Force and Pressure PDF

1 Force and Pressure A football...

1 Force and Pressure A football player kicks the ball with his foot, applying a force and directing the ball into a particular part of the ground. "Force is an action exerted on an object which may change the object’s state of rest or motion." 1. Introduction The word ‘force’ is frequently used in our day to day life. A football resting on the ground, remains motionless, until you kick it. Your science book rests on the table until you pick it up. If you hold your book above the ground, then let it go, gravity pulls it to the floor. In each of these cases, the motion of the ball or book was changed by something pushing or pulling on it. Thus, an object will speed up, slow down, or turn only if someone is pushing Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure or pulling on it. The magnitude of force is measured in terms of Newton, named after Sir Isaac Newton(1642–1727), whose work contributed much to the modern understanding of force and motion. Sir Isaac Newton    NCERT Basics : Class 8 We always see or feel the effect of a force. If can only be explained by describing what happens when a force is applied to an object. Pushing, hitting and pulling of objects are some ways of bringing objects in motion. They move because we make a force act on them. 2. Force A push or a pull acting on some object is called a force. SI unit of force – Newton ; C.G.S. unit of force – Dyne 1 Newton = 105 Dyne (a) Pushing a trolley (b) Pulling an aeroplane (c) Player kicking a football Force - a push or a pull 1 Table given below contains some examples of familiar situations involving motion of objects. Each situation has been classified and grouped as a push and/or a pull. Action : (pushing/ pulling /picking / Action can Description of the hitting / lifting / lowering/flying/ be grouped S. No. situation kicking/ throwing/shutting/ as a flicking) Push Pull Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure 1 Moving a book placed on Pushing Pulling Lifting — Yes Yes a table 2 Opening or shutting a Pushing Pulling Shutting — Yes Yes door 3 Drawing a bucket of Lowering Pulling — — No Yes water from a well 4 A football player taking a Kicking Pushing — — Yes No penalty kick 5 A cricket ball hit by a Hitting Pushing Flicking — Yes No batsman 6 Moving a loaded cart Pushing Pulling — — Yes Yes 7 Opening a drawer Pulling — — — No Yes   Physics 3. Forces are due to an interaction Let us consider a man standing behind a stationary car. Now, if the man 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 [see figure]. The man ‘pushes’ the car to make it move. A car being pushed by a man From this example, we can conclude 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. Some other examples of interaction between two objects are given in figure (a) and figure (b). Forces are always exerted by one object on another object i.e., to apply force atleast two objects are required. (a) Men pushing each other during arm wrestling (b) Two wrestlers applying force on each other while wrestling Interaction between two objects Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure 4. Exploring forces In the game of ‘tug-of war’, two teams pull a rope in opposite directions (see figure). Members of both the teams try to pull a rope in their direction. Sometimes the rope simply does not move. The team that pulls harder, that is, applies a larger force, finally wins the game. Tug of war : Rope will not move if the two teams pull with equal force NCERT Basics : Class 8 Quantities that have both magnitude and direction, are called ‘vectors’. Quantities that are just numbers without any direction, are called ‘scalars’. Since the effect of a force depends on both magnitude and direction, force is a vector quantity. A force vector points in the direction of the force, and its length is proportional to the magnitude of the force. 2 1. Take a heavy box which can be moved only by pushing hard. Try to push it by yourself [see figure]. Let's assume you are able to push the box with some difficulty. Now, ask one of your friends to help you in pushing it in the same direction. You can see that, now, it is easier to move the box. 2. Now, push the same box, but ask your friend to push it from the opposite side. You can see that, now, it is very difficult to move the box. The box will not move if both you and your friend apply same force. If the box moves, it will surely move in the direction of greater force. 300 N 400 N Active physics 2 Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Conclusion : Forces applied on an object in the same direction add to one another. 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. A force can be of different magnitudes i.e., one force can be greater or smaller than the other. The strength of a force is measured by its magnitude. Also, we should specify the direction in which a force acts. If the direction or the magnitude of the applied forces change, their effect also changes. If many forces are acting simultaneously on an object, the effect on the object is due to the net force acting on it. The combination of all the forces acting on an object is called net force. The net force acting on an object is also referred as the total force, the resultant force, or the unbalanced force acting on the object.   Physics 1 You are facing eastward, standing in front of a gate that can swing. In what direction is your force if you The ‘state of motion’ pull the gate or push the gate? of an object is described by its speed and the Explanation direction of motion. The Since you are facing eastward, to pull on the gate you state of rest can be have to apply force in west direction i.e., westward. To considered as ‘the state of motion with zero push on the gate, you have to apply force in east speed’. direction i.e., eastward. 