Two equal particles $A$ and $B$ are attached to the ends of a light rigid rod of length $2a$. The rod is supported in the position of unstable equilibrium with $A$ in contact with a smooth horizontal table and $B$ above $A$ making an acute angle $\alpha$ with the vertical axis. Find the velocity of $A$ when the rod has turned through angle $θ$ and thrust on the table.
Is this $\theta$ measured through vertical axis or from initial position? Can anyone help me to understand the problem?
Hint.
The gravity center maintains the $x$ coordinate constant along the movement. The initial potential energy is equal to the kinetic energy when the rod thrust on the table. Also there exists a holonomic constraint which is $y = a\sin\theta(t)$