Q.1 We can cut an apple easily with a sharp knife as compared to with a blunt knife. Explain why?
Answer: The area of a sharp edge is much less than the area of a blunt edge. For the same total force, the effective force per unit area (or pressure) is more for the sharp edge than the blunt one. Hence a sharp knife cuts easily than a blunt knife.
Q.2 Why are the bags and suitcases provided with broad handles?
Answer: Broad handles have large area. So the force per unit area or the pressure exerted on the hand will be small while carrying the bag or the suitcase.
Q.3 Railway tracks are laid on large sized wooden sleepers. Why?
Answer: This spreads force due to the weight of the train on a larger area and hence reduces the pressure considerably. This, in turn, prevents yielding of the ground under the weight of the train.
Q.4 It is painful to walk barefooted on the ground covered with edged pebbles. Why?
Answer: While walking, when entire weight of our body gets supported on the sharp edge of any pebble, it will exert a large pressure on our feet due to reaction. This causes considerable pain on our feet.
Q.5 It is difficult for a man to walk on sand. Why?
Answer: This is because sand yields under the weight of the man. This difficulty can be overcome by placing wooden plank on the sand. Then the weight of the man will act on larger area and hence pressure exerted on sand decreases. The sand does not yield.
Q.6 Water does not come out of a dropper unless its rubber bulb is pressed hard. Why?
Answer: Water is held inside the dropper against the atmospheric pressure. When the rubber bulb is pressed, pressure on water becomes greater than the atmospheric pressure and so water comes out.
Q.7 Why two holes are made to empty an oil tin?
Answer: When oil comes out through a tin with one hole, the pressure inside the tin becomes less than the atmospheric pressure, soon the oil stops flowing out. When two holes are made in the tin, air keeps on entering the tin through the other hole and maintains pressure inside.
Q.8 Why is a slight blow on a cork of bottle filled with a liquid sufficient to break the bottle?
Answer: The blow on the cork exerts a pressure (=f/a)on the liquid. According to Pascal's law, this pressure is transmitted to the entire bottle through the liquid. Since the surface area (A) of the bottle is large, the large force (=fA/a) so developed is sufficient to break the bottle.
Q.9 What is the force on a man due to atmospheric pressure? Why one does not feel it?
Answer: Atmospheric pressure on the surface of the earth is about 1 kg f per square centimetre. The surface area of the body of average man is about 2m2 and hence, a total force equal to about 2 x 105N, acts on the body of a man due to atmospheric pressure. But one does not feel this enormous force because his blood exerts a pressure slightly greater than the atmospheric pressure.
Q.10 How does the boiling point of a liquid vary with pressure?
Answer: The boiling point of a liquid increases with pressure. For example, if the pressure is more than the atmospheric pressure, water boils at a temperature higher than 100°C.
Q.11 Why the boiling point of a liquid varies with pressure?
Answer: At the boiling point, vapour pressure of the liquid is equal to the atmospheric pressure. Hence when the atmospheric pressure on the surface of the liquid increases, the liquid boils at higher temperature to generate greater vapour pressure.
Q.12 Why the food is cooked faster in the pressure cooker? Why it becomes difficult to cook food at the mountains?
Answer: The pressure inside the pressure cooker is very high. This raises the boiling point of water and the temperature inside the cooker is higher than 100∘C which results in faster cooking of food. At the mountains, the pressure is less, so the boiling point of water is less than 100∘C. This makes the cooking of food difficult.
Q.13 Why the passengers are advised to remove the ink from their pens while going up in an aeroplane?
Answer: We know that atmospheric pressure decreases with height. Since ink inside the pen is filled at the atmospheric pressure existing on the surface of earth, it tends to come out to equalise the pressure. This can spoil the clothes of the passengers, so they are advised to remove the ink from the pen.
Q.14 Why is it difficult to stop bleeding from a cut in human body at high altitudes?
Answer: The atmospheric pressure is low at high altitudes. Due to greater pressure difference in blood pressure and the atmospheric pressure, it is difficult to stop bleeding from a cut in the body.
Q.15 Why are straws used to suck soft drinks?
Answer: When we suck through the straw, the pressure inside the straw becomes less than the atmospheric pressure. Due to the pressure difference, the soft drink rises in the straw and we are able to take the soft drink easily.
Q.16 If a drop of water vapour is introduced in a mercury barometer, how will the barometric height change?
Answer: Due to pressure exerted by water vapour, the barometric height decreases.
Q.17 Why water is not used in barometers?
Answer: Water is not used in barometer due to following reasons:
(i) It sticks to the walls of the barometer tube.
(ii) It has low density. A barometer with water will require a tube length of about 11 m which is unmanageable.
Q.18 Why is mercury used in barometers?
Answer: Mercury is used in barometers due to the following reasons:
(i) It does not stick to the walls of the barometric tube.
