At first glance the concept of mechanical friction seems not at all odd. It is just another force. It must be considered (as friction) when surfaces slide over one another. It must be taken into account (as viscosity) when motion in liquid media are studied. It must always be included (as resistance) when analyzing motion through the air. But it has certain peculiarities that are most instructive. Provided that two contacting bodies do not move with respect to each other, the friction between them increases with the force tending to make them slide. Physicists call this the static or Coulomb dry friction. When the external forces acting on the contacting bodies reach a certain magnitude they begin to slide along one another. But there is still friction. This is dynamic friction. Like static friction it is greater as the pressure holding the two surfaces together increases, but is always less than the static friction in the same case. If we imagine one of the contacting surfaces to be stationary and the other to be moving or tending to move the force of friction is always opposed to the direction of actual or potential motion.