Quote Originally Posted by Poopadoop View Post
physical pushing and pulling which might happen if (e.g.) the arch of the foot were pushing against the laces.
You mean like the flexing of a foot operating a gas pedal?
Quote Originally Posted by Poopadoop View Post
vibration which might happen when (e.g.) the foot hits the ground on each step.
or maybe vibrations caused by being in contact with something attached to a running engine?
Quote Originally Posted by Poopadoop View Post
Similarly, what are the qualities, physically, that makes a knot resistant to these forces and stay done up?
Even I know that one!
Quote Originally Posted by Poopadoop View Post
Is it the friction between the laces of a tight knot that does it?
Yeah