A block rests on a horizontal table. A string tied to it runs up over a small fixed pulley above the table, and a person pulls the free end of the string down at a steady speed $u$, as shown in the figure. At the instant when the part of the string from the block to the pulley makes an angle of $60^\circ$ with the horizontal, the speed of the block along the table is

Fig. 1: A block on a table tied to a string that rises at 60 degrees to a pulley hanging from the ceiling. The other end of the string is pulled down at speed u.