Xiaodong wants to estimate the density of some kind of oil, ρ oil. The measuring tool he has is only a scale. Xiaodong uses the equipment around him to design an experimental scheme. First, find a straight hard rod, hang it at o point with a thin line, and the hard rod is balanced in the horizontal position. Then hang the metal block B with known density of ρ at C on the left end of the hard rod, and find a weight a at the right end of the hard rod, and adjust the weight a At this time, the position of the weight hanging on the hard rod is e, as shown in the figure. The following is part of the experimental steps for Xiaodong to measure ρ oil. Please complete the experimental steps according to Xiaodong's experimental ideas (1) Measure the length l0 of OE with a scale; (2) immerse the metal block B in the oil to be measured, and move the weight a from e to D, and the hard rod will be balanced in the horizontal position again; (3)___ (4) the formula for calculating ρ oil by using the physical quantity measured above and the known quantity in the question is as follows:___ ．

Xiaodong wants to estimate the density of some kind of oil, ρ oil. The measuring tool he has is only a scale. Xiaodong uses the equipment around him to design an experimental scheme. First, find a straight hard rod, hang it at o point with a thin line, and the hard rod is balanced in the horizontal position. Then hang the metal block B with known density of ρ at C on the left end of the hard rod, and find a weight a at the right end of the hard rod, and adjust the weight a At this time, the position of the weight hanging on the hard rod is e, as shown in the figure. The following is part of the experimental steps for Xiaodong to measure ρ oil. Please complete the experimental steps according to Xiaodong's experimental ideas (1) Measure the length l0 of OE with a scale; (2) immerse the metal block B in the oil to be measured, and move the weight a from e to D, and the hard rod will be balanced in the horizontal position again; (3)___ (4) the formula for calculating ρ oil by using the physical quantity measured above and the known quantity in the question is as follows:___ ．

(3) According to the lever balance condition, we can know that: when the metal block B is immersed in the oil to be measured and the weight a is moved from e to D, when the hard rod is balanced in the horizontal position again: (mb-m row) · OC = ma · OD. Therefore, we should use the scale to measure the length of OD L1. (4) according to the lever balance condition, we can get that: hang the metal block B with known density ρ at the left end C of the hard rod, and find another weight a to hang At the right end of the hard rod, adjust the position of the weight a to balance the hard rod in the horizontal position. At this time, when the position of the weight hanging on the hard rod is e: MB · OC = ma · OE. According to the density formula ρ = MV, M = ρ V is obtained. Then, measure the length of OE with a scale, l0. ρ B · VB · OC = ma · l0, The hard rod is balanced in the horizontal position again, and the length of OD is measured with the scale L1. ∧ (mb-m row) · OC = ma ∧ (ρ B · VB - ρ oil · VB) · OC = ma ∧ L1. ∧ ρ B · VB · oc - ρ oil · VB · OC = ma ∧ L1. ∧ Ma ∧ l0 - ρ oil · VB · OC = ma ∧ L1. ∧ ρ oil = ma (l0-l1) & nbsp; VB · OC = ma (l0-l1) · ρ BMA · l0 = l0-l1l0 ·ρ B. so the answer is: measure the length L1 of OD with a scale, ρ oil = l0-l1l0 ·ρ B