Jesse Maher of Balclutha Primary School asks :-

Why does water take away friction?

Pat Langhorne, a physicist at the University of Otago, responded.

I assume that you are thinking about why cars skid more easily on a wet road than on a dry road, or why licking your finger makes it easier to rub it over a bench. In each case there is a thin film of water between two solid objects: eg the tyres of the car and the road.

Friction between two solids arises because no solid surface is perfectly flat. If you examine an object at the small-scale you find that there are lumps and bumps on all its surfaces. When two such surfaces are placed together these lumps and bumps contact each other, resisting any sliding between the objects. The more we push the two objects together the greater the interaction between the lumps and the more difficult it becomes to slide objects over each other.

Now let’s imagine that the space is filled with a layer of liquid. A liquid has the property that it can flow to fill the space available. So water, for example, would go some way to filling up the gaps between the two surfaces caused by these surfaces not being perfectly flat. The water trapped adjacent to the solid must have the same speed as the solid. Now when we try to slide one object over the other we need only supply a force that makes sure the top of the water layer is moving, while the bottom of the water layer is still. Because water flows easily (technically we say it has a low viscosity) then this requires a much smaller force than the case where there is no water layer.