The glass rod rubbed by silk is positively charged. If the medulla tetrapanacis ball was originally charged, it must be that the medulla tetrapanacis ball was originally negatively charged, and different charges attracted each other.

In the process of contact with each other, due to the neutralization of charges (that is, the remaining electricity is evenly distributed after the positive and negative charges are equally offset), the negative charges of the original medulla oblongata ball and the positive charges of the same amount of glass rod cancel each other out, and after the cancellation, some positive charges remain on the glass rod and the medulla oblongata ball, which repel each other according to the same kind of charges. At this time, both objects are positively charged, so they will be bounced off.

This has been introduced in the section of friction electrification, just like the two metal foils of electroscope will open when charged, except that the positive charge of a glass rod is transferred to the grass ball, which is what we call the contact electrification process. In addition, this problem must require that the positive charge of the glass rod is more than the negative charge of the grass ball. If the two charges are equal, there will be no back bounce. If the grass ball is more negatively charged, it is found that both the glass rod and the grass ball should be negatively charged after spreading.

I didn't see the topic clearly last time. I thought it was another topic. I'm sorry. I hope the revised answer is detailed enough to satisfy the landlord.