The wood block moves on the inclined plane under the action of tensile force, and the wood block is subjected to tensile force F, friction force F and component G' of gravity downward along the inclined plane;

A. the installation of the experimental device is shown in figure a below. Rubber-tipped dropper continuously drips water into the empty plastic bag until the wooden block begins to slide, and the number of water drops is recorded as n 1.

At this time, the downward pulling force F 1, the component of gravity and the upward friction along the slope direction are balanced forces, so F 1+G'=f, where the pulling force F 1 is equal to the gravity of water in the light plastic bag: f1= g1= n/kloc-.

B. The installation of the experimental device is shown in Figure B below. The rubber-head dropper drips water into the empty plastic bag until the wooden block begins to slide, and the number of water drops n2 is recorded.

At this time, the upward components of gravity, friction and pulling force F2 along the slope direction are equilibrium forces, so F2=G'+f, where pulling force F2 is equal to the gravity of water in the light plastic bag: F2=G2=m2g=ρ water n2V0g, which is obtained by substituting it into F2, and ρ water N2v0g = g'+f-②;

①+②, it is concluded that ρ water n 1V0g+G'+ρ water n2V0g=f+G'+f, and the equation g' is equal on both sides and can be omitted. The solution is: f= 12ρ water V0g (N 1+N2).

So the answer is: (1) the mathematical expression of friction: f= 12ρ water v0g (n1+N2);

(2) Experimental steps: a. The experimental device is installed as shown in Figure A above, and the rubber-tipped dropper is continuously dropped into the empty plastic bag until the wood block begins to slide, and the number of water drops is recorded as n1;

B. The experimental device is installed as shown in Figure B above, and the rubber pointed dropper continuously drips water into the empty plastic bag until the wood block begins to slide, and the number of N2 drops is recorded.