Whatever you are doing, put your whole mind on it. If you are shooting, your mind should be only on the target. Then you will never miss. If you are learning your lessons, think only of the lesson. In India boys and girls are taught to do this."Swami Vivekananda 1.What is meant by the statement relative density of gold is 19.3?It means that gold is 19.3 times denser than an equalvolume ofwater. Those objects whose relative density is less than one will float in water and those greater than one will sink.

2.Why is it difficult to hold a school bag having a strap made of a thin and strong string?It is difficult to hold a school bag having a strap made of a thin and strong stringbecause it applythe 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. We feel uncomfortable.3.What do you mean by buoyancy?The tendency of a liquid to exert an upward force on an object placed in it is called buoyancy.4.Why does an object float or sink when placed on the surface of water?An object floats or sinks when placed on the surface of waterbecause of the buoyant force acting on the object.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.5.Why the cutting edge of a knife should be as sharp as possible?The cutting edge of a knife should as sharp to create greater pressure even for a very small applied force for quick action.6.You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?More than 42 kg, the weighing machine reads slightly less value due to upthrust of air acting on the body.7.You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?The cotton bag is heavier than iron bar. The cotton bag experiences larger upthrust than the iron bar. So the cotton bag indicates a smaller mass for cotton bag than actual mass.

8.In what direction does the buoyant force on an object immersed in a liquid act?Ans: The buoyant force acts on an object in the vertically upward direction through the center of gravity of the displaced liquid.9.Why does a block of plastic released under water come up to the surface of water?A block of plastic released under water come up to the surface of water because the up thrust or the buoyant force exerted by the water is greater than the downward gravitational force.10.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?Given the mass of the substance = 50 g Volume of the substance = 20 cm3Density of substance = Mass /Volume = (50 /20) g/cm3= 2.5 g/cm3Since the density of the substance2.5 g/cm3is greater than the density of water (1 g/cm3), so the substance will sink in the water.11.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 ?Mass of packet = 500 g Volume of packet = 350 cm3Density of packet = Mass Volume = (500 / 350) g/cm3= 1.43 g/cm3The sealed packet will sink in the water as its density1.43 g/cm3is more than that of water.The volume of water that will be displaced by the packet = 350 cm3And mass of this displaced water = Volume x Density = 350 x 1 = 350 gm.

12.State Archimedes principle. Give any two applications of Archimedes principle?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 milk13.How do a submarine sink and float on water?A submarine can both sink and float on water because of the ballast tank .To make a submarine sink into the water, these tanks are filled with water. This increases the weight of the submarine and it sinks.To make a submarine float on the water surface, these tanks are emptied by pumping out water from them. This decreases the weight of the submarine and it floats14.Why is iron heavier than cotton?5 kg of cotton occupies more space than 5 kg of iron. The particles of iron are closely packed while that of cotton are loosely packed. The amount of iron packed in a unit volume is more. This explains as to why iron is heavier than the same volume of cotton.Statement of Archimedes principle:When a body is immersed partially or completely in a fluid (liquid or gas), it experiences an up thrust or buoyant force which is equal to the weight of the fluid displaced by the body.Applications based on Archimedes principle are:a. Designing of ships and submarinesb. Lactometers, which are used to determine the purity of a sample of milkc. Hydrometers used for determining density of liquids

