Question about shuttle ball experiment. Very confused.?

When the ball reaches the earth plate, it becomes neutral, as electrons will flow from the earth-plate to the ball. But the ball is still under the action of the +ve plate (i.e. the ball is subject to the action of the electric field lines set up by the +ve plate), hence -ve charge is induced on the left-hand side of the ball and +ve charge on the right-hand side of the ball. The induce +ve charge is immediately neutralized by electrons from the ground (because it is earthed), leaving only the -ve charge, which is attracted by the +ve plate and cannot escape to ground.

Because unlike charges attract, the ball, which now carries only -ve charge, is attracted by the +ve plate and move towards it.

Note that when the ball touches the earth-plate, its potential is zero. But an object at earth potential doesn't necessarily mean that it must carry no charge.