# Day 44: Surface Area and Friction

College-Prep Physics: Much to most students’ surprise, surface area did not have an effect on the amount of friction between surfaces. So how can our “interlocking bumps”-model for friction explain this? It would seem that, for a greater surface areas, there are more bumps in contact, and therefore more friction. However, think about the compression of the 2 surfaces:

This demo is 2 identical metal drawers, one laying flat and the other standing up on one end. Inside both drawers is a 1 kg mass so there is noticeable compression. Although you can’t see it very well in the photo, the “no-bounce” foam is compressed a lot deeper on the right than on the left.

In other words, even though the situation on the right has fewer bumps in contact, it also has a larger compression, making it harder for any pair of interlocking bumps to slide past each other. And vice-versa: an increase in surface area leads more bumps in contact, but a decrease in surface compression, and therefore the total friction remains unchanged.

##BFPM