ncert-textbook-for-class-8-science-chapter-8.pdf

FORCE AND PRESSURE I n Class VII, you have learnt how by a batsman? (Fig. 8.1). In all these objects move. Do you recall how we situations the ball is either made to can decide whether an object is move faster or...

FORCE AND PRESSURE I n Class VII, you have learnt how by a batsman? (Fig. 8.1). In all these objects move. Do you recall how we situations the ball is either made to can decide whether an object is move faster or slower or its direction moving faster than the other? What does of motion is changed. the distance moved by an object in unit We often say that a force has been time indicate? You also know that a applied on a ball when it is kicked, moving object like a ball rolling on the pushed, thrown or flicked. What is a ground slows down. Sometimes it may force? What can it do to bodies on which change its direction of motion. It is also it is applied? We shall seek answers to possible that the ball may slow down such questions in this chapter. and also change its direction. Have you ever wondered what makes an object 8.1 Force – A Push or a Pull slow down or go faster, or change its direction of motion? Actions like picking, opening, Let us recall some of our everyday shutting, kicking, hitting, lifting, experiences. What do you do to make flicking, pushing, pulling are often a football move? What do you do to used to describe certain tasks. Each make a moving ball move faster? How of these actions usually results in does a goalkeeper stop a ball? A some kind of change in the state of hockey player changes the direction motion of an object. Can these terms of the moving ball with a flick of the be replaced with one or more terms? stick. How do fielders stop a ball hit Let us find out. (a) (b) (c) Fig. 8.1 : (a) A goal keeper saving a goal, (b) A hockey player flicking a ball, and (c) A fielder stopping a ball Rationalised-2023-24 Activity 8.1 Table 8.1 gives some examples of familiar situations involving motion of objects. You can add more such situations or replace those given here. Try to identify action involved in each case as a push and/or a pull and record your observations. One example has been given to help you. Table 8.1 : Identifying Actions as Push or Pull S. Description of Action : (pushing/ pulling/picking/ Action can be No. the situation hitting/lifting/ lowering/flying/ grouped as a kicking/ throwing/shutting/ flicking) Push Pull 1. Moving a book Pushing Pulling Lifting — Yes Yes placed on a table 2. Opening or shutting a door 3. Drawing a bucket of water from a well 4. A football player taking a penalty kick 5. A cricket ball hit by a batsman 6. Moving a loaded cart 7. Opening a drawer Do you notice that each of the actions can be grouped as a pull or a push or both? Can we infer from this, that to move an object, it has to be pushed or pulled? I learnt in Class VI that a In science, a push or a pull on an magnet attracts a piece of object is called a force. Thus, we can iron towards it. Is attraction say that the motion imparted to objects also a pull? What about was due to the action of a force. When repulsion between similar poles of two magnets? Is it a does a force come into play? Let us pull or a push? find out. 94 SCIENCE Rationalised-2023-24 8.2 Forces are due to an Fig. 8.3 shows three situations that Interaction may be familiar to you. Can you decide who is pulling and who is pushing in Suppose a man is standing behind a stationary car [Fig.8.2(a)]. Will the car these cases? In Fig. 8.3 (a), both the girls move due to his presence? Suppose the appear to push each other while man now begins to push the car [Fig.8.2(b)], that is, he applies a force on it. The car may begin to move in the Fig. 8.3 (b) : Who is pulling whom ? Fig. 8.2(a) : A man standing behind a stationary car the pair of girls in Fig. 8.3 (b) are trying to pull each other. Similarly, the cow and the man in Fig. 8.3 (c) appear to Fig. 8.2 (b) : A car being pushed by a man direction of the applied force. Note that the Fig. 8.3 (c) : Who is pulling whom? man has to push the car to make it move. pull each other. The girls in the two situations shown here are applying force on each other. Is it also true for the man and the cow? From these examples, we can infer that at least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the Fig. 8.3 (a) : Who is pushing whom? two objects. FORCE AND PRESSURE 95 Rationalised-2023-24 8.3 Exploring Forces Let us try to learn more about forces. Activity 8.2 Choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all Fig. 8.5 : The rope may not move if the two teams pull at it with equal force by yourself. Can you move it? Now ask one of your friends to help you rope in their direction. Sometimes the rope in pushing it in the same direction simply does not move. Is it not similar to [Fig. 8.4(a)]. Is it easier to move it the situation shown in Fig. 8.3 (b)? The now? Can you explain why? team that pulls harder, that is, applies a larger force, finally wins the game. Now push the same object, but What do these examples suggest ask your friend to push it from the about the nature of force? opposite side [Fig. 8.4 (b)]. Does the Forces applied on an object