# Doubt from work power energy

**Meena_2020**#1

**Shreyas_2021**#4

That is the constraint equation : x^2 + y^2 = l^2.

Here, the ring and the block are constrained to move in a such a way that their directions of motion are perpendicular and the length of the string joining them does not change, which in turn, on application of the pythagoras theorem, results in the above equation.