Q. What is the SI unit of buyont force? Ans: The S.I. unit of buoyant force is Newton. This upward force that acts on an object when immersed in a fluid is called buoyant force.
Q. What are the factor on which the buoyant force depends on?
Ans: Two main factors on which the buoyant force depends are: (i) Density of the fluid. (ii) Value of acceleration due to gravity at that place.
Hence, Buoyant force experienced by a body when submerged in a liquid depends on the volume of the body and the density of the liquid.
Q. When an iron nail is placed on the surface of water it sinks whereas ship made up of iron floats.
Ans: This is because size or volume of the ship is more. If the density of the liquid is more than the density of the material of the body then the body floats due to the buoyant force exerted by it and vice-versa.
Q. What is density?
Ans: Density of a substance is the mass of the substance contained in a unit volume of that substance. Density of an object is given as Mass/Volume The density of a substance is one of its characteristic properties and used to determine the purity of any substance.
Q. How does floating and Sinking depends on density of water and the density of an object?
Ans: If the density of an object is less than the density of water it floats. This means that the up thrust of water on an object is greater than the weight of the object. If the density of an object is more than the density of water it sinks. This means that the up thrust of water on an object is less than the weight of the object. Therefore objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid. Two forces act on an object immersed in water. One is the gravitational force, which pulls the object downwards, and the other is the buoyant force, which pushes the object upwards. If the upward buoyant force is greater than the downward gravitational force, then the object comes up to the surface of the water The density of the substance is more than the density of water ( 1 g/ cm3) than it will sink in water.
Q. What do you mean by relative density?
Ans: Relative density of a substance is the ratio of density of substance to the density of water. It is also called specific gravity.
Relative density = Density of substance/density of water
Relative density of a substance help us determine the purity of a substance by measuring its relative density Examples: We use lactometer to differentiate between pure and impure(measure the impurity) milk. We can measure the impurity of water using hydrometer.
Q. What is meant by the statement relative density of gold is 19.3?
Ans: It means that gold is 19.3 times denser than an equal volume of water. Those objects whose relative density is less than one will float in water and those greater than one will sink.
Q. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Ans: It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area . we know that the pressure is inversely proportional to the surface area on which the force acts
Q. A solid weigh 75 g in air and 55.6 g in water. Find the relative density?
Ans: The relative density = 75/(75-55.6)= 3.866
Q. A cubical block of side 2cm is lying on a table. If the density of the material of 10,000kg/m3.Find the pressure exerted by the block on the table. [Oswaal 2015]
Solution: For cubical block given that l = 2 cm = 0.02 m ; Volume, V = (l) 3 = (0.02)3 = 8 × 10– 6 m3
Area of each face A = (l)2 = (0.02)2 = 4 × 10– 4 m2 ; Density of block = ρ = 10000 kg/m3
Mass of the block = ρV = 10000 × 8 × 10– 6 kg ; g =10 m/s2
Pressure = F/A = mg/A = (10000 x 8 x 10– 6 x 10 ) / (4 × 10– 4) = 2000N/m2
Q. The volume of 50 gm of a substance is 20 cm3. If the density of water is 1 g/cm3, will the substance float or sink? Ans: Given the mass of the substance = 50 g Volume of the substance = 20 cm3 Density of substance = Mass /Volume = (50 /20) g/cm3 = 2.5 g/cm3 Since the density of the substance 2.5 g/cm3 is greater than the density of water (1 g/cm3), so the substance will sink in the water.
Q. The volume of a 50 gm sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g/cm3 ? What will be the mass of the water displaced by this packet ?
Ans: Mass of packet = 500 g Volume of packet = 350 cm3 Density of packet = Mass ÷ Volume = (500 / 350) g/cm3 = 1.43 g/cm3 The sealed packet will sink in the water as its density 1.43 g/cm3 is more than that of water. The volume of water that will be displaced by the packet = 350 cm3 And mass of this displaced water = Volume x Density = 350 x 1 = 350 gm.
Q. State Archimedes’ principle. Give any two applications of Archimedes principle?
Ans: Archimedes’ principle states that when a body is immersed partially or fully in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it. The two applications of Archimedes principle are given below: (i) Floating of ships and submarines (ii) The lactometers to test purity of milk
Q. A solid weights 100g in air, 75g in water and 70g in oil. Calculate the relative density of the oil and solid. Ans: The relative density of the oil = (loss of weight of solid in oil) / (loss of weight of solid in water) = (100 – 70) /(100 – 75) = 30/25 = 1.2 The relative density of the solid = weight of solid in air) / (loss of weight of solid in water) = 100/(100-75) = 100/25 = 4 Practice: A solid weighs 80g in air, 68g in water and 60g in oil. Calculate the relative density of oil and solid Ans: 6.67 and 1.67
Q. Relative density of a substance depends upon: (i) Mass of the substance. (ii ) Volume of the substance. (iii) Shape of the substance (iv) Material of the substance.
Ans: (iv) Material of the substance.
Q. The mass of a solid iron cube of side 4 cm is to be determined. Out of the four spring balances available, the one best suitable for this purpose would have what range? Density of iron is 7.8 g/cc.