Proof of Archimedes Principe :Consider a cylindrical body of cross-sectional area a submerged in a liquid of density r . Let the upper face of the body is at a depth h1 below the surface of the liquid and the lower face is at a depth h2 below the surface of the liquid. The pressure exerted by the liquid on the upper surface of the body is given by p1 = h1dg .Downward thrust on the upper surface of the body is ,F1 = p1 a = h1 d g aPressure exerted by the liquid at the lower surface of the body,P2 = h2 d gUpward thrust on the lower surface of the body is,F2 = P2 a = h2 d g aThe horizontal thrusts acting on the vertical sides of the body being equal and opposite from all the sides cancels out.Resultant upthrust or buoyant force acting on the bodyF = F2 - F1 = h2 d g a - h1 d g a = (h2 - h1) d g a Since volume of the body, V = (h2 - h1)aF = V dgThus, when a body is submerged in a liquid, it experiences an upward thrust equal to the weight of the liquid displaced by the body.Let we take two similar bricks lying on the ground, one in the lying position and another in the standing position. The two bricks exert the same force on the ground because they have the same weight.But the two bricks exert different pressures on the ground because their areas in contact with the ground are different.The brick in thelying position has a large areain contact with the ground. So, the force of the weight of the brick falls on a large area of the ground and the force per unit area or pressure on the ground is less.The brick in thestanding positionhas a small area in contact with the ground. So, the force of the weight of the brick falls on a smaller area of the ground and the pressure on the ground is more.A school bag has wide straps made of thick cloth= The weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure, it is more comfortable to carry the heavy school bag.= On the other hand, if the school bag has a strap made of thin string, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.A sharp knife cuts better than a blunt knife.A sharp knife hasa very thin edge to its blade.Due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily.On the other hand, a blunt knifedoes not cut an object easily because due to its thicker edge, the force of our hand falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty.The tip of a sewing needle is sharpso that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.The pressure on ground is more when a man is walking than when he is standing.When a man is walking, then at one time only his one foot is on the ground.Due to this, the force of weight of man falls on a smaller area of the ground and produces more pressure on the ground.On the other hand, when the man is standing, then both his feet are on the ground.Due to this the force of weight of the man falls on a larger area of the ground and produces lesser pressure on the ground.

The depression is much more when a man stands on the cushion than when he lies down on it.When a man stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion.On the other hand, when the same man is lying on the cushion, then his whole body (having large area) is in contact with the cushion. In this case the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.The tractors have broad tyresso that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.A wide steel belt is provided over the wheels of army tanks so that they exert less pressure on the ground and do not sink into it.Wooden sleepers (or concrete sleepers) are kept below the railway lineso that there is less pressure of the train on the ground and railway line may not sink into the ground.The snow shoes have large, flat solesso that there is less pressure on the soft snow and this stops the wearer from sinking into it.It is easier to walk on soft sand if we have flat shoes rather than shoes with small heels(or pencil heels). This is because a flat shoe has a greater area in contact with the soft sand due to which there is less pressure on the soft ground. Due to this the flat shoes do not sink much in soft sand and it is easy to walk on it.On the other hand, a small heel (or sharp heel) has a small area is contact with the soft sandand so exerts a greater pressure on the soft sand. Due to this greater pressure, the small heels tend to sink deep into soft sand making it difficult for the wearer to walk on soft sand.

The foundations of buildings and dams are laid on a larger area of groundso that the weight of the building or dam (to be constructed) produces less pressure on ground and the building or dam may not sink into the ground. Atmospheric pressure The pressure at any place due to the atmosphere is called atmospheric pressure. Its value varies from place to place and also with the time.Atmospheric pressure at the earths surface near the sea level is around 1.01x105 Pa. This value is known as 1atmosphere of pressure (1atmosphere = 760mm of Hg).

1. Units of pressure are :-

(a) N/m2 (b) m2/N

(c) Nm2 (d) Nm

2. Units of Relative Density are :-

(a) Kg/m3 (b) Unit less

(c) Depends on the density of the substance

(d) Depend on the density of water

3. Pressure at a point in the liquid is

(a) Same in all directions (b) Greater in the upward direction

(c) Grater in the downward direction (d) None of the Above

4. If the area of an object is less then the pressure acting on that object will be

(a) Less (b) More

(c) Independent of area (d) None of the above

5. If cross sectional area of an object is more than the pressure applied by the external force is :-

(a) Less (b) More

(c) Remains same (d) None of the above.

6. Mass per unit volume of a substance is called

(a)Density (b)Relative density

(c)specific gravity (d)None

7. A buoyant force of 200 g wt. acts on a body dipped completely in water. If the apparent weight of body is 100 g wt., then its actual weight is

(a)200 g wt. (b)100 g wt.

