Friction is the resistance to the relative motion between two surfaces. In other words, friction resists the relative motion between two surfaces. It is the reason why a book pushed on a table stops after moving some distance. There are a large number of other examples of friction in our day to day life. It can be of two types: static and kinetic. Static friction keeps the bodies at rest, at rest. That is, it prevents motion of a resting body. Kinetic friction resists the motion of a moving body. The friction between two surfaces can be measured by the coefficient of friction (`mu`), which is the ratio of friction force and the normal force.
The lower the value of coefficient of friction (whether static or kinetic), the lower the friction is. Here is a list of coefficient of friction between two materials in contact:
- Teflon on Teflon: `mu_s` = 0.04; `mu_k` = 0.04
- Metal on metal (lubricated): `mu_s` = 0.0.15; `mu_k` = 0.06
- synovial joints in human beings: `mu_s` = 0.01; `mu_k` = 0.003
- Vehicle tires on concrete: `mu_s` = 1.00; `mu_k` = 0.80
- Steel on steel: `mu_s` = 0.74; `mu_k` = 0.54
- Glass on glass: `mu_s` = 0.94; `mu_k` = 0.4
From this list, we can see that teflon (and other plastics as well) are low friction surfaces, while the metals are high friction surface. Also note that lubrication, whether it is between metals or in our joints, reduces friction.
Hope this helps.