(a) Range = 0 to 1000 g, and least count = 10g (b) Range = 0 to 100 g, and least count = 5g
(c) Range = 0 to 100 g, and least count = 25g (d) Range = 0 to 100 g, and least count = 1g
Ans: Mass of the iron cube should come within the range of the spring balance.
mass = d x v = 4 x4 x4 x 7.8 = 499.2 kg
Correct option will be (a) Range = 0 to 1000 g, and least count = 10g
Q. Balance A has a range of 0 to 250 g and a least count of 2.5 g, while balance B has a range of 0 to 1000 g and a least count of 10g. Which balance would be the preferred option for mass measurement of the cubes? (a) Balance A for both the cubes (b) Balance B for both the cubes (c) Balance A for the Aluminium cube and balance B for Iron cube (d) Balance A for the iron cube and balance B for aluminium cube.
Ans: as the density of iron is greater than Aluminium so correct option will be (c) Balance A for the Aluminium cube and balance B for Iron cube.
Q. State the Archimedes ‘principle. How will you verify it experimentally?
Ans. Archimedes' principle: when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Verification of Archimedes' principle: Apparatus: Spring balance, measuring cylinder, any irregular small object, thread, water Procedure: i. Fill the measuring cylinder with water till some level. Note this reading as v1 ii. Weigh the object in air using a spring balance and note the reading w1. iii. Immerse the object into the water in the measuring cylinder. Note the weight, w2 , from the spring balance and the volume of water,v2 in the cylinder. According to Archimedes' principle Weight loss of an object when immersed in water = Weight of the water displaced w =V x D w1- w2 = (v2 - v1) x D w1- w2 = (v2 - v1 ) x 1 (Density of water = 1 g/cmi) w1- w2 = (v2 - v1 ) Q. A cube of side 5cm is immersed in water and then in saturated salt solution. In which case will it experience a greater buoyant force. If each side of the cube is reduced to 4cm and then immersed in water compare the force experienced by the cube, as compared to the first case. Give reasons for each case.
Ans: We know buyant force = V.P g Here - V = Volume of body ; P = density of liquid and g = acceleration Since density of saturated salt solutions is more than water the cube will experience greater upthrust. In second case as volume of cube is less then first so in 2nd case it will experience less upthrust
Q. In an experiment finding volume of a solid by immersing into water, the initial reading of water level in graduated cylinder was 16.2 ml. On immersing the solid completely in water, the water level in graduated cylinder rose to 19.7ml.Find the volume of the solid. Ans: final volume- initial volume =19.7-16.2 = 3.5cm3
Q. A 200 g wt . Solid block of Aluminium (density AI= 2.70 g / cc ) is placed in a break of water filled to the brim. Some water over flows. The same is done in another beaker with a 200 g solid block of lead (density lead = 11.3 g / cc ). Compare amount of water displayed by block of Aluminium and block of lead. Ans: Volume of water displaced by body = v of body object immersed into water Volume of water displaced by Solid block of Aluminium = Volume of solid block of Aluminium immerged into water = m/d = 200/2.7 = 74.07 cm3 Volume of water displaced by solid block of lead = Volume of body solid block of lead immerged into water = m/d = 200/11.3 = 17.70 cm3 Thus, Aluminium displaces more water than lead.
Q. if you are given a spring of volume 33cm3 . if you are asked to select a best suited spring balance to determine its weight then what should be the range of the spring balance ( the sphere is made of alloy of density 7 g cm-3
Ans: Mass of sphere = d x v = 33 x 7 = 231g
Weight of solid = 231gwt.
Thus , Range of spring balance should be 0-250gwt
Q. An iron ball of density 7800kg/m3 and volume 200cm3 is totally immersed in water ( take g = 10m/s2) Find: 1.weight of iron ball in water 2. calculate upthrust 3. apparent weight in water 4. Apparent density in water
Ans: Volume V = 200cm3 = 200 × 10– 6 m3 and density = 7800kg/m3
Mass of iron ball = 7800 × 200 × 10 – 6 = 78 × 2 × 10– 2 kg So, m = 1.56 kg g = 10m/s2
Since density of iron is greater than water. It will sink
(1) Weight of iron ball in air = mg = 1.56 × 10 = 15.6 N
(2) Upthrust = weight of displaced water = (ρw Vg)= 1000 × 10– 6 × 10= 10– 2 = 0.01 N
(3) Apparant weight in water = (weight of ball ) – weight of displaced water = 15.6 = 0.01= 15.59 N
(4) Apparent density in water = Density of iron/ Density of water = 7800/1000=7.8 Q. A loaded test-tube placed in pure milk sinks to a certain mark (M). Now some water is mixed with the milk. Will the test tube sink more or less ? Explain.
Ans. When water is added, the density of milk decreases. So the weight of milk water mixture displaced by immersed part of the tube sink more also decrease and becomes than the weight of loaded tube. So the tube sinks more and will float only when the weight of this milk-water mixture displaced by the tube is equal to the weight of loaded tube.