(c)300 g wt. (d)None of these

8. If the weight of the floating body is equal to the buoyant force then body

(a)Sinks (b)Rises

(c)Floats (d) First floats and then sinks

9. When a body is placed in a liquid, the buoyant force experienced by it is equal to the

(a)weight of liquid displaced (b)Density of the body

(c)volume of body (d)Density of liquid

10. A body immersed in a liquid will rise to the surface if the buoyant force acting on it is

(a)greater than its actual weight (b)less than its actual weight

(c)equal to its actual weight (d)equal to Zero

11. When a body is submerged in a liquid, its weight

(a)Remains same (b)Increases

(c)Decreases (d)Reduces to zero

12. A body weighing 200 g wt. is dipped in water. Its weight in water as indicated by the spring balance is 150 g wt. The buoyant force acting on the body is

(a)200 g wt (b)1000 g wt

(c)50 g wt (d)250 g wt

13. A body is floating in upright position in water. Then force of gravity and buoyant force acting on the body are

(a) In the same direction long the same line

(b) In opposite direction but along the same line

(c) At right angle to each other.

(d) In opposite to direction and from couple which turns and only anti-clockwise.

14. A body floats in vertical position, when its centre of buoyancy is

(a) Below C.G. of the body (b) Above the C.G. of the body

(c) At some place where the C.G. of the body lies (d) None of these.

15. In case of a body placed in liquid if the buoyant force is equal to its acutal weight, then its apparent weight

(a) Is equal to the actual weight (b) Is equal to buoyant force

(c) Is greater than its actual weight (d) Is equal to zero

16. A block of ice is floating on water contained in a beaker. When all the ice melts, the level of water

(a)rises (b)falls

(c)remains unchanged(d)None

17. A boat floating in a tank is carrying passengers. If the passengers drink water, the water level of the tank

(a) rises (b) falls

(c) remains unchanged (d) depends upon the atmospheric pressure

1.Give reason why, a block of plastic when released under water comes up to the surface of water.Ans: It is because of buoyancy force2.Define density. What is the SI unit of density?Ans: Mass per unit volume of a substance is called density.The SI unit of density is kg/m3 or gm/cm33.Name and define SI unit of pressure.Ans: Pascal or n/m21 Pascal is the pressure act when 1 N force acts perpendicular on unit area.4.What do you understand by the term buoyancy?Ans: Tendency of fluid to exert an upward force when a body is immersed in to it is called buoyancy5.Why does a mug full of water feel lighter inside water?Ans: a mug full of water feels lighter inside water because of buoyancy force6.A perpendicular force of 50 N acting on a surface generates a pressure of 250 Pa. Calculate the area of cross-section of the surface on which pressure is acting.Ans: p = F/AA = F/p50 N/250 Pa = 0.2 m27.What is a lactometer used for?Ans: lactometer used for finding purity of milk8.State the unit of density and relative density in SI system.Ans: The unit of density is kg/m3There is no unit of relative as it is ratio of same physical quantity. 9.State Archimedes principle.Ans:When a body is immersed partially or completely in a fluid (liquid or gas), it experiences an up thrust or buoyant force which is equal to the weight of the fluid displaced by the body.10.What is meant by buoyant force?Ans: if a body is immersed partially or completely in a fluid, it experiences an up thrust called buoyant force.11.A cork floats while the nail sinks in water. Give reason.Ans: Weight of cork is less than buoyant force acting on it so it floats Where as : Weight of the nail is more than buoyant force acting on it so it sink12.What is the principle of floatation?Ans: A body floats is a liquid if weight of the liquid displaced by the immersed portion of the body is equal to the weight of the body.13.A body of mass 400 g has a volume of 300 cm3. Will body sink in water or not? [Density of water = 1000 kgm3]Ans: density of body =m/v= 400/300=1.3gm/cm3 which is greater than density of water 1gm/cm3Hence, Body will sink in water.14. When we stand on loose sand, our feet go deep into the sand. But when we lie down on the sand our body does not go that deep in the sand. Why?Ans: when we lie down on the sand our body exert same force on large are that reduce pressure and our body does not go that deep in the sand.15. Why is it easy to walk on sand with flat shoes, then with high heel shoes?Ans: flat shoes exert force on large surface area and less force act on sand this make easy to walk on sand.

