题目

读写综合，信息归纳 ，请仔细阅读下面材料，并按信息表中的项目要求填写信息。 Little Bob was born in a small village. His dad has been ill for a long time and his mom is always busy working, so Bob often helps his parents with housework. Though Bob is a very good boy, his parents are still very strict with him. There are a lot of rules at Bob’s home. There is a cat and a dog at Bob’s house. Bob must feed them twice a day and give them water three times a day. He is allowed to play with them for a short time every day. However, Bob is not allowed to wat如图所示，杠杆AB可绕O转动，绳AD连在以A为圆心的弧形导轨MN上，D可在MN上自由滑动，当绳的D端从N向M滑动过程中，杠杆仍保持平衡，则AD对杠杆的拉力变化情况是    ．