A tractor moves along a straight line on a flat horizontal field with a constant speed $v$. A sled is attached to the tractor by a taut, inelastic rope of length $L$. The sled has runners that force it to always move in the direction the rope is pointing (i.e., directly toward the tractor). At a certain moment, the rope makes an angle $\theta$ (where $0 < \theta < 90^\circ$) with the tractor's path. What is the magnitude of the sled's acceleration at this moment?