Fluid Mechanics

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Kinematics

Introduction to Kinematics

Fluid Mechanics - Short Question Answers 3

1. Small air bubbles rise slowly while big bubbles rise rapidly through liquid, why?
The terminal velocity of a body through a fluid is given by, v = 2r2 (ϱ – σ)g/9η. This shows that when the radius increases, the terminal velocity also increases. If the air bubbles are large, they have larger radius, so the terminal velocity will be more. However, if the radius is small, as in the case of small bubble, the terminal velocity will be less. Hence, small air bubbles rise slowly while big bubbles rise rapidly through the liquid.

2. Why is a suction effect experienced by a person standing close to the platform at a station when a fashion when a fast train pass?
When a fast-moving train passes to a person standing nearby its track, the air between the person and the train moves with a high speed. According to the Bernoulli formula, P + 1/2 ϱv2 = constant, the air pressure becomes low near him/her compared to the pressure at other regions including behind the person. So, the air moves from back side of the person to front side pushing him/her on the way.

3. Why are light roofs blown off during cyclones or strong wind storms?
During windstorm, the velocity of wind suddenly decreases when enters into the houses, but its velocity remains high above the roof. According to Bernoulli’s theorem, P + 1/2  ϱv2 = constant, the pressure on the top of region is lower than that at the bottom region. The inside pressure becomes very much larger than the outside pressure of the roof because the inside air has lower kinetic energy than that of outside air of the roof. This difference of pressure results in an increment of upthrust. As a result, the roof is lifted up and blown off by the wind.

4. Machine parts are jammed in cold day. Why?
The viscosity of a liquid varies inversely with its temperature. As the temperature decreases in winter, the coefficient of viscosity of the lubricant increases. Due to the increase in viscosity of lubricant, the machine parts are jammed in winter.

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