Team Quanta gladly presents all possible short questions of BS Physics book Mechanics – II’s Chapter#04: Bulk Properties of Matter for the students.
Q.1 While swimming below surface of water in a lake, a swimmer let out a small bubble of air from his mouth. As bubble rises towards surface, what happens to its diameter?
Answer: When bubble rises towards surface of water, the pressure in surrounding water decreases due to which air in bubble expands and its size increases. In other words, diameter of bubble increases.
Q.2 How pressure varies with depth?
Answer: If pressure at one point in a fluid is p1 , the pressure at depth h below that point is,
$$p_2=p_1+\ \rho gh$$
Q.3 What is Torricelli’s law and how it can be deduced from Bernoulli’s equation?
Answer: According to Torricelli’s law, if a hole is poked in a container at depth h below the surface , the fluid exits with speed given by,
$$v=\sqrt{2gh}$$
This same speed as an object has during free fall through height h.
Q.4 State Bernoulli’s equation in words. What happens to this equation when only speed changes? When only height changes? Mention some applications of Bernoulli’s equation.
Answer: Bernoulli’s equation is a statement of law of conservation of energy. It states that, “For an incompressible, non-viscous and steady flow of fluid the sum of pressure, kinetic energy and potential energy per unit volume always remains constant.”
If speed of fluid changes from to then Bernoulli’s equation becomes,
$$p_1+\frac{1}{2}\rho v_1^2=p_2+\frac{1}{2}\rho v_2^2$$
If height of fluid changes from h1 to h2 then Bernoulli equation becomes,
$$p_1+\rho gh_{1\ }=p_2+\rho gh_2$$
Some applications of Bernoulli’s equation are:
- The swing of ball
- Working of carborator
- Up thrust of aero plane
- Blood flow
Q.5 A cylinder containing water rests on a scale. If a boy dips his finger into water without touching the cylinder, does the reading on scale stay the same?
Answer: No, when a boy dips his finger in water, its depth increases and as a result pressure at bottom of flask increases, hence reading on the scale increases.
Q.6 When we drink cold drink through straw, explain why the liquid moves upward against gravity into our mouth?
Answer: When we drink liquid through a straw, our lungs expand due to which air pressure reduces in our mouth than atmospheric pressure and resulting pressure difference produces an upward force on liquid in straw.
Q.7 A sheet of water passing over a water fall is thicker near top than bottom. Similarly a steam of water emerging from a water faucet becomes narrower as it falls. Why?
Answer: As water falls, its speed increases. Hence thickness of water decreases to offset the increase in speed.
Q.8 A ranger car has a soft ragtop roof. When car is at reset, roof is flat. When car is cruising at high speed with its window rolled up, what happens to flat roof?
Answer: When car is in motion, air flows over top of roof and air inside car is at rest since windows are closed. So, there is low pressure over roof than inside car due to which roof of cars bows upward.
Q.9 Two light objects are suspended. When a stream of air passes through the space between them, what happens to distance between objects?
Answer: When air is blown between objects, due to high speed air pressure decreases between objects than outside the objects and hence distance between objects increases.
Q.10 When you are driving a small car on motor-way and a truck passes you at a high speed, you feel pulled towards truck. Explain.
Answer: From Bernoulli equation, when speed is high, pressure is low. As pressure between car and truck is low, so car is pulled towards truck due to net force because of pressure difference.
Q.11 A cup is filled to brim with water and a floating ice cube. When ice melts, which of the following occurs?
(a)- Water overflows the cup. (b)- The water level remains same.
Answer: When ice cube melts, it becomes water and its weight is same. So melted water fills same volume that the ice cube displaced when floating, therefore water level remains unchanged.
Q.12 Consider a physical balance in equilibrium, if steam of air is blown under one of the pan, will pan goes up or down?
Answer: When steam of air is blown under than pan, pressure below the pan decreases as speed of air increases. Thus pan will go down.
Q.13 A passenger is smoking in a back seat of moving car. To remove the smoke, driver opens his window just a bit. Why smoke is drawn to and out of the driver’s window?
Answer: When car is moving, air outside the car is moving at very high speed, so by Bernoulli equation pressure is low. When driver opens the window a bit, by equation of continuity this small are increases speed of air in open window portion due to which pressure decreases. Due to this pressure difference smoke is drawn out of window.
Q.14 Houses roofs are often lifted due to hurricanes at high altitude areas. Why should we keep windows open under these conditions?