(ii) Its density is high, so the length of the tube used is conveniently small.
(iii) Its vapour pressure is quite small.
Q.19 Why is the reading of a mercury barometer always less than actual pressure?
Answer: Due to property of surface tension, mercury in the tube gets depressed. Consequently, the observed height of mercury in the barometer tube is less than its actual height. Thus, the reading of mercury barometer is always less than the actual pressure.
Q.20 How can we check whether the barometric tube contains air or not?
Answer: If on raising or lowering the barometric tube in a trough of mercury, the height of mercury column changes, then air is present in the barometric tube.
Q.21 The dams of water reservoir are made thick near the bottom. Why?
Answer: Pressure exerted by a liquid column of height h=hρg. As the value of h is greatest near the bottom, so pressure exerted by water is greatest near the bottom. So dams are made thick near the bottom.
Q.22 Why an air bubble in water rises from bottom to top and grows in size?
Answer: The fluids move from higher pressure to lower pressure and a fluid pressure increases with depth. Hence pressure at the top is less than that at the bottom and so the air bubble will rise from bottom to top. When bubble moves from bottom to top, pressure decreases and in accordance with Boyle's law volume V will increase i.e., bubble will grow in size.
Q.23 A beaker containing a liquid is kept inside a big closed jar. If the air inside the jar is continuously pumped out, how will the pressure change inside the liquid near the bottom?
Answer: The total pressure at a point near the bottom is equal to the sum of the pressure due to liquid column and due to air inside the jar. When air of the jar is pumped out, the pressure of air inside the jar decreases. So pressure near the bottom of the liquid also decreases.
Q.24 Why does a siphon fail to work in vacuum?
Answer: As siphon works on account of atmospheric pressure, hence it fails to work in vacuum.
Q.25 A barometer kept in an elevator accelerating upwards reads 76 cm of Hg. What will be the possible air pressure inside the elevator?
Answer: The net acceleration of the elevator accelerating upwards=g+a ∴ Pressure inside the elevator Clearly, this pressure will be greater than.
Q.26 A barometer accelerating downwards reads 76 cm of Hg. What will be the possible air pressure inside the jar?
Answer: The net acceleration of the elevator accelerating downwards =g−a ∴ Pressure inside the elevator = learly, this pressure will be less than 76 cm of Hg.
Q.27 In a mercury barometer, at sea level, the normal pressure of the air (one atmosphere) acting on the mercury in the dish supports a 76 cm column of mercury in a closed tube. If you go up in the air, until the density has fallen to half its sea level value, what height of mercury column would you expect?
Answer: Pressure exerted by a gas is directly proportional to its density. When we go high up in air at a point where the density of air falls to half its sea level value, the pressure also reduces to half its sea-level value. Hence the height of mercury column is also halved i.e., it becomes 38 cm.
Q.28 A liquid cannot withstand a shear stress. How does this imply that the surface of a liquid at rest must be level, i.e. normal to the gravitational force?
Answer: If the free surface of the liquid is not perpendicular to the gravitational force, then there will be a component of force along the surface. The liquid will not be at rest in that case.
Q.29 A wooden block is on the bottom of a tank when water is poured in. The contact between the block and the tank is so good that no water gets between, them. Is there a buoyant force on the block?
Answer: Since there is no water under the block to exert an upward force on it, therefore, there is no buoyant force.
Q.30 A piece of iron sinks in water, but a ship made of iron floats in water. Why?
Answer: The weight of water displaced by iron piece is less than its own weight, so it sinks. On the other hand, the ship displaces water more than its own weight, so it floats.
Q.31 A man is sitting in a boat, which is floating in a pond. If the man drinks some water from the pond, will the level of water in the pond fall?
Answer: No. When the man drinks water, say m kg, he displaces m kg of water and hence the level tends to increase. But m kg of water has already gone inside his stomach. So the level remains the same.
Q.32 An ice cube floats in a glass of water filled to the brim. What happens when the ice melts?
Answer: The water level remains unchanged. The ice cube displaces a weight of water equal to its own weight. When the ice cube melts, the volume of water produced equals the volume of water it displaced when frozen.
Q.33 An ice piece with an air bubble in it is floating in a vessel containing water. What will happen to the level of water when the ice melts completely?
Answer: The level of water does not change. The mass of air bubble is negligible. So the volume of water produced during melting is equal to the volume of water displaced by ice piece with air bubble.
Q.34 Ice floats in water with about nine-tenths of its volume submerged. What is the fractional volume submerged for an iceberg floating on a fresh water lake of a (hypothetical) planet whose gravity is ten times that of earth?
Answer: The fractional volume submerged does not depend upon the value of acceleration due to gravity. So, on the new planet, the ice will float in water with nearly nine-tenths of its volume submerged.
