Example 3: (200 1 Qingdao) The teacher gave Xiaoli the following equipment: a beaker of 100ml, a spring dynamometer and some fine wires, and asked Xiaoli to modify them into devices that can directly measure the density of liquid. Xiaoli is puzzled. Please help her design it. Please write down the modified structure, process and usage of the dial.
Guidance: This topic is a research-oriented examination of exploration and design, which requires students to design experiments to determine the density of liquids by using existing equipment. How to convert the spring dynamometer for force measurement into a density measuring device requires comprehensive application of knowledge such as buoyancy and density, and at the same time, the dial of the spring dynamometer is converted into the dial of the densimeter.
Solution: (1) Structure: As shown in the figure, hang the beaker on the lower end of the spring dynamometer with a rope.
(2) Scale transformation: gravity scale translation. When the beaker is suspended, the indicator number of the spring dynamometer is 0N, and the other scales move down in turn, and then 1N, 2N, 3N…… ...........................................................................................................................
Mark the density scale value. Because ρ = m/v = g/gv = g/(10n/kg×10×10-4m3) = g/10-3g.
According to the above relationship, the density corresponding to the gravity scale is obtained, and the corresponding relationship is as follows:
When G=0N, ρ = 0; G= 1N，ρ= 1.0× 103kg/m3，G=2N，ρ=2.0× 103kg/m3……
G=0. 1N，ρ=0. 1× 103kg/m3，G=0.2N，ρ = 0.2× 103kg/m3 ...
(3) Usage: When measuring the density of liquid, fill the beaker with liquid (100ml) and read the density value directly from the density dial.
Point essence: this question, entitled inquiry design in research questions, involves students' knowledge of buoyancy and density, as well as students' divergent thinking and technological innovation. It is beneficial to cultivate students' inquiry design ability, improve students' technical design ability and promote students' application ability of physical knowledge.
Misunderstanding analysis: The main mistake of students is that they can't understand the problem and don't convert the measurement of force in mechanics into the measurement of density. The main reason is that students can't translate the scale zero, and at the same time use the density formula flexibly to express the density. The solution is that students should master the transformation of ρ=m/v=G/gv formula.