(2) from the 1, 2, 3 times the experiment, we can see that when the same degree of roughness of the contact surfaces, the larger the pressure, the greater the sliding friction force;
< p>(3) comparing the number of experiments 1 and 4 (2 and 5, 3 and 6), found that the wooden block was subjected to different sliding friction, Xiaoming think it is possible that it is due to the different contact area. In order to study this problem, Xiaoming will be placed on the side of the block along the vertical direction after cutting off half of the spring loaded dynamometer pulling on the board to do uniform linear motion, the spring loaded dynamometer for the original number of half the number of signals, concluded that "the size of the sliding friction and the size of the contact area," the conclusion that his practice is wrong. The reason is that there is no control of the same size of the pressure, the process of the existence of two variables;(4) in the course of the experiment, must be pulled at a constant speed horizontal block, so that the block to do a uniform linear motion, when not to do a uniform linear motion, the dynamometer is no longer equal to the size of the sliding friction. According to the known physical knowledge to analyze the number of experiments 1 and 4 (2 and 5, 3 and 6) in the block by the sliding friction force is different reasons, may be pulling the dynamometer, to do the speed of motion.
The answer is: (1) the block to do uniform linear motion, in the horizontal direction of the balance of two forces; (2) 1, 2, 3; (3) did not control the same size of the pressure, the process of the existence of the two variables; (4) pulling the dynamometer, to do the speed of motion.