(20 13? As shown in the figure, a square one-turn wireframe abcd is placed on a smooth insulating horizontal plane, with the wireframe side length L, the mass M and the resistance R.

Displacement of wireframe after (1) time t x= 12at2

According to Newton's second law: F=ma

Force does work f: w = FX.

Solution: w = 12ma2t2.

(2) The speed of time t is v=at.

The induced electromotive force at this time E=BLv.

Induced current I = er

Ampere: Fan An =BIL

According to Newton's second law, F-F An =ma.

Simultaneous solution: f = B2L2 ATR+Ma.

Answer: (1) The work done by tension in time t is 12ma2t2. ..

(2) When the cd edge of the wireframe just leaves the boundary MN, the horizontal pulling force F is B2L2atR+ma. ..