A force $$F = - K\left( {y\widehat i + x\widehat j} \right)$$ (where K is a positive constant) acts on a particle moving in the xy plane. Starting from the origin, the particle is taken along the positive x axis to the point $$\left( {a,0} \right)$$, and then parallel to the y axis to the point $$\left( {a,a} \right)$$. The total work done by the force F on the particle is

A stone tied to a string of length L is whirled in a vertical circle with the other end of the string at the center. At a certain instant of time, the stone is at its lowest position, and has a speed u. The magnitude of the change in its velocity as it reaches a position where the string is horizontal is