in the object move? If it does, note the same direction add to one another. Now direction in which it moves. Can recall what happened when you and you guess which one of you is your friend pushed the heavy box in the applying a larger force? same direction in Activity 8.2. (a) If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces. What did you observe in Activity 8.2 when both of you were pushing the heavy box from opposite directions? Recall that in the tug-of-war when two teams pull equally hard, the rope does not move in any direction. (b) So, we learn that a force could be larger or smaller than the other or equal to each other. The strength of a force is usually expressed by its magnitude. We have also to specify the direction in which a force acts. Also, if the direction Fig. 8.4 : Two friends pushing a heavy load or the magnitude of the applied force (a) in the same direction, (b) in opposite direction changes, its effect also changes. Have you ever seen a game of tug-of Does it mean that the net war? In this game two teams pull at a force on an object is zero if the rope in opposite directions (Fig. 8.5). two forces acting on it in Members of both the teams try to pull the opposite directions are equal? 96 SCIENCE Rationalised-2023-24  In general, more than one force may You might recall similar situations. be acting on an object. However, the For example, while taking a penalty kick effect on the object is due to the net in football, the player applies a force on force acting on it. the ball. Before being hit, the ball was at rest and so its speed was zero. The applied force makes the ball move 8.4 A Force can Change the towards the goal. Suppose, the State of Motion goalkeeper dives or jumps up to save the Let us now find out what happens when goal. By his action the goalkeeper tries a force acts on an object. to apply a force on the moving ball. The force applied by him can stop or deflect Activity 8.3 the ball, saving a goal being scored. If the goalkeeper succeeds in stopping the Take a rubber ball and place it on a ball, its speed decreases to zero. level surface such as a table top or These observations suggest that a a concrete floor. Now, gently push force applied on an object may change the ball along the level surface its speed. If the force applied on the object (Fig. 8.6). Does the ball begin to is in the direction of its motion, the speed move? Push the ball again while it of the object increases. If the force is is still moving. Is there any change applied in the direction opposite to the in its speed? Does it increase or direction of motion, then it results in a decrease? decrease in the speed of the object. Next, place your palm in front of the moving ball. Remove your I have seen palm as soon as the moving ball children competing with one touches it. Does your palm apply a another in moving a rubber tyre force on the ball? What happens to or a ring by pushing it the speed of the ball now? Does it (Fig. 8.7). I now understand increase or decrease? What would why the speed of the tyre increases whenever it is happen if you let your palm hold pushed. the moving ball? Fig. 8.6 : A ball at rest begins to move Fig. 8.7 : To move a tyre faster it has to be when a force is applied on it pushed repeatedly FORCE AND PRESSURE 97 Rationalised-2023-24  Paheli is curious to know whether Let us consider some more application of a force can only change the examples. In a game of volleyball, speed of an object. Let us find out. players often push the moving ball to their team mates to make a winning Activity 8.4 move. Sometimes the ball is returned to the other side of the court by Take a ball and place it on a level pushing or smashing it. In cricket, a surface as you did in Activity 8.3. batsman plays his or her shot by Make the ball move by giving it a applying a force on the ball with the push [Fig. 8.8(a)]. Now place a ruler bat. Is there any change in the in its path as shown in Fig. 8.8(b). direction of motion of the ball in these In doing so, you would apply a force cases? In all these examples the speed on the moving ball. Does the ball and the direction of the moving ball continue to move in the same change due to the application of a direction after it strikes the ruler? force. Can you give a few more Repeat the activity and try to examples of this kind? obstruct the moving ball by placing A change in either the speed of an the ruler in such a way that it object, or its direction of motion, or both, makes different angles to its path. is described as a change in its state of In each case note your observations motion. Thus, a force may bring a about the direction of motion of the change in the state of motion of an ball after it strikes the ruler. object. State of Motion The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero (a) speed. An object may be at rest or in motion; both are its states of motion. Does it mean that the application of a force would always result in a change in the state of motion of the object? Let us find out. It is common experience that many a time application of force does not result (b) in a change in the state of motion. For Fig. 8.8 : (a) A ball set in motion by example, a