My question probably doesn’t even make sense so here is an example. If you pour water over a massive boulder and throw sand at it, the sand will stick to the rock better than it would without the water. For larger objects like your hand, it would make it much harder to climb that rock than without the water poured on it. Why is it the opposite?
In: Physics
Three terms – cohesion, adhesion, and friction.
Cohesion is a material’s tendency to stick to itself, for example, water likes to stick to itself in drops.
Adhesion is a material’s tendency to stick to other things. Water has decent adhesion, this is why water will stick to the sides of cup in tiny droplets (the droplets held together in cohesion).
Friction is the force responsible for the “stickiness” between objects rubbing against each other.
The issue here is water has pretty high strength in co- and adhesion. So water poured on a rock will stick a little bit and keep the rock wet, as opposed to all running off and leaving the rock dry. Adhesion can “hold” small things onto that water, like the sand in your example. Think of it as a rock/water/sand sandwich. The water sticks to the rock and the sand sticks to the water.
These “hesion” forces are really weak though. A large force, like your hands grabbing the rock, overwhelm the “hesion” forces and now Friction comes into play. Water acts as a weak lubricant, meaning it slightly reduces friction. If you imagine a rough material, like sandpaper, water’s ad/cohesive properties effectively allow the water fill in the gaps and *smooth* the surface on a microscopic level. This smoothness makes it harder to grab and makes you slip. A more highly co/adhesive material, like oil, does this even more noticeably.
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