Q.35 Does the Archimedes' principle hold in a vessel in free fall?
Answer: No. The vessel in free fall is in a state of weightlessness i.e., the value of g is zero. The buoyant force does not exist. Hence Archimedes principle does not hold good.
Q.36 What is the fractional volume submerged of an ice cube in a pail of water placed in an enclosure which is falling freely under gravity?
Answer: Since the pail of water is falling freely, therefore, it will be in state of weightlessness. Both the weight of the ice cube and the up thrust would be zero. So, the ice cube can float with any value of fractional volume submerged in water.
Q.37 A piece of cork is floating in water contained in a beaker. What is the apparent weight of the cork piece?
Answer: The weight of the cork piece acting vertically downwards is balanced by the up thrust due to water. So the apparent weight of the floating cork piece is zero.
Q.38 One small and one big piece of cork are pushed below the surface of water. Which has greater tendency to rise swiftly?
Answer: The up thrust on a piece of cork is equal to the weight of water displaced by it. So up thrust is greater on the bigger piece of cork and it has greater tendency to rise swiftly.
Q.39 Why is it easier to swim in sea water than in river water?
Answer: The sea water has many salts dissolved in it. So the density of sea water is greater than that of river water. Consequently, the sea water exerts greater up thrust on the swimmer than the river water. Hence it is easier to swim in sea water than in river water.
Q.40 Two bodies of equal weight and volume and having the same shape, except that one has an opening at the bottom and the other is sealed, are immersed to the same depth in water. Is less work required to immerse one than the other? If so, which one and why?
Answer: More work is required in case of the body having hole at its bottom. As the liquid enters the hole, more work is required in compressing the air, less work required in case of the sealed body.
Q.41 A wooden cylinder floats in a vessel with its axis vertical. How will the level of water in the vessel change if the cylinder floats with its axis horizontal?
Answer: The level of water will not change because in both cases, the cylinder displaces equal volume of water.
Q.42 A block of ice is floating in a liquid of specific gravity 1.2 contained in a beaker. When the ice melts completely will the level in the beaker change?
Answer: The level of liquid in the beaker will rise. This is because the density of water formed by melting of ice is less than the density of liquid in the beaker. Consequently, the volume of water formed by melting of ice will be more than the volume of the portion of the liquid displaced by ice while floating.
Q.43 A boy is carrying a fish in one hand and a bucket full of water in the other hand. He then places the fish in the bucket and thinks that in accordance with Archimedes' principle he is now carrying less weight as weight of fish will reduce due to up thrust. Is he right?
Answer: No. When he places the fish in the water in the bucket, the weight of fish is reduced due to upthrust but the weight of water is increased by the same amount so that the total weight carried by the boy remains the same.
Q.44 A solid body floats on mercury with a part of its volume below the surface. Will the fractional volume of the body immersed in mercury increase or decrease, if a layer of water poured on the top of mercury covers the body completely?
Answer: In the first case, the weight of the body is balanced by the weight of mercury displaced. But when water is poured to cover the body completely, the weight of (mercury + water) displaced by the body is equal to the weight of the body. The weight of mercury displaced is now less than that in the first case. So the fractional volume of the body inside the mercury will decrease.
Q.45 A bucket of water is suspended from a spring balance. Does the reading of balance change (a) When a piece of stone suspended from a string is immersed in water without touching the bucket? (b) When a piece of iron or cork is put in water in the bucket?
Answer: (a) Yes, the reading of balance will increase but the increase in weight will be equal to the loss in n weight of stone and not the weight of stone.
(b) Yes, the reading of balance will increase but increase in weight will be equal to weight of iron or cork piece.
Q.46 Why a sinking ship often turns over as it becomes immersed in water ?
Answer: When the ship is floating, the meta-centre of the ship is above the centre of gravity. While sinking, the ship takes in water. As a result, the centre of gravity is raised above the meta-centre. The ship turns over due to the couple formed by the weight and the buoyant force.
Q.47 A soft plastic bag weighs the same when empty as when filled with air at atmospheric pressure. Why?
Answer: The weight of air displaced by the bag is same as the weight of air inside it. The increase in weight due to the filled air gets cancelled by the up thrust of air. So weight remains same when air is filled in the bag.
Q.48 Stirred liquid comes to rest after son time. Why?
Answer: Different layers of a stirred liquid destroy the relative motion among themselves due to internal viscous force.
Q.49 What is the reason that a constant driving force is always required for the maintenance of the flow of oil through the pipe lines in the oil refineries?
Answer: Due to viscous force between the layers of oil, the motion of liquids gets retarded after a certain distance. Hence, a constant driving force is required for maintaining the flow of oil through the pipe lines.
Q.50 What is the effect of temperature on the viscosity of liquids and gases?
Answer: The viscosity of liquids decreases with rise in temperature while that of gases increases with an increase in temperature.