mg= 13qE
Solution: q=3mgE
(2) Based on the kinetic energy theorem, the linear change of electric field force with displacement is considered:
mgl3？ 0+qE/32？ l3= 12mv2m
Replace vm=gl3 with q = 3mge.
(3) When the speed of the rod drops to zero, the falling distance is h, which is obtained from the kinetic energy theorem:
mgh？ 12qE？ hl？ h=0
De: h = 2mglqe
From Newton's second law: qlhE? Mg = mA
Bring h into the above formula: a = g.
Answer: (1) The charge of the thin rod is 3mgE.
(2) The maximum speed of rod dropping is gl3；;
(3) At this time, the acceleration of the rod is g..