Questions and discussion for this lecture live here. Fire away by hitting Reply below 
Just quick question about stability of the structure here. Since we have horizontal force acting on the system and both of the supports can not resist rotation, system should be unstable as the acting horizontal force will push the truss to the right side.
Hi @Div - no that’s not quite the right interpretation of the behaviour of this structure. You’re correct in that neither of these supports will generate a rotational resistance - but acting together, the vertical reactions at each support form a couple (equal magnitude forces with parallel lines of action acting in opposite directions) and this couple (or moment) resists the overturning moment developed by the horizontal force.
Let me know if this raises any more questions for you.
Seán