[填空题]
A 2.30-m-long pole is balaw1ypr + n+ajb)skbb (8ze.)46gmpmf oncedzf2f,4x, 2pph lbpws 5 vertically on its tip. It starts to fall and its lowep,5w2hz fl2 x fp4,spbr end does not slip. What will be the speed of the upper end of the pole just before it hits the ground? [Hint: Use conservation of energy.] $m/s$