Force and Pressure

Welcome dear students! Today we are going to learn about Force and Pressure from Class 8 Science.

In your previous class, you learned how objects move. Do you recall how we can decide whether an object is moving faster than another? What does the distance moved by an object in unit time indicate? You also know that a moving object, like a ball rolling on the ground, slows down. Sometimes it may change its direction of motion. It is also possible that the ball may slow down and also change its direction. Have you ever wondered what makes an object slow down or go faster, or change its direction of motion? Let us recall some of our everyday experiences. What do you do to make a football move? What do you do to make a moving ball move faster? How does a goalkeeper stop a ball? A hockey player changes the direction of the moving ball with a flick of the stick. How do fielders stop a ball hit by a batsman? In all these situations, the ball is either made to move faster or slower, or its direction of motion is changed. We often say that a force has been applied on a ball when it is kicked, pushed, thrown, or flicked. What is a force? What can it do to bodies on which it is applied? We shall seek answers to such questions in this chapter.

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Let us begin with section eight point one, Force, a Push or a Pull. Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, and pulling are often used to describe certain tasks. Each of these actions usually results in some kind of change in the state of motion of an object. Can these terms be replaced with one or more terms? Let us find out through Activity eight point one. Table eight point one gives some examples of familiar situations involving motion of objects. You can add more such situations or replace those given here. Try to identify the action involved in each case as a push or a pull and record your observations. One example has been given to help you. For moving a book placed on a table, the actions are pushing, pulling, and lifting, which can be grouped as both a push and a pull. For opening or shutting a door, the action is pushing or pulling. For drawing a bucket of water from a well, the action is pulling. For a football player taking a penalty kick, the action is kicking, which is a push. For a cricket ball hit by a batsman, the action is hitting, which is a push. For moving a loaded cart, the action is pushing or pulling. For opening a drawer, the action is pulling. 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? In science, a push or a pull on an object is called a force. Thus, we can say that the motion imparted to objects was due to the action of a force. When does a force come into play? Let us find out. You learnt in Class Six that a magnet attracts a piece of iron towards it. Is attraction also a pull? What about repulsion between similar poles of two magnets? Is it a pull or a push?

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Now let us move to section eight point two, Forces are due to an Interaction. Suppose a man is standing behind a stationary car. Will the car move due to his presence? Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the direction of the applied force. Note that the man has to push the car to make it move. In the diagram, we see a man standing behind a stationary car, and in the next part, the same man is pushing the car forward. Another figure shows three familiar situations. In the first, two girls appear to push each other. In the second, a pair of girls are trying to pull each other. Similarly, a cow and a man appear to pull each other. The girls in the two situations 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 two objects.

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Let us explore forces further in section eight point three. In Activity eight point two, choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all by yourself. Can you move it? Now ask one of your friends to help you in pushing it in the same direction. Is it easier to move it now? Can you explain why? Now push the same object, but ask your friend to push it from the opposite side. Does the object move? If it does, note the direction in which it moves. Can you guess which one of you is applying a larger force? The diagram shows two friends pushing a heavy load in the same direction, and in the second part, they push from opposite directions. Have you ever seen a game of tug of war? In this game, two teams pull at a rope in opposite directions. Members of both teams try to pull the rope in their direction. Sometimes the rope simply does not move. Is it not similar to the situation where two people pull each other? The team that pulls harder, that is, applies a larger force, finally wins the game. What do these examples suggest about the nature of force? Forces applied on an object in the same direction add to one another. Now recall what happened when you and your friend pushed the heavy box in the same direction. 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 when both of you were pushing the heavy box from opposite directions? Recall that in tug of war, when two teams pull equally hard, the rope does not move in any direction. 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 or the magnitude of the applied force changes, its effect also changes. Does it mean that the net force on an object is zero if the two forces acting on it in opposite directions are equal? In general, more than one force may be acting on an object. However, the effect on the object is due to the net force acting on it.

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Next, we will learn in section eight point four how a force can change the state of motion. In Activity eight point three, take a rubber ball and place it on a level surface such as a table top or a concrete floor. Now, gently push the ball along the level surface. Does the ball begin to move? Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease? Next, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it. Does your palm apply a force on the ball? What happens to the speed of the ball now? Does it increase or decrease? What would happen if you let your palm hold the moving ball? The diagram shows a ball at rest that begins to move when a force is applied on it. You might recall similar situations. For example, while taking a penalty kick in football, the player applies a force on the ball. Before being hit, the ball was at rest and so its speed was zero. The applied force makes the ball move towards the goal. Suppose the goalkeeper dives or jumps up to save the goal. By his action, the goalkeeper tries to apply a force on the moving ball. The force applied by him can stop or deflect the ball, saving a goal from being scored. If the goalkeeper succeeds in stopping the ball, its speed decreases to zero. I have seen children competing with one another in moving a rubber tyre or a ring by pushing it. I now understand why the speed of the tyre increases whenever it is pushed.