3 1. Suppose you and your friend are pushing a door of a room from opposite sides. Let you are trying to close the door while your friend is trying to open it. Initially, the door is going nowhere i.e., neither it is getting closed nor it is opening [see figure (a)]. This is because you and your friend are applying same force on the door. 2. Now try to increase your force and ask your friend to do the same. Suppose at some stage your force becomes more than your friend then, the door will get closed. This is because the door will move in the direction of net force acting on it [see figure (b)]. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure (a) Door is not moving because the (b)Door is closing because the force forces exerted on it are equal pushing the door to close is greater but in opposite directions. than the force pushing it open. Active physics 3 If the resultant of all forces acting on a body is zero, the forces are called balanced forces. If the resultant of all forces acting on a body is not zero, the forces are called unbalanced forces. NCERT Basics : Class 8  Unit of force should always be taken in MKS system. 1 1. Arun pushes a box with a force of 50 N [see figure (a)]. The box moves slightly. He requests Varun to help him. Varun also pushes with a force of 50 N [see figure(b)]. The box now moves easily. Now, what is the net force acting on the box? 50 N 50 N 50 N (a) (b) Numerical ability 1 (1) Decode the problem Solution Analyse the direction of Since, the forces applied by Arun and Varun are in same forces. directions, the net force acting on the box is equal to sum of Same✓/Opposite forces applied by Arun and Varun. If forces are in same Net Force = force applied by Arun + force applied by Varun direction, add them. = 50 N + 50 N = 100 N 2. (a) If Arun and Varun push the box from opposite ends with the same force Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure [see figure (a)], what do you think will happen? What would be the net force? (b) Suppose Arun pushed extra hard with a force of 60 N [see figure (b)]. What do you think will happen? What will be the net force? 50 N 50 N 50 N 60 N (a) (b) Numerical ability 1 (2)   Physics Solution (a) The box will not move because the forces acting on it are equal in magnitude but opposite in direction. Decode the problem Since, the forces are in opposite directions, the net Analyse the direction of force will be the difference of the forces acting on forces. the object. Same/Opposite✓ The net force on the box = 50 N – 50 N = 0 N. (b) Again, in this case, the forces are in opposite If forces are in opposite directions. direction, subtract them. Thus, the net force on the box = 60 N – 50 N = 10 N. The box will move but with some difficulty as the force acting on it is quite less. 3. In a tug-of war (see figure), the three members of team A pull with a force of 120 N, 100 N and 135 N. The three members of team B pull with a force of 120 N, 105 N and 130 N. Which team will win? Team A Team B Numerical ability 1 (3) Solution Total force applied by team A = 120 N + 100 N + 135 N Decode the problem Analyse the direction of = 355 N forces. Total force applied by team B = 120 N + 105 N + 130 N Same/Opposite✓ Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure = 355 N Net force on the rope = Force applied by A – Force If fores are in opposite applied by B direction, subtract them. = 355 N – 355 N = 0 N Since, the net force on the rope is zero, it will not go either way i.e., no team will win, its a tie !! 5. Effects of force Force can change the state of motion 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. A force can bring a change in the state of motion of an object. NCERT Basics : Class 8 4 1 1. Take a rubber ball and place it on a table top. Now, gently push the ball along the level surface. You will observe that the ball begins to move (see figure). Now, push the ball again while it is still moving. There will be a change in its speed, it may increase or decrease depending on the direction of force Active Physics 4 applied by you. 2. Now, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it. Here, your palm does not apply a force on the ball. Thus, the speed of the ball does not change. 3. Now, let your palm hold the moving ball. You will observe that the ball stops i.e., the speed of ball decreases to zero. Conclusion : These observations suggest that a force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object. Force can move a stationary object A force can cause a stationary object to move, as when you throw a ball. Some examples of this are shown in figure (a) and figure (b). (a) A player kicking the ball (b) A hockey player hitting the ball Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Making an object move from rest. Force can stop a moving object A force can cause a moving object to stop, as when you catch a ball. Some examples of this are shown in figure. (a) A goalkeeper stopping the ball (b) A cyclist applying brakes to stop bicycle Stopping an object by applying force on it.   Physics 1 1. How does the speed of an object travelling in a straight line change if a non zero net force acts in the direction of motion of the object? 2. Suresh and Ramesh are applying forces on a table. If the table is not moving, what can you predict about their forces? Neglect any other forces (like friction) acting on the table. 