A person blows across the top of one arm of a U-tube partially filled with water. The water on that arm:
Correct answer: B. rises slightly
- A. Drops slightly
- B. rises slightly
- C. rises if the blowing is soft but drops if it is hard
- D. remains at the same height
Explanation
When a person blows across the top of one arm of a U-tube partially filled with water, the water level in that arm actually rises slightly. According to venturi relation "where speed is high pressure will be low". So, blowing across the tube creates a region of lower pressure due increased speed of air, causing the water in that arm to be pushed up by the relatively higher atmospheric pressure on the other side of the U-tube.
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Fluid flow is described through speed, pressure and density, including steady flow and the equation of continuity for conserving mass. Bernoulli's equation relates pressure, speed and height, while fluid drag and terminal velocity explain why a falling object eventually moves at constant speed when drag balances its weight.
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