1. A research group discussed whether sliding friction is related to contact area and humidity of contact surface. They chose a rectangle with the same roughness as the hook as the research object.

(1) The following table is the experimental record table. By analyzing the experimental records with the serial number () in the table, we can preliminarily draw the conclusion that the sliding friction has nothing to do with (). The experimental records numbered 3, 4 and 5 in the table can show that the sliding friction is related to ().

Contact area s Contact surface dry-wet friction f/N

1 Xiaogan 0.5

2 dry 0.5

3 dry 0.5

4 wet 0.6

5 wet 0.8