Points to ponder:

1. The formula for induced e.m.f = Blv where v = ds/dt. Hence the gradient of s-t graph determines the value of E. The gradient change must be reflected in the E -t graph. Hence there are positive and negative values of E.

2. The current flow clockwise as seen from the leftside. Otherwise the current flow as shown. Current flow anticlockwise when viewed from the right side.

3. The current flow up and so it exit from the top end, hence the top end is positive. The external force needs to be applied to the right as the repulsive magnetic force is to the left. This is to enable to rod to keep moving at constant speed.The external force is removed , the repulsive force will slow down the rod , decreasing speed and in turn reducing repulsive force. This will continue all the ke converted to electrical energy and the rod comes to a half.

The power can be calc using P = Fv or P = E2/R.