Xiao Ming watched the badminton match and found that the athletes' soles had deep patterns. He guessed that the friction between the sole and the ground might be

(1) The shoe moves in a straight line at a uniform speed on the horizontal plane, and the pulling force and sliding friction force acting on the shoe in the horizontal direction are a pair of equal balancing forces.

(2) Explore the relationship between sliding friction and pattern depth, and control the pressure constant. The pressure of new shoes and old shoes is different, and the experimental conclusion is not accurate, so the pressure is not a control constant in the experiment.

(3) When exploring the relationship between friction and the depth of the sole pattern, we should control the pressure of the object on the contact surface to be the same, and only change the depth of the pattern. Therefore, experiments 3 and 4 explore the relationship between friction and the depth of sole pattern.

By comparing the experimental data of serial numbers 1 and 3, it can be seen that the roughness of the contact surface is the same, but the pressure on the contact surface is different, so the data in the comparison table show that the friction force is related to the pressure.

So the answer is: (1) even straight line; (2) The pressure on the contact surface is not controlled to be the same; (3) depth; Stress.