Leonardo da Vinci is usually credited with conducting the first systematic study of friction in the late 15th century, a subfield now known as tribology that deals with the dynamics of interacting surfaces in relative motion. Da Vinci's notebooks depict how he pulled rows of blocks using weights and pulleys, an approach that is still used in frictional studies today, as well as examining the friction produced in screw threads, wheels, and axles. The authors of this latest paper used an experimental setup similar to da Vinci's.
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但這位總統面臨的挑戰在於，他的公眾支持率徘徊在40%左右，而美國民眾希望他採取更多行動來解決他們的擔憂。上個月，他在白宮發表全國演說時，也使用了類似主題與統計數據——但未能說服公眾。總統與他的幕僚似乎寄望於國情咨文更大的觀眾群（預計數千萬人）能帶來不同結果。