(20 1 1? Xiaoming explored the relationship between sliding friction and roughness of contact surface with a graphic device. The following operations are correct: () A.

A, putting heavy objects on the block increases the pressure of the block on the plane, which is to study the relationship between friction and pressure, which is inconsistent with the meaning of the question;

B, the experiment is to study the relationship between the magnitude of friction and the roughness of contact surface. The roughness of the contact surface should be a variable, which obviously does not meet the problem.

C, put the wooden block on the side to reduce the contact area, and study the relationship between the friction force and the contact area, which is inconsistent with the meaning of the question;

D, No matter which of the three variables is studied, such as pressure, roughness of contact surface and contact area, the pulling force of the wood block in the horizontal direction should be uniform, because according to the knowledge of the balance of two forces, only when the wood block moves in a straight line at a uniform speed can the friction force be equal to the pulling force of the spring dynamometer.

So choose D.