Answer: Opening the window results in a smaller pressure difference between outside and inside of house and hence less net force on the roof. This reduces the chance of damaging the structure of building.
Q.15 Unit of coefficient of viscosity is poise. Relate it to Newton and dyne.
Answer: 1poise = 1 dyne-s/cm2 = 0.1 N-s/m2
Q.16 The cross-sectional area of a stream of water becomes smaller as the water falls from tap according to equation of continuity. What would you expect to happen to cross-sectional area when water is not shot upward, as it is in fountain?
Answer: When a stream of water moves upward, its velocity decreases as it moves against force of gravity. Therefore cross-sectional area of stream increases.
Q.17 When we drink cold drink through straw, the liquid moves upward against gravity into our mouth. Can we sip water with straw on moon?
Answer: We cannot sip liquid with straw on surface of moon because there is no atmosphere and hence no pressure difference to sip liquid?
Q.18 Many peoples believe that vacuum created in a vacuum cleaner causes particle of dust to be drawn in. Actually however the dirt is pushed in. Explain.
Answer: Speed of air inside cleaner is made very high which decreases pressure inside. So dirt particles at high pressure are pushed inside towards the low pressure. This pressure difference creates the net force on particles towards inside of vacuum cleaner.
Q.19 Suppose a bowler wants to throw a base so that it rises as it approaches the batter. How should the ball be spinning to accomplish this feat?
Answer: The ball should spin so that the side facing the pitcher is moving downward and the side facing batter is moving upward.
Q.20 To change oil in car, you remove a plug beneath the engine and the let the oil run out. Let your car be in a garage on a cold day before changing the oil , it is advisable to run the engine for a while. Explain.
Answer: When we run the engine for a while, the engine produces heat and increases temperature of oil, which in turn will decrease viscosity of oil. This will help the oil to run out of engine easily.
Q.21 You have noticed that smoke rising from a smoldering piece of wood, first rises straight then start swirling. Why?
Answer: As speed of smoke rises, by equation of continuity cross-sectional area of stream decrease due to which smoke raises first straight. When smoke gains a high speed, it changes from streamline flow to turbulent flow and start moving in circles.
Q.22 Is it best for an aero-plane to take off against the or with the wind ? Explain your answer.
Answer: It is better for aero- plane to take off into wind because more lift is produced in this case, which is better situation.
Q.23 Two rowboats moving parallel to one another in the same direction are pulled to one another. Two automobiles moving parallel are also pulled together. Explain such phenomenon on the basis of Bernoulli’s Equation.
Answer: In both cases, the air speed between vehicles is very high as compared to outer sides. From Bernoulli’s equation when speed is high, the pressure will be low due to which vehicles are pulled towards each other because of pressure difference.
Q.24 A person floats in a boat in a swimming pool. Inside the boat with person are some bricks. If person drops the bricks overboard to bottom of pool, does the water level in lake increase?
Answer: No, the water level decreases when brick is thrown overboard.
Q.25 A tarpaulin is a piece of canvas that is used to cover the cargo. When the truck is stationary, the tarpaulin is flat but it bulges outward when the truck is speeding sown the highway. Why?
Answer: When truck is stationary, there is no pressure difference on both sides of tarpaulin, therefore tarpaulin lies flat. When truck moves, pressure difference is produced as air moves very fast over the truck. This pressure difference creates a force due to which tarpaulin bulges.
Q.26 What is meant by blood pressure? Explain systolic and diastolic pressure?
Answer: The force exerted by blood per unit area on walls of vessel is known as blood pressure. When heart pumps blood into body, is squeezes and pressure in arteries rises, this pressure is called systolic pressure its normal value is 120 torr. When heart relaxes, then pressure decreases which is called diastolic pressure. For a healthy person its normal value is 80 torr.
Q.27 An apple is held completely submerged just below the surface of a container of water. The apple is then moved to a deeper point in the water. Compared to the force needed to hold the apple just below the surface, is the force needed to hold it at a deeper point the same?
Answer: Yes, since the buoyant force is independent of the depth in an incompressible fluid.
Q.28 A student observes two helium balloons floating next to each other at the ends of strings secured to a table. The facing surfaces of the balloons are speeded by 10-20 mm. He blows through the small space between the balloons. What happens to the balloons?
Answer: Between the balloons the high speed air results in low pressure in this region whereas the higher pressure on the outer surfaces of the balloons pushes them toward each other.
