Wet and dry Friction

We are learning about how different conditions on a surface change the speed of it. I was wondering how a wet surface versus a dry surface creates more or less friction?
- Julia (age 11)
NY, NY

Water can either increase or decrease the friction between two surfaces. Water reduces the friction between rubber tires and road pavements. That's why drivers have to be extra careful not to skid when roads are wet. Water can increase the friction between your fingers and a sheet of plastic. That's why people sometimes moisten their fingers when trying to open the seam of a plastic vegetable bag. Water can also increase the friction between two sheets of glass. 

I think the reason is more or less this. The rubber tires are a bit squishy. The rubber mushes around to fit the little cracks and bumps in the road. That makes a lot of friction, since for the rubber to slide it has to change shape to fit a new set of little bumps. Also the molecules of rubber have to unstick from the road surface as it slides.  For really smooth surfaces like a plastic bag, your fingers don't touch many spots and don't form many bonds. There aren't little bumps for your fingers to squash around. A thin layer of water allows your fingers to bond with the water which also bonds with the plastic, since the water can fit in the little open spaces. That doesn't give a lot of friction, but it does give more than without the water.

Mike W.

(published on 01/23/2014)