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Paheli is curious to know whether application of a force can only change the speed of an object. Let us find out in Activity eight point four. Take a ball and place it on a level surface as you did in Activity eight point three. Make the ball move by giving it a push. Now place a ruler in its path. In doing so, you would apply a force on the moving ball. Does the ball continue to move in the same direction after it strikes the ruler? Repeat the activity and try to obstruct the moving ball by placing the ruler in such a way that it makes different angles to its path. In each case, note your observations about the direction of motion of the ball after it strikes the ruler. The diagram shows a ball set in motion by pushing it along a level surface, and in the second part, the direction of motion changes after it strikes a ruler placed in its path. Let us consider some more examples. In a game of volleyball, players often push the moving ball to their team mates to make a winning move. Sometimes the ball is returned to the other side of the court by pushing or smashing it. In cricket, a batsman plays his or her shot by applying a force on the ball with the bat. Is there any change in the direction of motion of the ball in these cases? In all these examples, the speed and the direction of the moving ball change due to the application of a force. Can you give a few more examples of this kind? A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object. 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 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 in a change in the state of motion. For example, a heavy box may not move at all even if you apply the maximum force that you can exert. Again, no effect of force is observed when you try to push a wall.

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Now, let us see how force can change the shape of an object in section eight point five. In Activity eight point five, some situations have been given in Column one of Table eight point two in which objects are not free to move. Column two of the table suggests the manner in which a force can be applied on each object, while Column three 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 four and five of the table. The situations are: a lump of dough on a plate, pressed down with your hands; a spring fixed to the seat of a bicycle, compressed by sitting on the seat; a rubber band suspended from a hook or nail fixed on a wall, stretched by hanging a weight or pulling its free end; and a plastic or metal scale placed between two bricks, bent by putting a weight at the centre of the scale. What do you conclude from the observations noted in the table? What happens when you apply a force on an inflated balloon by pressing it between your palms? What happens to the shape of a ball of dough when it is rolled to make a chapati? What happens when you press a rubber ball placed on a table? In all these examples you saw that the application of force on an object may change its shape. Having performed all the above activities, you would have realised that a force may make an object move from rest, may change the speed of an object if it is moving, may change the direction of motion of an object, may bring about a change in the shape of an object, or may cause some or all of these effects. While a force may cause one or more of these effects, it is important to remember that none of these actions can 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.

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Let us discuss contact forces in section eight point six. First, 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 object. The contact may also be with the help of a stick or a piece of rope. When we push an object like a school bag or lift a bucket of water, where does the force come from? This force is caused by the action of muscles in our body. The force resulting due to the action of muscles is known as the muscular force. It is the muscular force that enables us to perform all activities involving movement or bending of our body. In the process of digestion, the food gets pushed through the alimentary canal. Could it be a muscular force that does it? You also know that lungs expand and contract while we inhale and exhale air during breathing. Animals also make use of muscular force to carry out their physical activities and other tasks. Animals like bullocks, horses, donkeys and camels are used to perform various tasks for us. In performing these tasks they use muscular force. The diagram shows animals using their muscular force to carry out difficult tasks. Since muscular force can be applied only when it is in contact with an object, it is also called a contact force. Are there other types of contact forces? Let us find out. Next is Friction. Recall some of your experiences. A ball rolling along the ground gradually slows down and finally comes to rest. When we stop pedalling a bicycle, it gradually slows down and finally comes to a stop. A car or a scooter also comes to rest once its engine is switched off. Similarly, a boat comes to rest if we stop rowing it. Can you add some more such experiences? In all these situations no force appears to be acting on the objects, yet their speed gradually decreases and they come to rest after some time. What causes a change in their state of motion? Could some force be acting on them? 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 nine.

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You may be wondering whether it is essential for the agent applying a force on an object to be always in contact with it. Let us find out in section eight point seven, Non-contact Forces. First is Magnetic Force. In Activity eight point six, take a pair of bar magnets. Place the longer side of one of the magnets over three round shaped pencils or wooden rollers. Now bring one end of the other magnet near the end of the magnet placed on the rollers. Make sure that the two magnets do not touch each other. Observe what happens. Next, bring the other end of the magnet near the same end of the magnet placed on the rollers. Note what happens to the magnet placed on the rollers every time another magnet is brought near it. The diagram shows observing attraction and repulsion between two magnets. 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 Six 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 you have to bring the magnets in contact for observing the force between them? A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force. Similarly, the force exerted by a magnet on a piece of iron is also a non-contact force.