Q.29 The pressure at the bottom of a filled glass of water having density 1000kg/m3 is p. The water is poured out and the glass is filled with another material with density 806kg/m3. Is the pressure as the bottom of the glass equal to p?
Answer: No, the material with smaller density will exert the smaller pressure.
Q.30 Lead has a greater density than iron, and both are denser than water. Is the buoyant force on a lead object equal to the buoyant force on an iron object of the same value?
Answer: Yes, the buoyant force on a lead object is equal to the buoyant force on an iron object of the same value.
Q.31 One tapes two different soda straws together end-to-end to make a loner straw with no leaks. The two straws have radii of 3.5 mm and 6mm. He drinks a soda through combination straw. In which straw is the speed of the liquid the highest?
Answer: The liquid moves at the highest speed in the straw with the smaller cross sectional area.
Q.32 (a)- Explain why a sealed bottle partially filled with a liquid can float in a basin of the same liquid.
(b)- A flat plate is immersed in a liquid at rest. For what orientation of the plate is the pressure on its flat surface uniform?
Answer: (a)- The density of glass is greater than the density of liquid and the air inside has smaller density, so total weight of the bottle may be less than the weight of an equal volume of the liquid. Thus a sealed bottle partially filled with a liquid can float in a basin of the same liquid.
(b)- The pate must be horizontal. Since with increase in the depth, the pressure of a fluid increases, other orientations of the plate with given a non-uniform pressure on the flat faces.
Q.33 A person in a boat floating in a same pond throws an anchor overboard. Does the level of the pond rise, fall, or remain the same?
Answer: A floating object displaces a volume of water whose weight is equal to the weight of the object. As the density of the anchor is greater than the density of water, a volume of the water that weight the same as the anchor will be greater than the volume of anchor and the level of pond will fall.
Q.34 Smoke rises in a chimney faster when a breeze is blowing. Use the Bernoulli’s equation to explain this phenomenon.
Answer: When the wind blows over the chimney lower pressure is created at the top of the chimney. Thus smoke flows from the relatively higher pressure in front of the fire place to the low pressure outside.
Q.35 To drinking glasses having equal weights but different shapes and different cross sectional areas are filled to be same level with water. According to the expression the pressure is same at the bottom of both glasses, then why does one weigh more than the other?
Answer: Two glasses weigh equal because the weight depends upon the total volume of glass whereas pressure depends only on the depth.
Q.36 When an object is immersed in a liquid at rest, why is the net force on the object in the horizontal direction equal to zero?
Answer: Since the horizontal force exerted by the outside fluid on an area element of the side wall of object is equal and opposite to the horizontal force the fluid exerts on another element, so net force on the object in the horizontal direction equal to zero.
Q.37 Suppose wind is blowing across the roof of a house at 15m/s. The density of air is 1.29kg/m3. Determine reduction in pressure that accompanies this wind.
Answer: Using,
$$p_1-p_2=\frac{1}{2}\rho v^2$$
$$p_1-p_2=\frac{1}{2}\times1.29kgm^{-3}\times\left(15\sfrac{m}{s}\right)^2=145.12\ Pa$$
Q.38 It is often seen that leaves lying on road start following the fast moving vehicle when it passes through the road. Explain.
Answer: When vehicle moves, speed of air closer to car is high and pressure is low. The high pressure pushes the leaves towards low pressure and hence it looks that leaves follow vehicle.
Q.39 A barometer kept in a stationary elevator reads h cm of mercury column. Find pressure in barometer when it is moving down with acceleration ‘a’.
Answer: When elevator is at rest, barometer measures the pressure . When elevator moves down with acceleration ‘a’ then resultant acceleration is and pressure observed by barometer is,
$$p=\rho\left(\ g-a\right)h\$$
Q.40 Does the drag force be position dependent, time dependent or velocity dependent? Explain with an example.
Answer: Drag force is velocity dependent.
Example:- When we stick our hands out of window of fast moving car, then force on hands increases as velocity of car increases.
Q.41 (a)- Is it possible that speed of emergence of liquid through a pipe can ever greater than the speed of entrance?
(b)- Why the design of car an submarine is of oblong shaped?
Answer: (a)- According to equation of continuity, it is possible. As emergent end has smaller area then speed of fluid at that end is greater and product of Av remains constant.
(b)- Because oblong shape helps for smoothing of car and reduction of drag force.
