p.17
Note that if a particle is constrained to the surface and that is itself
moving in time, the force of constraint is instantaneously perpendicular to
the surface and the work during a virtual displacement is still zero even
though the work during an actual displacement in the time dt does not
necessarily vanish.
疑問
1. 第一句是在說質點被限制在一個本身會移動的面上嗎？
這樣跟假設點在面上移動有什麼不一樣呢？
2.既然力都垂直於表面,那為什麼actual displacement 中的work有可能不為0呢?
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