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Consider a snake climbing up the wall or ant climbing up the wall, which force is responsible for it?

An obvious answer is frictional force but my question is that if there is a normal reaction between the wall and snake, the snake should then be accelerating horizontal direction; which is obviously not the case, which external force is balancing the normal reaction applied by the wall.

If no Normal force between the wall and snake, there is no friction as well.

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    $\begingroup$ This is geckos, not snakes or ants. But you might find it interesting. The Stickiest Non-Sticky Substance $\endgroup$
    – mmesser314
    May 16 at 2:59
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    $\begingroup$ For insects it's more like a human mountain climbing than friction on a flat surface $\endgroup$
    – RC_23
    May 16 at 3:12
  • $\begingroup$ @RC_23 I also didn’t understand the free body diagram of a human climbing a mountain .. $\endgroup$
    – Kampann
    May 16 at 4:24
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    $\begingroup$ In many cases, snakes do not even have to climb, even though it looks like it: As long as they are in contact with level ground above or below the wall, they can basically give themselves a hand climbing the wall. Vertical force on the level ground plus muscle tension to keep their body in the correct shape. They are basically thick walled tubes made from muscle tissue, with additional support from their spine, that surrounds some inner organs in the middle. $\endgroup$ May 16 at 13:48
  • $\begingroup$ Back when we had (wookpecker-induced) siding issues on my house, I once had the pleasure of watching a snake emerge from my siding around the 2nd floor with a bird egg in its mouth (never cheered so hard for a snake in my life), and then watched it slither its way down the side of the house with its prize. It was definitely using its whole body, and being very windy with it, like road switchbacks up and down a big hill. I suspect the physical principles are similar. $\endgroup$
    – T.E.D.
    May 16 at 13:48

4 Answers 4

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You might be picturing the whole thing wrong. The walls are not smooth in the real world. Normal reaction has a vertical component, and the weight of the snake is balanced by this.


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Snake locomotion

Let me first say that I am not an expert on snakes, so please do your own research before citing or sharing this answer.

Due to the lack of legs, snakes have to employ a variety of methods to move around. Each of these methods have their own advantages, and snakes choose between them depending on the topography and texture of surfaces they are on. One of those methods is called Lateral undulation, which might be used to climb though rough walls.

In this mode, the snake climbs the wall by hanging on to it, either using its body as a whole or using its scales to cling on surface irregularities on the wall, like a mountaineer climbs using an axe.


This study shows that snakes can actuate their belly scales to actively generate friction for moving forward.


This is an animation I made using Scratch, which might give an idea of how they can move in this position. Check it out!

However, as this study points out, Lateral undulation is one of the slowest mode of locomotion for snakes, and they rarely choose to perform this. If the surface is rough enough or if there is a groove, a snake finds locations where it can latch on to, and pushes its way through (Concertina movement, B).

For climbing poles and trees, they mostly use concertina movements.

Conclusion

In conclusion, for all these methods to work, you need irregularities in the surface, which brings us to the take home message of this discussion. Snakes cannot climb smooth walls. So if you need to keep snakes from climbing into your house, put smooth ceramic tiles/wallpapers on the walls.

References:

  1. Snakes move their scales to increase friction - Biotribology
  2. Why arboreal snakes should not be cylindrical: body shape, incline and surface roughness have interactive effects on locomotion. - Journal of Experimental Biology
  3. Snakes' scales propel them up tree trunks - NBC news
  4. Snake locomotion - Wikipedia
  5. Generating Snake Robot Concertina Locomotion Using a New Dynamic Curve - International Journal of Modeling and Optimization
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The answers vary tremendously by species, but speaking from experience, a sufficiently wet young snake can use the surface tension of the water on their body to pull them to the surface they are climbing.

On a related note, keep a lid on snake enclosures, even if they're flat vertical glass much taller than the snake.

