Comparing the experimental data of 1 and 3, it is found that 1 is aluminum block and 3 is pine, with the same volume, different mass and different mass-volume ratio, and it can be concluded that the mass-volume ratio of different substances is generally different;
(2) density is a characteristic of the substance itself, and its size is related to the substance, but has nothing to do with mass and volume;
(2)① In order to accurately measure the friction force, the wood block should be pulled to move in a straight line at a uniform speed on the horizontal plane. At this time, tension and friction are a pair of balancing forces, and the magnitude of friction is equal to the magnitude of tension;
(2) Adding different weights to the wood blocks changes the pressure of the wood blocks on the horizontal plane;
③ To explore the relationship between friction and contact surface roughness, we must make the pressure the same and change the friction, so we should choose A and C for comparison.
(3) In this problem, the influence of pressure is expressed by the depth of sand surface subsidence. By comparing the changes of pressure effect in Figure A and Figure B, a conclusion can be drawn.
So the answer is: (1)① The mass-volume ratio of the same substance is certain; The mass-volume ratio of different substances is generally different; It doesn't matter; (2)① horizontal plane; A uniform straight line; A pair of scales; ② Change the pressure; ③A； c； When the pressure is constant, the rougher the contact surface, the greater the friction force; (3) the depth of sand surface subsidence; When the pressure is constant, the smaller the stress area, the more obvious the pressure effect; When the stress area is fixed, the greater the pressure, the more obvious the effect of pressure.