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## Main Question or Discussion Point

in the "derivation" of 'principle of vitual work' Goldstein started with this

he wrote the force F acting on each particle as the sum of [tex]F_{a}[/tex] (Force applied) and [tex]F_{i}[/tex](Constraint force)

ok i have some three queries

1)What are constraint forces?

2)Are all internal forces constraint forces

3)He said the cond. of equilibriia of a system is

[tex]\sum \vec{F_{a,i}} .d \vec{r_{i}} =0[/tex]

that is the virtual work is [tex]0[/tex] i don't get how that happens(because virtual work being 0 doesn't imply force on each particle of the system is [tex]0[/tex] or is it just by definition?

he wrote the force F acting on each particle as the sum of [tex]F_{a}[/tex] (Force applied) and [tex]F_{i}[/tex](Constraint force)

ok i have some three queries

1)What are constraint forces?

2)Are all internal forces constraint forces

3)He said the cond. of equilibriia of a system is

**equivalent**to[tex]\sum \vec{F_{a,i}} .d \vec{r_{i}} =0[/tex]

that is the virtual work is [tex]0[/tex] i don't get how that happens(because virtual work being 0 doesn't imply force on each particle of the system is [tex]0[/tex] or is it just by definition?

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