heavy box may not move at pushing it along a level surface all even if you apply the maximum force and (b) the direction of motion that you can exert. Again, no effect of of the ball after it strikes the force is observed when you try to push ruler placed in its path a wall. 98 SCIENCE Rationalised-2023-24 8.5 Force can Change the Shape of an Object Activity 8.5 Some situations have been given in Column 1 of Table 8.2 in which objects are not free to move. Column 2 of the Table suggests the manner in which a force can be applied on each object while Column 3 shows a diagram of the action. Try to observe the effect of force in as many situations as possible. You can also add similar situations using available material from your environment. Note your observations in Columns 4 and 5 of the Table. Table 8.2 : Studying the Effect of Force on Objects Description of How to Apply Diagram Action of Force Situation Force Change in Change in State of Shape Motion Yes No Yes No A lump of Pressing it down dough on a with your hands. plate. Spring fixed to By sitting on the the seat of a seat. bicycle. A rubber band By hanging a suspended from weight or by a hook/nail pulling its free end. fixed on a wall. A plastic or By putting a weight metal scale at the centre of the placed between scale. two bricks. FORCE AND PRESSURE 99 Rationalised-2023-24  What do you conclude from the help of a stick or a piece of rope. When observations noted in Table 8.2? What we push an object like a school bag or happens when you apply a force on lift a bucket of water, where does the an inflated balloon by pressing it force come from? This force is caused between your palms? What happens by the action of muscles in our body. to the shape of a ball of dough when it The force resulting due to the action of is rolled to make a chapati? What muscles is known as the muscular happens when you press a rubber ball force. placed on a table? In all these examples you saw that the application It is the muscular force that enables of force on an object may change us to perform all activities involving movement or bending of our body. In its shape. Class VII you have learnt that in the Having performed all the above process of digestion the food gets activities, you would have realised that pushed through the alimentary canal. a force Could it be a muscular force that does it? You also know that lungs expand l may make an object move from rest. and contract while we inhale and l may change the speed of an object exhale air during breathing. Where if it is moving. are these muscles located which l may change the direction of motion make breathing possible? Can you list a few more examples of the force of an object. exerted by the muscles in our body? l may bring about a change in the shape of an object. Animals also make use of muscular l may cause some or all of these force to carry out their physical activities effects. and other tasks. Animals like bullocks, horses, donkeys and camels are used to While a force may cause one or more per for m various tasks for us. In of these effects, it is important to per for ming these tasks they use remember that none of these actions can muscular force (Fig. 8.9). take place without the action of a force. Thus, an object cannot move by itself, it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself. 8.6 Contact Forces Muscular Force Can you push or lift a book lying on a table without touching it? Can you lift a bucket of water without holding it? Generally, to apply a force on an object, your body has to be in contact with the Fig. 8.9 : Muscular force of animals is used to object. The contact may also be with the carry out many difficult tasks 100 SCIENCE Rationalised-2023-24  Since muscular force can be applied on an object to be always in contact with only when it is in contact with an object, it. Let us find out. it is also called a contact force. Are there other types of contact forces? Let us find 8.7 Non-contact Forces out. Magnetic Force Friction Activity 8.6 Recall some of your experiences. A ball rolling along the ground gradually slows Take a pair of bar magnets. Place down and finally comes to rest. When the longer side of one of the magnets we stop pedalling a bicycle, it gradually over three round shaped pencils or slows down and finally comes to a stop. wooden rollers as shown in A car or a scooter also comes to rest Fig. 8.10. Now bring one end of the once its engine is switched off. Similarly, other magnet near the end of the a boat comes to rest if we stop rowing magnet placed on the rollers. Make it. Can you add some more such sure that the two magnets do not experiences? touch each other. Observe what In all these situations no force happens. Next, bring the other end appears to be acting on the objects, yet of the magnet near the same end their speed gradually decreases and of the magnet placed on the rollers they come to rest after some time. What (Fig.8.10). Note what happens to causes a change in their state of motion? the magnet placed on the rollers Could some force be acting on them! every time another magnet is Can you guess the direction in which brought near it. the force must be acting in each case? The force responsible for changing the state of motion of objects in all these examples is the force of friction. It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest. Similarly, friction between water and the boat brings it to a stop once you stop rowing. