It's perhaps the second week of your introductory physics course. Your instructor starts talking about friction and writes the following two formulas on the board. Then there is probably some sort of ...

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Friction is a force that exists when two surfaces rub up against each other. For example, this cup on ...

One of the dirty little secrets of physics is that there is no generally accepted explanation of the basic laws of friction. An advance in the theory of cracks will stimulate fresh thinking on the ...