Why is friction called as a self adjusting force?
I can give you two reasons, one of which is kind of obvious and the other much more interesting if you’re into machines. The 1st ref. describes static friction as self-adjusting because as long as you don’t push too hard, the friction force is exactly equal to the pushing force and the object doesn’t move. Just like pushing against a wall. Big deal. The more interesting reason concerns an invention called the Prony brake, used to apply a known torque load on a motor whose power output is being measured. Originally it was just a brakeband held rigidly against a rotating drum. Unfortunately, when the brake heated up, the coefficient of kinetic friction would change, which changed the torque load. Improvements were made to arrange the band restraints and the geometry relative to the rotation direction such that a larger torque tended to reduce the tension of the brakeband which reduced the normal force. This created a feedback mechanism that produced self-regulation of friction torque. Se