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is also an example of a contact force. You will learn more about this force in Chapter 9. You may be wondering whether it is Fig. 8.10 : Observing attraction and repulsion between two magnets essential for the agent applying a force FORCE AND PRESSURE 101 Rationalised-2023-24  Does the magnet on the rollers begin to move when the other magnet is brought near it? Does it always move in the direction of the approaching magnet? What do these observations suggest? Does it mean that some force must be acting between the two magnets? You have learnt in Class VI that like poles of two magnets repel each other and unlike poles attract each other. Attraction or repulsion between objects can also be seen as another form of pull or push. Do Fig. 8.11 : A straw rubbed with paper attracts you have to bring the magnets in contact another straw but repels it if it has for observing the force between them? A also been rubbed with a sheet magnet can exert a force on another of paper magnet without being in contact with it. The force exerted by a magnet is an A straw is said to have acquired example of a non-contact force. electrostatic charge after it has been Similarly, the force exerted by a rubbed with a sheet of paper. Such a magnet on a piece of iron is also a straw is an example of a charged body. non-contact force. The force exerted by a charged body on another charged or uncharged body Electrostatic Force is known as electrostatic force. This Activity 8.7 force comes into play even when the bodies are not in contact. The Take a plastic straw and cut it into electrostatic force, therefore, is another nearly two equal pieces. Suspend one example of a non-contact force. You will of the pieces from the edge of a table learn more about electric charges in with the help of a piece of thread Chapter 12. (Fig. 8.11). Now hold the other piece of straw in your hand and rub its Gravitational Force free end with a sheet of paper. Bring the rubbed end of the straw near the You know that a coin or a pen falls to the suspended straw. Make sure that the ground when it slips off your hand. two pieces do not touch each other. Leaves and fruits also fall to the ground What do you observe? when they get detached from the plant. Next, rub the free end of the Have you ever wondered why it is so? suspended piece of straw with a When the coin is held in your hand it sheet of paper. Again, bring the is at rest. As soon as it is released, it piece of straw that was rubbed begins to move downwards. It is clear that earlier with paper near the free end the state of motion of the coin undergoes of the suspended straw. What do you a change. Can this happen without a observe now? force acting on it? Which is this force? 102 SCIENCE Rationalised-2023-24  Objects or things fall towards the Try cutting vegetables with a blunt earth because it pulls them. This force knife and then with a sharp knife. Which is called the force of gravity, or just is easier? gravity. This is an attractive force. The Do you get the feeling that the area force of gravity acts on all objects. The over which the force is applied (for force of gravity acts on all of us all the example, the pointed end of the nail) time without our being aware of it. Water plays a role in making these tasks easier? begins to flow towards the ground as The force acting on a unit area of a soon as we open a tap. Water in rivers surface is called pressure. flows downward due to the force of gravity. pressure = force / area on which it acts At this stage we consider only those Gravity is not a property of the earth forces which act perpendicular to the alone. In fact, every object in the surface on which the pressure is to universe, whether small or large, be computed. exerts a force on every other object. This force is known as the gravitational force. I now understand why porters place a round piece 8.8 Pressure of cloth on their heads, Is there any relation between pressure when they have to carry and force? Let us find out. heavy loads (Fig. 8.13). By Try to push a nail into a wooden doing this they increase the plank by its head. Did you succeed? Try area of contact of the load with their head. So, the now to push the nail by the pointed end pressure on their head is (Fig. 8.12). Could you do it this time? reduced and they find it easier to carry the load. Fig. 8.12 : Pushing a nail into a wooden plank Fig. 8.13 : A porter carrying a heavy load FORCE AND PRESSURE 103 Rationalised-2023-24  Note that the area is in the this process a few more times. Can denominator in the above expression. you see any relation between the So, the smaller the area, larger the amount of the bulge in the rubber pressure on a surface for the same force. sheet and the height of the water The area of the pointed end of the nail column in the pipe? is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank. Can you explain now why shoulder bags are provided with broad straps and not thin strap? And, why the tools meant for cutting and piercing always have sharp edges? Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out. 8.9 Pressure Exerted by Liquids and Gases Fig. 8.14 : Pressure exerted by water at the Activity 8.8 bottom of the container depends on the height of its column Take a transparent glass tube or a plastic pipe. The length of the pipe/ tube should be about 25 cm and its Activity 8.9 diameter should be 5-7.5 cm. Also Take a plastic bottle. You can take take a piece of thin sheet of a good a discarded water or soft drink quality rubber, say, a rubber bottle. Fix a cylindrical glass tube, balloon. Stretch the rubber sheet a few cm long near its bottom as tightly over one end of the pipe. shown in Fig. 8.15. You can do so Hold the pipe at the middle, keeping by slightly heating one end of the it in a vertical position (Fig. 8.14). glass tube and then quickly Ask one of your friends to pour some inserting it near the bottom of the water in the pipe. Does the rubber bottle. Make sure that the water sheet bulge out? Note also the does not leak from the joint. If there height of the water column in the is any leakage, seal it with molten pipe. Pour some more water. wax. Cover the mouth of the glass Observe again the bulge in the tube with a thin rubber sheet as you did in Activity 8.8. Now fill the bottle rubber sheet and the height of the upto half with water. What do you water column in the pipe. Repeat observe? Why does the rubber sheet 104 SCIENCE Rationalised-2023-24  fixed to the glass tube bulge this time? Pour some more water in the bottle. Is there any change in the bulge of the rubber sheet? Fig. 8.16 : Liquids exert equal pressure at the Fig. 8.15 : A liquid exerts pressure on the walls same depth of the container Note that the rubber sheet has been Can you now say that liquids exert fixed on the side of the container and not pressure on the walls of the at the bottom. Does the bulging of the container? rubber sheet in this case indicate that water Do gases also exert pressure? Do exerts pressure on the sides of the container they also exert pressure on the walls of as well? Let us investigate further. their containers? Let us find out. Activity 8.10 Take an empty plastic bottle or a cylindrical container. You can take I have seen fountains of water a used tin can or a used plastic coming out of the leaking joints bottle. Drill four holes all around or holes in pipes supplying near the bottom of the bottle. Make water. Is it not due to the sure that the holes are at the same pressure exerted by water on the walls of the pipes? height from the bottom (Fig. 8.16). Now fill the bottle with water. What do you observe? When you inflate a balloon, why do Do the different streams of water you have to close its mouth? What coming out of the holes fall at the happens when you open the mouth of same distance from the bottle? What an inflated balloon? Suppose you have does this indicate? a balloon which has holes. Would you FORCE AND PRESSURE 105 Rationalised-2023-24 be able to inflate it? If not, why? Can we But, how large or small is the say that air exerts pressure in all atmospheric pressure? Let us get an directions? idea about its magnitude. Do you recall what happens to the air in the bicycle tube when it has a Activity 8.11 puncture? Do these observations suggest that air exerts pressure on the inner walls Take a good quality rubber sucker. of an inflated balloon or a tube? So, we It looks like a small rubber cup (Fig. find that gases, too, exert pressure on 8.18). Press it hard on a smooth the walls of their container. plane surface. Does it stick to the surface? Now try to pull it off the 8.10 Atmospheric Pressure surface. Can you do it? You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure. We know that pressure is force per unit area. If we imagine a unit area and a very long cylinder standing on it filled with air, then the force of gravity on the air in this cylinder is the atmospheric pressure (Fig. 8.17). Fig. 8.18 : A rubber sucker pressed on a surface When you press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the sur face because the pr essure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the Unit area atmospheric pressure. This activity might give you an idea about the magnitude of atmospheric pressure. In Fig. 8.17 : Atmospheric pressure is the force of fact, it would not be possible for any gravity on air in a column of unit area human being to pull the sucker off the 106 SCIENCE Rationalised-2023-24 surface if there were no air at all between the sucker and the surface. Does it give you an idea how large the atmospheric pressure is? If the area of my head were 15 cm × 15 cm, how much force air will exert on my head? The force due to air in a column of the height of the atmosphere and area 15 cm × 15 cm (Fig. 8.19) is nearly equal to the force of gravity on an object of mass 225 kg (2250N). The reason we are not crushed under this force of gravity is that the pressure inside our bodies is also equal to the atmospheric Fig. 8.19 : Pressure of atmosphere on pressure and balances the pressure from your head outside. Did