Next is Electrostatic Force. In Activity eight point seven, take a plastic straw and cut it into nearly two equal pieces. Suspend one of the pieces from the edge of a table with the help of a piece of thread. Now hold the other piece of straw in your hand and rub its free end with a sheet of paper. Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other. What do you observe? Next, rub the free end of the suspended piece of straw with a sheet of paper. Again, bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw. What do you observe now? The diagram shows a straw rubbed with paper attracts another straw but repels it if it has also been rubbed with a sheet of paper. A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body. The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. This force comes into play even when the bodies are not in contact. The electrostatic force, therefore, is another example of a non-contact force. You will learn more about electric charges in Chapter twelve.

Next is Gravitational Force. You know that a coin or a pen falls to the ground when it slips off your hand. Leaves and fruits also fall to the ground when they get detached from the plant. Have you ever wondered why it is so? When the coin is held in your hand it is at rest. As soon as it is released, it begins to move downwards. It is clear that the state of motion of the coin undergoes a change. Can this happen without a force acting on it? Which is this force? Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or just gravity. This is an attractive force. The force of gravity acts on all objects. The force of gravity acts on all of us all the time without our being aware of it. Water begins to flow towards the ground as soon as we open a tap. Water in rivers flows downward due to the force of gravity. Gravity is not a property of the earth alone. In fact, every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force.

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Now let us explore section eight point eight, Pressure. Try cutting vegetables with a blunt knife and then with a sharp knife. Which is easier? Do you get the feeling that the area over which the force is applied plays a role in making these tasks easier? Try to push a nail into a wooden plank by its head. Did you succeed? Try now to push the nail by the pointed end. Could you do it this time? The diagram shows pushing a nail into a wooden plank. The force acting on a unit area of a surface is called pressure. Pressure equals force divided by area on which it acts. At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed. I now understand why porters place a round piece of cloth on their heads, when they have to carry heavy loads. By doing this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load. The diagram shows a porter carrying a heavy load with a cloth on his head. Note that the area is in the denominator in the above expression. So, the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail 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 straps? And, why the tools meant for cutting and piercing always have sharp edges?

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Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out in section eight point nine, Pressure Exerted by Liquids and Gases. In Activity eight point eight, take a transparent glass tube or a plastic pipe. The length of the pipe or tube should be about twenty five centimeters and its diameter should be five to seven point five centimeters. Also take a piece of thin sheet of a good quality rubber, say, a rubber balloon. Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe at the middle, keeping it in a vertical position. Ask one of your friends to pour some water in the pipe. Does the rubber sheet bulge out? Note also the height of the water column in the pipe. Pour some more water. Observe again the bulge in the rubber sheet and the height of the water column in the pipe. Repeat this process a few more times. Can you see any relation between the amount of the bulge in the rubber sheet and the height of the water column in the pipe? The diagram shows that pressure exerted by water at the bottom of the container depends on the height of its column.

In Activity eight point nine, take a plastic bottle. Fix a cylindrical glass tube, a few centimeters long near its bottom. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet as you did in Activity eight point eight. Now fill the bottle up to half with water. What do you observe? Why does the rubber sheet 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? The diagram shows a liquid exerting pressure on the walls of the container. Note that the rubber sheet has been fixed on the side of the container and not at the bottom. Does the bulging of the rubber sheet in this case indicate that water exerts pressure on the sides of the container as well? Let us investigate further.

In Activity eight point ten, take an empty plastic bottle or a cylindrical container. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom. Now fill the bottle with water. What do you observe? Do the different streams of water coming out of the holes fall at the same distance from the bottle? What does this indicate? The diagram shows that liquids exert equal pressure at the same depth. Can you now say that liquids exert pressure on the walls of the container? Do gases also exert pressure? Do they also exert pressure on the walls of their containers? Let us find out. I have seen fountains of water coming out of the leaking joints or holes in pipes supplying water. Is it not due to the pressure exerted by water on the walls of the pipes? When you inflate a balloon, why do you have to close its mouth? What happens when you open the mouth of an inflated balloon? Suppose you have a balloon which has holes. Would you be able to inflate it? If not, why? Can we say that air exerts pressure in all directions? Do you recall what happens to the air in the bicycle tube when it has a puncture? Do these observations suggest that air exerts pressure on the inner walls of an inflated balloon or a tube? So, we find that gases, too, exert pressure on the walls of their container.

