Study Lagrangian Mechanics: D'Alembert's Principle

[KT KIM]

Studying Lagrangian mechanics starting from D'Alembert's principle.
I just ran into this.

Why virtual displacement doesn't contain time t for it's coordinates
What does that mean 'take place at a fixed time'?

 

[drvrm]

Why virtual displacement doesn't contain time t for it's coordinates

The simplest answer is that its not 'real' displacement therefore it must not contain change in time - and that's why its called 'virtual'

if a geometrical change in the configuration of the physical system is done consistent with the constraints operating on the system then one can formulate a principle which helps in weaning away the force of constrains from equation of motion. and that has certain advantage over Newtonian description.

 

[KT KIM]

Thank you much for fine answer, helped me a lot. Guess need some time to grasp that notion well.