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    $\begingroup$ I don't think surface tension alone will be sufficient - Sample Problem #1 here (fsz.ifas.ufl.edu/surfacetensionandcapillarity/html/…) suggests that horizontal adhesion force would be equivalent to the weight of perhaps a couple grams. That would not provide sufficient normal force for friction to support a snake weighing more than a couple grams on a smooth surface. Surface tension of water is unusually strong among liquids, but still quite weak at only a few millinewtons per meter of contact. $\endgroup$ May 16 at 14:52
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    $\begingroup$ It would be rather odd to find a coefficient of friction >1 with a wet surface, and with a coefficient <1, the adhesion force must be greater than the snake's own weight. But that would imply surface tension alone can support the snake's weight, which as far as I know doesn't happen - snakes swim through water, they don't skim along the top of it like a water strider does (and which has only about 0.1% of the mass of even a tiny snake). $\endgroup$ May 16 at 16:08
  • $\begingroup$ @CortAmmon "the normal force from the wall pushing it away from the wall" the wall resists but I don't think there's a normal force unless there's an opposing force against it and I'm not seeing where that would come from in the given scenario. $\endgroup$
    – JimmyJames
    May 17 at 19:32
  • $\begingroup$ @JimmyJames It's opposing the force of the water pulling it towards the wall. Probably not the most important force in the whole picture, but its one that's there. $\endgroup$
    – Cort Ammon
    May 18 at 3:52
  • $\begingroup$ snakes have a huge, absolutely enormous, contact surface (probably something like 50x more sq cm than, say, a racing pushbike). you can easily stick (say) a longish sausage to a vertical piece of glass, at least momentarily, with any random fat oir moisture on the sausage $\endgroup$
    – Fattie
    May 18 at 14:55
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Principally it is due to surface tension (not friction force). Googling something like surface tension insects wall would produce many hit of different level of difficulty (in terms of knowing physics.)

Here is, e.g., the abstract to article Insect-Inspired Wall-Climbing Robots Utilizing Surface Tension Forces:

This paper describes insect-inspired wall-climbing robot that is capable of walking on a smooth vertical surface utilizing surface tension forces. The adhesion mechanism of the robot is inspired from the attachment system of ants, which is responsible for a thin film of secreted liquid between the adhesive organs and the surface. Two kinds of adhesive pads made of PDMS and glass were fabricated using MEMS techniques and adhesive properties were measured. Furthermore, a hexapod robot with the adhesive pads installed on its feet was developed. The robot weighs 9.5g and walks in the alternating tripod gait. It successfully walked on vertical and inverted glass surfaces.

(emphasis is mine)

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    $\begingroup$ As far as I know, snakes don't secrete adhesive fluid to help them stick to walls like some other reptiles or insects do. The article you link to uses a specialized fluid with surface tension several orders of magnitude greater than water to enable the wall-sticking. What fluid are snakes supposed to be sticking to walls with? The surface tension of water is simply not strong enough. $\endgroup$ May 16 at 15:41
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    $\begingroup$ @NuclearHoagie this answer makes no claims about snakes. On the other hand, the question does ask about ants. $\endgroup$
    – Roger V.
    May 16 at 16:00
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I used to keep a range of reptiles when I was younger with the goal of becoming a herpetologist. Snakes possess unique scales on their bellies known as scutes, which aid in gripping surfaces like the ground or tree bark to assist in crawling or climbing. One interesting fact about these scutes is that they are made of keratin, the same material found in human fingernails and hair.

The ability of arboreal snakes to climb trees is facilitated by their scales. The scales on their ventral side, or belly, are modified to provide extra grip, allowing them to grasp onto the rough texture of tree bark or vines. Additionally, their scales have small, hook-like structures that further enhance their ability to cling to branches and prevent them from slipping or falling.

From https://vitalfrog.com/snake-climbing-branch-how-snakes-climb-trees-and/

For a more formal, physics-ey discourse, see https://www.pnas.org/doi/10.1073/pnas.0812533106.

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