【Haichuan Teahouse】Brain-teasing Series: A Matter of One Finger
Thread Content
As shown in the figure, a beaker filled with water is placed on one end of the balance, and weights are placed on the other end to keep the balance in equilibrium. If a finger is then placed in the water, and if the finger remains completely still without touching the beaker or its bottom, and no water overflows, what change will occur in the balance’s state at this point? This question was part of the final physics exam in a certain prefecture-level city. After reading it, the students were very confused and did not know how to approach it; most of them felt that insufficient information was available. According to statistics, over 90% of the students chose option D. Many junior high school physics teachers considered this question to be imprecise, and extensive discussions were held on it.
C. When the finger is placed in the beaker, it experiences buoyant force from the water. According to Newton’s third law, forces act in pairs; therefore, the finger also exerts a reaction force on the water. Therefore, as long as the fingers remain in the water and no water overflows, there is buoyancy from the water on the fingers, and thus there is pressure exerted by the fingers on the water. Specifically, the fingers exert a downward pressure on the liquid, causing the left side of the balance, which was previously in equilibrium, to tilt downward.
Initial state: C； Final state: A
Choose C. How can it not be precise? Can fingers lift a beaker when water freezes?
This topic was debated before; if the debate didn’t reach a conclusion, one could try guessing by pointing at an option – and the result was: C! ;P
C. . . AO, CAO;P