Gravity pulls a car down. A wall can't hold it up the way a road can. So what's the trick? Here is the answer in one line:
Side view: the wall is the gray bar. Arrows are drawn to the same scale. Dots are air molecules, drawn bigger and slower than real.
Gravity pulls the car straight down. That pull is its weight. On a flat road, the road pushes up and the car stays put. A wall is vertical, so the wall can't push the car up.
Only one force can point up along a wall: friction. It's the grip between two surfaces rubbing together. If friction is as big as the weight, the car stays. Try it!
But friction doesn't just appear. Where does it come from? That's the next step.
Press a book flat against a wall with your hand. Let go and it drops. Push harder and it stays. Friction only shows up when something pushes the two surfaces together.
Science has a name for that push into the surface: the normal force. ("Normal" means at a right angle to the surface.) We'll just call it the push.
In plain words: a grippier surface or a harder push gives more friction. No push means no friction. (This is the most friction you can get. Friction never needs to be more than the weight.)
If the friction available is at least as big as the weight, the object stays. If it's smaller, the object slides.
So to hold a car on a wall we need a huge push into the wall. With very grippy tires (stickiness about 1), the push has to be about as big as the car's weight. Where could a push like that come from?
Air looks like nothing, but it has weight. Above every square meter of ground sits a tall column of air, all the way up to space. It presses with about 101,000 newtons on every square meter.
Why don't we feel it? Air pushes on all sides of us, and the air inside our bodies pushes back. Pressure only shows up when the two sides are different. That is the secret of suction.
Air molecules zoom around and bump into everything. Billions of bumps every second add up to a steady push: air pressure.
The air outside still pushes on the outer faces of the plates. But between them, there's less air pushing back. The plates get squeezed together.
Squeeze the air out, and the outside air presses the cup onto the glass.
Pull up and the pressure inside drops. The air outside pushes the rubber down.
In 1654, Otto von Guericke pumped the air out of two metal half-spheres. Teams of horses could not pull them apart.
Now use that idea on a car. Fit soft skirts around the underside so the car and the wall trap a pocket of air. Then add a fan or pump that pulls air out of the pocket. Step through it:
The air thinning is drawn bigger than real so you can see it. In real life the pressure only drops by a few percent.
Pressure is push per square meter. More sealed area, or a bigger pressure drop, means more push. 1 kPa (kilopascal) is 1,000 N on every square meter.
| Sealed area | Drop to just stick | Drop with 2× margin |
|---|
Big idea: a big sealed area needs only a small pressure drop. That is why fan cars used big skirts.
Change the sliders. The car sticks when stickiness × push is at least the weight. Try the buttons too: what happens if the fan stops, or the skirt has a gap?
Model: a skirt gap lets air leak back in, so only 15% of the pressure drop is left. The car's real-world motion is simplified. The numbers are illustrative.
Race car engineers have used suction to stick cars to the track. Pressed down harder, the tires grip more and the car turns faster.
Fans at the back pulled air out from under the car, and flexible skirts sealed the sides. It pressed itself onto the track far harder than wings could. It was soon banned.
A big rear fan, officially for cooling, also pulled air out from under the car. It won the only race it entered, the 1978 Swedish Grand Prix, and was then withdrawn.
Suction works even when parked, because a fan, not speed, makes the pressure difference.
Using it on a wall is harder. The skirt must stay sealed on a rough surface, the fan needs lots of power, and if the fan fails there is no second chance.
A viral video shows a sports car driving up a vertical wall. It looks computer-generated, and we can't verify what the video shows or how it was made. We don't know what method, if any, is used in the clip.
What we can say is about the physics: of all the ways to push a car into a surface, suction is the sensible one for holding on at any speed. A good habit when you see a wild video: ask who made it, and whether anyone has repeated it for real.
Friction holds the car up, but friction needs a push into the wall (the normal force).
Suction is outside air pushing. Air push = pressure difference × sealed area.
A big sealed area needs only a small pressure drop, which is why fan cars used big skirts.
Do it over a table, not near anything fragile, and ask an adult first.