For a ceiling fan I am confused about how the blades cut through air to cool off things in a room?
See, basically it is an application of Bernoulli's Theorem if you already didn't know it. Bernoulli's Theorem states that $P + \rho gh + \frac{1}{2} \rho v^2$ = constant where the symbols have the usual meanings.You can check it here if you want. For constant $h$, we can say that $P + \frac{1}{2} \rho v^2$ = constant i.e. $P$ varies as $\frac{1}{v^2}$.
Now consider the blades of the fan to be rotating. The air column above the fan can be considered to have streamline flow and same elevation. So as the fan blades rotate, they are specially angled to make the air column move and this air column attains a high velocity. So from Bernoulli's Theorem, we can say the pressure of the air column decreases i.e. it has low air pressure.
However the air column below the ceiling fan has high pressure since it is not agitated by the fan blades and has low or no velocity.And we know air moves from high to low pressure. So air from above the fan rushes down and creates winds which increase the rate of evaporation of any liquid on any surface, creating a cooling effect.
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.
Best Answer
When it's off, there is only gravity pulling down, and the mount pulling up, so it remains stationary. While it is on, there is also a torque from pushing sideways through the air, which the mount must carry, and depending on the direction of rotation, also either an additional lift or downwards force from moving air down or up, respectively; this is also countered by the mount on the ceiling.