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Let us move to section eight point ten, Atmospheric Pressure. You know that there is air all around us. This envelope 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. The diagram illustrates that atmospheric pressure is the force of gravity on air in a column of unit area. But, how large or small is the atmospheric pressure? Let us get an idea about its magnitude. In Activity eight point eleven, take a good quality rubber sucker. It looks like a small rubber cup. Press it hard on a smooth plane surface. Does it stick to the surface? Now try to pull it off the surface. Can you do it? The diagram shows 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 surface because the pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. This activity might give you an idea about the magnitude of atmospheric pressure. In fact, it would not be possible for any human being to pull the sucker off the 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 fifteen centimeters by fifteen centimeters, 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 fifteen centimeters by fifteen centimeters is nearly equal to the force of gravity on an object of mass two hundred twenty five kilograms, which is two thousand two hundred fifty newtons. The reason we are not crushed under this force of gravity is that the pressure inside our bodies is also equal to the atmospheric pressure and balances the pressure from outside. The diagram shows the pressure of atmosphere on your head.

Did you know? Otto von Guericke, a German scientist of the seventeenth 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 fifty one centimeters diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart. So great is the force of air pressure that the hemispheres could not be pulled apart. The diagram shows horses pulling the hemispheres.

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Let us review the key concepts you have learned. Force could be a push or a pull. A force arises due to the interaction between two objects. Force has magnitude as well as direction. A change in the speed of an object or the direction of its motion or both implies a change in its state of motion. Force acting on an object may cause a change in its state of motion or a change in its shape. A force can act on an object with or without being in contact with it. Force per unit area is called pressure. Liquids and gases exert pressure on the walls of their containers. The pressure exerted by air around us is known as atmospheric pressure.

Now, let us solve the exercises together. Exercise one: Give two examples each of situations in which you push or pull to change the state of motion of objects. For pushing, examples include a football player kicking a stationary ball to make it move, and a person pushing a heavy cart to start its motion. For pulling, examples include drawing a bucket of water from a well by pulling the rope, and a horse pulling a cart to move it forward.

Exercise two: Give two examples of situations in which applied force causes a change in the shape of an object. Examples include pressing an inflated balloon between your palms, which flattens it, and rolling a ball of dough into a flat chapati, which changes its shape from spherical to flat.

Exercise three: Fill in the blanks in the following statements. (a) To draw water from a well we have to pull at the rope. (b) A charged body attracts an uncharged body towards it. (c) To move a loaded trolley we have to push or pull it. (d) The north pole of a magnet repels the north pole of another magnet.

Exercise four: 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 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 shape. (b) The force applied by the archer to stretch the bow is an example of muscular force. (c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force. (d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

Exercise five: 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. Agent: fingers. Object: lemon. Effect: change in shape of the lemon, causing juice to come out. (b) Taking out paste from a toothpaste tube. Agent: hand or fingers. Object: toothpaste tube. Effect: change in shape of the tube, forcing paste out. (c) A load suspended from a spring while its other end is on a hook fixed to a wall. Agent: load due to gravity. Object: spring. Effect: stretching of the spring, changing its shape. (d) An athlete making a high jump to clear the bar at a certain height. Agent: athlete's legs applying muscular force. Object: athlete's body. Effect: change in state of motion, lifting the body upward against gravity.

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Exercise six: A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron? The force of hammering changes the shape of the hot iron, allowing it to be molded into the desired tool shape.

Exercise seven: 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? The electrostatic force is responsible for the attraction. Rubbing the balloon charges it, and this charged balloon exerts an electrostatic force on the wall, causing it to stick.

Exercise eight: 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. The forces acting are the upward muscular force exerted by your hand and the downward gravitational force, which is the weight of the bucket and water. These two forces are equal in magnitude and opposite in direction, so the net force is zero. Therefore, there is no change in the state of motion, and the bucket remains stationary.

Exercise nine: 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. The two forces are the upward thrust force generated by the rocket engines and the downward gravitational force of the earth.

Exercise ten: 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 atmospheric pressure. When the bulb is released, the pressure inside decreases, and the higher external atmospheric pressure pushes the water up into the dropper.

For extended learning, you can try these activities. First, make a fifty centimeter by fifty centimeter bed of dry sand about ten centimeters thick. Level the top surface. Take a stool, cut two strips of graph paper one centimeter wide, and paste them vertically on one leg. Place the stool on the sand. Put a school bag on the seat and mark the depth the legs sink. Turn the stool upside down so it rests on its seat, add the same load, and note the sinking depth. Compare the pressure in both situations to see how area affects pressure. Second, take a tumbler filled with water, cover it with a thick card, and turn it upside down while holding the card. Gently remove your hand. The atmospheric pressure will hold the card in place, preventing water from spilling. Third, take four or five plastic bottles of different shapes and sizes, join them at the bottom with tubes, and place them on a level surface. Pour water into one bottle and observe that the water level rises equally in all bottles simultaneously, demonstrating that liquids seek their own level regardless of container shape.

Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]