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In technical terms, its dynamic friction must be lower than its static friction.

The traditional style of cross-country skiing works on the same principle—the downward kick stroke requires high static friction, while the glide is possible thanks to low dynamic friction.

"The candidate should then be able to explain why both fingers reach the centre of the rule at the same time as observed. "We might even discuss the fact that the coefficient of static friction is higher than the coefficient of dynamic friction.

Taylor Swift is executing this bit of strategy with ruthless precision as part of her 1989 tour: bring out a new batch of celebrities every few nights, keep your name in the headlines, and you never have to overcome static friction to promote a new single or LP.

It is called the “stick-slip” phenomenon, says David Bowen, director of the noise and vibration group at Acentech, an acoustics consulting firm in Cambridge, Mass. The ultimate culprit is friction: the parts “stick” to one another because of static friction but then suddenly “slip” by one another, which causes the panel to vibrate and radiate sound.