I’m not sure how they got the name “human fly” for this guy climbing Trump Tower. It appears he was using suction cups for his climbing device, so maybe a name like “Suction Cup Boy” would work better. Also, I’m not sure of his motive—but I’m sure it was super important.

Whatever the reason for this crazy Gecko Man stunt (that’s what I’m going to call him), there’s still some cool science here.

Suction cups don’t really suck

Actually, I guess this depends on your definition of “suck.” In a physics-view, suction cups are all about the atmosphere, and not the cup. On the surface of Earth, we are surrounded by air. It’s actually quite a significant amount of air. This air consists of mostly nitrogen and oxygen molecules moving around and colliding with things. They collide with you, they collide with me, and they collide with the wall. They even collide with other gas particles.

These particle collisions exert a force. If there are more particle collisions, there is a greater force. So a bigger wall would have a greater force (from air collisions) than a smaller wall. However, we like to talk about just the collisions independent of the area—and that is why we have pressure. Pressure is the force per unit area due to this gas.

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For our Earth-atmosphere, this pressure is usually about 14.7 psi (pounds per square inch) or 105 N/m2 (105 Pascals). Also, this air doesn’t just push down on things. It pushes in all directions.

OK, now for the suction cup. You take one of these things and push it up against a smooth wall. In this process the air between the cup and the wall is forced out leaving an area of lower pressure. This means the total force from the air pushing out of the cup would be less than the force from the atmosphere pushing in. The result is the cup is forced up against the wall with the force from the wall providing the extra force to make the cup in equilibrium.

Spring 2016 Sketches key

So, they don’t really “suck.” In fact, they wouldn’t even work without the atmosphere. Here is a fun demo to show the importance of the air. I use a small suction cup to lift a block—but I put the block and cup inside a vacuum bell. When the air is pumped out, the block falls.

But this suction device is probably how the Gecko Man uses his super climbing power.

It’s about friction

Notice that the guy is climbing a vertical wall. While it’s true that the surface needs to be smooth in order to have properly functioning suction cup, it’s actually friction that keeps him up. Let’s draw a force diagram for Gecko Man.

Spring 2016 Sketches key

If he wants to be at rest on the wall, all of these forces have to add up to zero. The force from the atmosphere (the suction cup) would push him towards the wall, not up so that it can’t balance with the gravitational force. But as the suction cups pushes into the wall, it makes a contact force that causes friction. The harder these two surfaces are pushed together, the greater the frictional force. But it is this frictional force that keeps the guy from falling and not the “suction force.”

It’s easier to climb upside down

What if he wanted to support himself underneath a smooth surface? In this case it would be the suction force holding him up (acting against gravity). Suppose that the inside of the suction cups was at 25 percent pressure of the atmosphere. In this case, there would be a net force pushing up on the cup equivalent to 75 percent of the atmosphere (so 7.5 x 104 N/m2). If the guy plus all his equipment was 100 kg, the downward gravitational force would be 980 Newtons. That means we need 980 Newtons from the suction cups. Now let’s find out the size of these cups.

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If this was just one circular suction cup, it would have a diameter of 12.8 cm (5 inches). Yes, it would be that small. Well, maybe a bit bigger—you want a safety margin.

These are probably powered suction cups

If you want to be a professional Gecko Man, you need professional gear. It’s going to be uncool to keep sticking and unsticking suction cups (plus you would get tired). A better method would be to have a small battery powered pump. This pump would remove air from inside the suction cup to make it “stick” to the wall.

When you want to move the suction cup, you would then have a small value that lets air back into the cup so that it’s easy to remove. This isn’t some new idea though. There have been several other people to build such a system and use it successfully—including the MythBusters.

How does a gecko climb a wall?

Some animals can climb walls. I think squirrels are a good example. With the squirrels it’s clear how they do this—they just stick their clawed feet into the surface of the wall and up they go. Of course, this method doesn’t work on smooth and hard surfaces. But where a squirrel can’t go, a gecko can. Geckos can stick to smooth walls using the Van der Waals force.

The Van der Waals force is an electrostatic interaction between molecules. The geckos can accomplish this molecular interaction with super tiny hairs that can get close enough to the surface so that this electrostatic force is big enough to support them. That’s pretty cool.

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How That Chucklehead Climbed Trump Tower Without Splattering