1. The Temperature Of Water Is Kept Constant During Swimming

VISCOSITY 1. The temperature of water is kept constant during swimming in international competition. Why? The viscosity of water changes with temperature. The rise in temperature lowers the viscosity and vice versa. This gives the swimmers in hot pool unfair advantage compared with times set in a cooler water pool. 2. Why do large rain drops hurt more than small ones when they fall on you? The terminal velocity of a sphere falling under viscous fluid is directly proportional to the square of the radius of the sphere. The large rain drops, therefore, have very high velocity compared to the smaller one. Thus, larger drops have more mass and velocity. When rain drop strike us, the rate of change of momentum, i.e. force for large drop is very high compared to the smaller one. Hence, they hurt us more. 3. Machine parts are jammed in cold days. Why? ([HSEB 2057] Machine parts are supplied with viscous oil to reduce mechanical friction between them. When temperature of the environment decreases, the viscosity of the oil increases. In very cold day, the viscosity becomes so high that the parts are jammed. 4. Why does the acceleration of a sphere falling through a fluid vary with the distance that it has fallen in the fluid? There are three forces acting on a sphere falling through a fluid. The weight acts vertically down but upthrust and viscous drag act up. The weight and upthrust are constant for the sphere and the liquid but viscous drag varies with speed of falling sphere. Initially due to the net downward force, the velocity increases but at the same time, the net force changes due to increase of viscous force. Obviously, the acceleration also changes. 5. Why do jet air planes usually fly at altitudes of about 9000m even though it takes a lot of fuel to climb to that altitude? The density of atmosphere is very low at higher altitude in comparison to that of lower altitude. This certainly reduces the viscous drag on the airplanes flying at higher altitude. The surplus of fuel thus becomes large while flying at the higher altitudes that while flying at the lower altitudes. Therefore, the jet companies never mind to spend the fuel while taking that much height. 6. Why does air escaping from the open end of a small pipe make a lot more noise than air escaping at the same volume rate from large pipe? From the equation of continuity, we know that the speed of fluid molecules increase with the decrease of cross section of pipe from same flow rate. Therefore, air escaping from the open end of a small pipe must have larger velocity than compared to the escaping from larger one. Larger the velocity, higher will be the kinetic energy. Hence, they produce lot more noise. 7. When smooth flowing stream of water comes out of faucet, it narrows as it falls. Why? For smooth flowing stream of water, the volume of water flowing per second at a point is equal to that of another point. Hence, from the equation of continuity, we know that the velocity of flow increases with decrease of cross section and vice-versa. When the stream of water falls

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from the faucet, its velocity of flow increases due to gravity and hence its cross section must decrease. That’s why the stream narrows as it falls. A stone and a soldier are dropped from the same height. Which one falls with less velocity and why? The viscous force is directly proportional to the area. The parachute has larger area, therefore, it experiences greater viscous force and reaches terminal velocity earlier, i.e. has a smaller terminal velocity. Small rain drops falling under gravity do not acquire very high velocity. Why? When part of the mouth of water tap or pipe is made smaller with fingers, the velocity of the water flow increases. Why? According to equation of continuity, velocity of liquid flowing through a pipe is inversely proportional to the cross-sectional area. [ of the pipe. When water tap or pipe is closed with fingers, area of cross-section of outlet is reduced. So the velocity of water increases and comes out as a fast jet. It is dangerous to stand near the edge of the platform when a fast moving train is crossing it. Why? When a fast moving train crosses the platform, air between the person and train moves along the train, i.e. with high speed. According to Bernoulli’s principle, if high pressure becomes low, air from surrounding moves from high pressure to low pressure. Therefore, a person standing near the platform may be pushed toward the train. Why do ships and motor boats have sharp edges on the front? Viscous force experienced by a body when moving through a fluid is minimum for a pin-shape. Such bodies can acquire maximum possible velocity, through a fluid, than any other shape. Therefore, high speed objects like rockets, aero planes, submarines, etc. have sharp front edge.