1. An object is made to fall from different heights 20 cm, 40 cm and 60 cm on wet sand.(a) What do you observe on the sand? Ans. (a) These ojects create depressions in the wet sand. The largest depression is created by theobject falling from largest height i.e. 60 cm and the smallest depression by the objectfalling from lowest height i.e., 20 cm. 2. A solid object of mass 50 g and volume 100 cm3 is put in water. Will the object float or sink? Give reasons for your answer.3.Name the instrument used to determine the density of liquid in which balance and graduated cylinder are not required. Why this instrument is made heavy near the bottom?Ans: The instrument used to determine the density of liquids is hydrometer. It consists of a uniform test tube. The base of the tube is made heavier by taking few lead shots in it so that the tube remains vertical while floating in water or a given liquid.4.A sphere of mass 5 kg and volume 2.2 104m3 is completely immersed in water. Find the buoyant force exerted by water on the sphere. Density of water = 1000 kgm3. (Given: g =9.8m/s2)Ans. Here, volume of sphere = Volume of water displaced = 2.2 104 m3( sphere is completely immersed in water)Mass of the water displaced = Volume Density = 2.2 104m3 1000 kg m3 = 2.2 101 kgThus, buoyant force exerted by water = weight of water displaced = mg= 2.2 101 kg 9.8 m/s2 = 21.56 101 N = 2.156 N5. What happens when :(a) Buoyant Force exerted by the fluid is less than the weight of the body?(b) Buoyant Force exerted by the fluid is equal to the weight of the body?6.What is Relative Density? The relative density of a substance is greater than 1, w hat does it signify?7.If relative density of aluminum is 2.7 and density of water is 1000 kg/m3. What is the density of aluminum in SI unit?Ans. Relative density of aluminium = Density of aluminium/Density of water Density of aluminium = Relative density of aluminium Density of water= 2.7 1000 kg /m3 = 2700 kg/m38.The volume of a solid block is 300 cm3, find the mass of water displaced when it is immersed in water? (Density of water is 1 g/cm3)9. (a) A floating boat displaces water weighing 6000 N.(i) What is the buoyant force on the boat? (ii) What is the weight of boat?(b) What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid?10. (a) Define relative density.(b) Relative density of gold is 19.3. The density of water is 10.3 kg/m3. What is the density of gold in S.I. units?

Questions : 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. [2011 (T-II)]

Ans. As density of milk is greater than density of water, so when some water is mixed with milk,then amount of buoyant force is decreased at some extent. Consequently test tube will sinkmore.1. The force acting on an object perpendicular to the surface is calleda. Pressure c. thrustb. Weight d. weight2. SI Unit of pressure isa. Newton c. Pascalb. Dyne d. m/s3. The upward force exerted by the liquid displaced by the body when it isplaced inside the liquid is calleda. Buoyant force c. Gravitational forceb. Centripetal force d. Force of gravitation

Remember:(i)The liquid at rest exerts equal pressure in all direction s at a point inside the liquid.(ii)Total pressure at a depth h below the liquid surface = P + hrg where P = atmospheric pressure.(i ii)Liquid pressure is independent of shape of the liquid surface, butdependsupon the height of liquid column.Archimedes principle is applicable to objects in fluids, i.e. liquids as well as gases.Gases (like air) exert an upward force (or buoyant force) on the objects placed in them but in most cases it is so small that we usually ignore it. It is the buoyant force due to displaced air which makes a balloon rise in air. The relative density of a substance expresses the heaviness (or density) of the substance in comparison to water e.g. the relative density of iron is 7.8, which means iron is 7.8 times as heavy as an equal volume of water. The relative density of water is 1. If the relative density of a substance is more than 1, then it will be heavier than water and hence it will sink in water. On the other hand, if the relative density of a substance is less than 1, then it will be lighter than water and hence float in water. e.g. Ice has a density of about 900 kg/m3 and water has a density 1000kg/m3. Thus an ice cube has a relative density of 0.9 so it floats in water. The relative density of iron is7.8, so an iron nail sinks in water.Relative density also determines the proportion of a floating body that will be submerged in a fluid. If the body is two thirds as dense as the fluid, then two thirds of its volume will be submerged, displacing in the process a volume of fluid whose weight is equal to the entire weight of the body. In the case of a submerged body, the apparent weight of the body is equal to its weight in air less the weight of an equal volume of fluid.

