(2) Measuring the molecular force per unit area, we can know the gravity balance between the lower lead block B and the hook code from the experiment, so we can get the resultant force by calculating the gravity g of the lead block B and the hook code, and the average force per unit area by measuring the contact area.
If the contact surface diameter is d, the contact area S=π(d2)2, and the force F=G,
Then the molecular force of a to b per unit area: p = fs = gπ (D2) 2 = 4gπ d2.
So the answer is: ① AC; (2) Gravity G of lead block B and hook code, and cross-sectional diameter D of lead block A or B; ? ③4Gπd2。