Steps:
(1) Fix one end of the rope on the hook of the spring scale and the other end on the pencil case.
(2) On the smooth desktop, gently pull the pencil case to find the force that can just make the pencil case move, record the reading on the spring scale, and repeat it for 3 times.
(3) After spreading a dry towel on the desktop, gently pull the pencil case on the rough towel to find out the force that can make the pencil case move, and make records. Repeat three times.