Q1. Calculate the mass of a body whose volume is 2 m3and density 0.52 g/cm3. (ans. 1040 kg)

Q2. A dining hall has dimensions 50m 15m 3.5m. Calculate the mass of air in the hall. Given, density of air =1.30kg/m3. (ans. 3412.5 kg)

Q3. A thread of mercury of 10.2 g is in a tube of uniform cross section 0.1cm3. Calculate the length of thread. The density of mercury is 13.6g/cm3. (ans. 7.5cm)

Q4. A cubical block of water is dipped completely in water. Each edge of the block is 1cm in length. Find the buoyant force acting on the block. (ans. 10-2N)

Q5. A body of mass 2.0 kg and density 8000 kg/m3is completely dipped in a liquid of density 800 kg/m3. Find the force of buoyancy on it. (ans. 2N)

Q6. A piece of iron of density 7.8 103kg/m3and volume 100 cm3is totally immersed in water. Calculate (a) the weight of the iron piece in air (b) the upthrust and (c) apparent weight in water. (ans. (a) 7.8N (b) 1N (c) 6.8 N)

Q7. A solid body of mass 150g and volume 250cm3is put in water. Will the body float or sink.

Q8. A solid of density 5000kg/m3weights 0.5 kg in air. It is completely immersed in water of density 1000kg/m3.(a)Calculate the apparent weight of solid in water.(ans. 0.4 kg)(b)What will be its apparent weight if water is replaced by a liquid of density 8000kg/m3? (ans. 0)

Q9. The mass of a block made of certain material is 13.5 kg and its volume is 15 10-3m3. Will the block float or sink in water. Give reason for your answer.

Q10. (a) When does a body sinks in a fluid? (b)Why does a balloon filled with hydrogen gas rise up against gravity?Q11. (a) Which has greater density: 1 kg of iron or 2 kg of iron?(b)If a hollow sphere and a solid sphere are both made of the same amount of iron, which sphere has greater average density?

Q12. . A body weighs 10 N in air and 8 N when fully immersed in water. How much is the buoyant force acting on the body?Q13. Where does a solid weigh more- in air or in a liquid?

Q14. What is the relationship between the buoyant force on an object and the liquid displaced by it?

Q15. The relative density of mercury is 13.6. What does this statement mean?

Q16. The density of turpentine oil is 840 kg/m3. What will be its relative density?

Q17. Explain why big boulders can be moved easily by floods.

Q18. Why is it easier to walk on soft sand with a flat shoe than a pencil-heeled shoe?

Q19. Why do you feel lighter when you swim?

Q20. Why is a bucket of water lighter when in water than when it is taken out of water?

Q21. Why it is easier to swim in sea water tan in river water?

