tires and road to accelerate, it's limited by the coefficient of friction. Using a coefficient of friction at 0.6 (reasonable), the best you can do is 0 to 60 mph in 4.56 seconds. Yes, you can get better tires and you can also increase the frictional force once the car moves faster and interacts with the air (pushing it down)—but those improvements still won’t get you to 60 mph in a second. The Roadster 2 relies on another form of force: compressed air shooting out the back. This is how rockets work. Since most of us don’t have rockets handy, consider the simple experiment of a toy car propelled by an inflated balloon. Once the balloon is released, the stretched rubber acts as a force that pushes the air out of the balloon and through an attached tube. But since this air has mass, it will take a force to accelerate it from rest to its exhaust velocity. Here's the cool thing about forces—they are an interaction between two things (like the balloon and the air). That means that when the balloon’s walls push on the air, the air pushes back with an equal force in the opposite direction—an example of Newton’s third law. This push-back force on the balloon is what makes the toy car accelerate. Instead of a wheezing balloon, the Roadster 2 relies on a tank of compressed air.

Flying Cars (Sort Of)

Now, we get to the flying car part. The Roadster 2 uses the same compressed-air method to push up on the car and get it off the ground. That’s stretching the limits of the term “flying,” especially because there's only a few seconds of thrust that it can get from the tank before the pressure drops.