you know? Otto von Guericke, a German scientist of the 17th century, invented a pump to extract air out of a vessel. With the help of this pump, he demonstrated dramatically the force of the air pressure. He joined two hollow metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart (Fig. 8.20). So great is the force of air pressure that the hemispheres could not be pulled apart. Fig. 8.20 : Horses pulling the hemispheres FORCE AND PRESSURE 107 Rationalised-2023-24  KEYWORDS WHAT YOU HAVE LEARNT ATMOSPHERIC Ü Force could be a push or a pull. PRESSURE Ü A force arises due to the interaction between CONTACT FORCE two objects. Ü Force has magnitude as well as direction. ELECTROSTATIC Ü A change in the speed of an object or the di- FORCE rection of its motion or both implies a change FORCE in its state of motion. Ü Force acting on an object may cause a change FRICTION in its state of motion or a change in its shape. Ü A force can act on an object with or without GRAVITATIONAL being in contact with it. FORCE Ü Force per unit area is called pressure. GRAVITY Ü Liquids and gases exert pressure on the walls of their containers. MAGNETIC FORCE Ü The pressure exerted by air around us is MUSCULAR FORCE known as atmospheric pressure. NON-CONTACT FORCE PRESSURE PULL PUSH Exercises 1. Give two examples each of situations in which you push or pull to change the state of motion of objects. 2. Give two examples of situations in which applied force causes a change in the shape of an object. 3. Fill in the blanks in the following statements. (a) To draw water from a well we have to __________ at the rope. (b) A charged body __________ an uncharged body towards it. (c) To move a loaded trolley we have to __________ it. (d) The north pole of a magnet __________the north pole of another magnet. 108 SCIENCE Rationalised-2023-24  EXERCISES 4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms. muscular, contact, non-contact, gravity, friction, shape, attraction (a) To stretch the bow, the archer applies a force that causes a change in its __________. (b) The force applied by the archer to stretch the bow is an example of __________ force. (c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force. (d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air. 5. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case. (a) Squeezing a piece of lemon between the fingers to extract its juice. (b) Taking out paste from a toothpaste tube. (c) A load suspended from a spring while its other end is on a hook fixed to a wall. (d) An athlete making a high jump to clear the bar at a certain height. 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron? 7. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall? 8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion. 9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad. 10. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to (a) pressure of water. (b) gravity of the earth. (c) shape of rubber bulb. (d) atmospheric pressure. FORCE AND PRESSURE 109 Rationalised-2023-24  Extended Learning — Activities and Projects 1. Make a 50 cm × 50 cm bed of dry sand about 10 cm in thickness. Make sure that its top surface is levelled. Take a wooden or a plastic stool. Cut two strips of graph paper each with a width of 1 cm. Paste them vertically on any leg of the stool - one at the bottom and the other from the top. Now gently put the stool on the sand bed with its legs resting on the sand. Increase the size of sand bed if required. Now put a load, say a school bag full of books, on the seat of the stool. Mark the level of sand on the graph strip. This would give you the depth, if any, to which the legs of stool sink in sand. Next, turn the stool upside down so that now it rests on its seat on the sand bed. Note the depth to which the stool sinks now. Next, put the same load on the stool and note the depth to which it sinks in the sand. Compare the pressure exerted by the stool in the two situations. 2. Take a tumbler and fill it with water. Cover the mouth of the tumbler with a thick card similar to that of a postcard. Hold the tumbler with one hand while keeping the card pressed to its mouth with your other hand. Turn the tumbler upside down while keeping the card pressed to its mouth. Make sure that the tumbler is held vertical. Gently remove the hand pressing the card. What do you observe? Does the card get detached allowing the water to spill? With a little practice you will find that the card continues to hold water in the tumbler even after it is not supported by your hand. Also try this activity by using a piece of cloth to hold the tumbler in an upside down position (Fig. 8.21). Fig. 8.21 110 SCIENCE Rationalised-2023-24  3. Take 4-5 plastic bottles of different shapes and sizes. Join them together with small pieces of glass or rubber tube as shown in Fig. 8.22. Keep this arrangement on a level surface. Now pour water in any one of the bottles. Note whether the bottle in which water is poured gets filled first or all the bottles get filled up simultaneously. Note the level of water in all the bottles from time to time. Try to explain your observations. Fig. 8.22 FORCE AND PRESSURE 111 Rationalised-2023-24