9. Why does a buffalo float on the river but not the man?

10. Why does ice float on the water?

11. Why does a ship made up of iron floats but the iron sinks?

12. How do submarines float or sink as desired?

13. Why does a mug full of water appear lighter inside the water?

14. A dead body floats in water with its head immersed in water. Explain?

15. In what direction does the buoyant force on an object, due to liquid act?

16. Why does a block of plastic piece left under water cone to the surface of water?

17. Write the condition under which the body would float on a liquid?

hrust The force acting on a body perpendicular to its surface is called thrust.The S.I. unit of thrust is Newton (N).e.g. For fixing a poster on a bulletin board one has to press drawing pins with the thumb.When pressing a drawing pin, force is applied on the surface area of its head.The force is directed perpendicular to the surface of the board. This force is called thrust.Pressure The thrust per unit area is called pressure.Pressure = Thrust/ AreaThe S.I. unit of pressure is Newton per square metre (N/m2) which is also called pascal (Pa).Many times a bigger unit of pressure called kilopascal (kPa) is used.The pressure depends on two factors:(a). Force applied (b). Area over which force acts.The same force can produce different pressures depending on the area over which it actse.g. when a force acts over a large area of an object, it produces a small pressure.But if the same force acts over a small area of the object, it produces a large pressure.Let we take two similar bricks lying on the ground, one in the lying position and another in the standing position. The two bricks exert the same force on the ground because they have the same weight.But the two bricks exert different pressures on the ground because their areas in contact with the ground are different.The brick in thelying position has a large areain contact with the ground. So, the force of the weight of the brick falls on a large area of the ground and the force per unit area or pressure on the ground is less.The brick in thestanding positionhas a small area in contact with the ground. So, the force of the weight of the brick falls on a smaller area of the ground and the pressure on the ground is more.A school bag has wide straps made of thick cloth= The weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure, it is more comfortable to carry the heavy school bag.= On the other hand, if the school bag has a strap made of thin string, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.A sharp knife cuts better than a blunt knife.A sharp knife hasa very thin edge to its blade.Due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily.On the other hand, a blunt knifedoes not cut an object easily because due to its thicker edge, the force of our hand falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty.The tip of a sewing needle is sharpso that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.The pressure on ground is more when a man is walking than when he is standing.When a man is walking, then at one time only his one foot is on the ground.Due to this, the force of weight of man falls on a smaller area of the ground and produces more pressure on the ground.On the other hand, when the man is standing, then both his feet are on the ground.Due to this the force of weight of the man falls on a larger area of the ground and produces lesser pressure on the ground.

The depression is much more when a man stands on the cushion than when he lies down on it.When a man stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion.On the other hand, when the same man is lying on the cushion, then his whole body (having large area) is in contact with the cushion. In this case the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.The tractors have broad tyresso that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.A wide steel belt is provided over the wheels of army tanks so that they exert less pressure on the ground and do not sink into it.Wooden sleepers (or concrete sleepers) are kept below the railway lineso that there is less pressure of the train on the ground and railway line may not sink into the ground.The snow shoes have large, flat solesso that there is less pressure on the soft snow and this stops the wearer from sinking into it.It is easier to walk on soft sand if we have flat shoes rather than shoes with small heels(or pencil heels). This is because a flat shoe has a greater area in contact with the soft sand due to which there is less pressure on the soft ground. Due to this the flat shoes do not sink much in soft sand and it is easy to walk on it.On the other hand, a small heel (or sharp heel) has a small area is contact with the soft sandand so exerts a greater pressure on the soft sand. Due to this greater pressure, the small heels tend to sink deep into soft sand making it difficult for the wearer to walk on soft sand.

The foundations of buildings and dams are laid on a larger area of groundso that the weight of the building or dam (to be constructed) produces less pressure on ground and the building or dam may not sink into the ground.

Qn A piece of stone is tied at one end of a rubber string and holding from other end, it is allowed to immerse partially then fully into water. What difference if any, you will observe, and why ?.Qn An object of 40N weight when immersed in water losses 10N weight. Will the object float or sink why?Nails and pins have pointed tips. Give reasons?

2. State Archimedes principle.A thrust of 200N exerts a pressure of 10Pa on a body. Calculate the area on which the pressure is exerted?3 MARKS QUESTIONS1. An object of mass 80g has a volume of 20cm3. Will the object float on the water? Why? What will be the mass of water displaced by the object?2. Give reasons:a)Balloons filled with hydrogen rise to the ceiling.b)Tractors have wide tyres.c)A mug full of water appears light as long as it is under water in the bucket than when it is outside water.Qn How much will a body of weight 35N weigh in water if it displaces 1 litre of water ?(g = 9.8m/s2)Qn . a) A body is floating on the surface of a liquid. With the help of a diagram

show the two forces acting on it that are responsible for its floatation.

State the relationship between these two forces in this case?

b)What is relative density? If an object is immersed wholly in a liquid causing up thrust equal to the weight of the body then what will be the relation between the relative densities of liquid and the object?