As shown above, are four rigid rods connected by hinges whose lengths are , and it's obvious that quadrilateral is a rhombus.
Initially, the diagonal is longer than , and the rhombus is put on the horizontal ground, then is moving to the opposite side along line with the same magnitude of velocity .
What's the acceleration of point relative to the ground at the moment when the rhombus becomes a square (see the picture on the right)?
Submit the value when .