I've only taken two semesters of calculus-based physics as part of my general education credits at university. Please keep this in mind when reading this question and framing answers.
Today I observed a gecko on a ceiling and began wondering how it managed to stay put. A bit of googling revealed that the magic is actually van der Waals forces, not an unusually high coefficient of static friction or something.
So I tried to draw a mental free body diagram of a gecko at rest on the ceiling. I drew a downwards arrow representing the gravitational force, and an upwards arrow for the van der Waals force. And then I wondered — is that it? Is the van der Waals force upwards equal to the downwards force of gravity?
When I studied physics at university, we always drew a normal force perpendicular to the surface the object in question was resting on. We were told this represented the force exerted "upwards" by the surface to counteract gravity. By doing this we could calculate the magnitude of an unknown force by having the normal counteract gravity.
In my gecko situation, my unknown force is acting counter to gravity. Is there a normal force at play here too? Or is the van der Waals force taking its place?
Best Answer
Firstly, the 'normal' force is a contact force which opposes two surfaces being forced together. It is not caused by gravity, but it can be the response to gravity (eg when gravity pushes a book into contact with a table). It is caused by inter-molecular repulsive forces.
Secondly, the van der Waals force is an attractive force. This is the only upward force here, pulling the gecko towards the ceiling.
However, does this mean that the pads on the gecko's feet are so finely controlled that the van der Waals force exactly balances the weight of the gecko? ie If the gecko has a larger breakfast than usual the pads somehow exert a stronger van der Waals force to compensate? I am not sure this is the case.
Even if the gecko can control the van der Waals force, I doubt that it can be done so precisely. I think it more likely that the van der Waals attractive force is always slightly greater than gravity, pulling the gecko into the ceiling. The usual repulsive inter-molecular forces then come into play to oppose the net upward force on the gecko. The normal contact force is reactive and adjusts to provide a perfect balance of forces.
So in addition to its weight (downward) and the van der Waals force of attraction from the ceiling (upward) I think there is also a small additional 'normal' (in both meanings of the word!) contact force from the ceiling acting downward on the gecko. This is where I disagree with Harm Moolenaar. (I do not understand what Joce is saying - possibly the same as Harm?)
To some extent the issue might be one of semantics - ie whether 'van der Waals force' includes the inter-molecular repulsion or is separate from it. Nature does not distinguish one force from another, only we do that, for our own convenience. So I think it is an issue of how forces are distinguished and classified.