3. A fielder catches a ball in the game of cricket. How are the directions of force and the velocity of ball related? Force can increase or decrease the speed of a moving object A force can cause a moving object to increase or decrease its speed. For example, when you kick a moving football in the direction of its motion, its speed increases. When you apply a gentle brake in your bicycle, its speed decreases as the applied force is opposite to the motion of the cycle. Some examples of this are shown in figure. Push F F Speed Speed Pull (a) Force exerted in the direction of velocity (b) Force exerted in opposite direction of velocity Changing the speed of an object by applying force on it 5 Take a ball and place it on a table top. Make the ball move by giving it a push [see figure (a)]. Now place the ruler from your geometry box in its path. In doing so, you will apply a Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure force on the moving ball. You will observe that the ball changes its direction after it strikes the ruler [see figure (b)]. Now, repeat this activity of obstructing the moving ball by the ruler by making different angles to its path. In each case you will observe that the direction of motion of the ball is different after it strikes the ruler which depends on the placement of the ruler. (a) A ball set in motion by pushing it. (b) Direction of motion of the ball Active physics 5 changes after it strikes the ruler placed in its path NCERT Basics : Class 8 Conclusion : A force can change the direction of moving object. A force can be applied to obtain any desired direction of an object. Force can change the direction of a moving object A force can cause a moving object to change its direction. For example, in the game of cricket, a batsman applies a force to change the direction of the ball coming towards him. Some examples of this are shown in figure. A batsman changing the direction of the ball using his bat Force can make an object to rotate If an object (like a fan), is free to rotate about an axis, a force applied on it can cause its rotation. For example, force of wind causes a windmill to rotate [see figure (a)] ; a person driving a car rotates the steering through the force applied by his hands [see figure (b)]. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure (a) A windmill (b) A person rotating a car steering Causing rotation by the application of a force 1 1. Whenever the net force applied on an object is in the direction of its motion, the speed of the object increases. 2. Both are applying forces which are equal in magnitude but opposite in direction. So, the net force on the table is zero. 3. Velocity and force are in opposite directions, that is why the ball comes to rest (or stopped).   Physics Force can change the shape of an object A force may bring about a change in the shape of an object. For example, stretching or compressing a spring changes its shape, pressing a rubber ball changes its shape. An example of this is shown in figure. (a) Original shape of ball (b) Pressing the ball changes its shape Force can change the shape of an object. 6 Take a lump of dough (wheat flour mixed with water) on a kitchen platform (see figure). Now, press it down with your hands. You will observe that there will be the change in the shape of dough whenever you press it. Also, the dough remains at rest before and after applying a force i.e., in this case there is a change in the shape of an object but there is no change in its state of rest or motion. Active physics 6 Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Conclusion : An object can not change its shape by itself, a force is required to change its shape. A force does not necessarily cause a body to change its state of rest or motion. 6. Types of forces Contact forces When you press the keys on a computer keyboard, your fingers exert a force on the keys. This force can be exerted only when your fingers are touching the keys.  A force that is exerted only when two objects are touching is called contact force. A contact force can be small, such as the force you exert to push a pencil across a sheet of paper, or large, such as the force exerted by a traffic crane as it pulls a car along a street. Some examples of contact forces are shown in figure. NCERT Basics : Class 8 (a) A karate fight (b) Stretching a bowstring Examples of contact forces 1. Muscular force When you push an object like a school bag or lift a bucket of water, the force comes from the action of muscles in your body. (see figure) Examples of muscular forces The force resulting due to the action of muscles is known as the muscular force. 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. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Muscular force of animals is used to carry out many difficult tasks If you pull on a spring, the spring stretches. If you pull hard enough on a wagon, the wagon moves. When you kick a football, it deforms briefly and is set in motion. These are all examples of contact forces, so named because they result from physical contact between two objects. 2. Friction You must have seen that a ball rolling along the ground gradually slows down and finally comes to rest. When you stop pedaling a bicycle, it gradually slows down and finally comes to stop. A car or a scooter also comes to rest once its engine is switched off. Similarly, a boat comes to rest if you stop rowing it.   Physics In all the above situations no force appears to be acting on the objects, yet their speed gradually decreases and they come to rest after some time. Since, there is a change in their state of motion, some force must be acting on them. This force which is responsible for changing the state of motion of objects in all the above examples is the force of friction. Friction is a force that resists motion. Friction is found everywhere in our world. You feel the effects of friction when you swim, ride in a car, walk, and even when you sit on a chair. Friction can act when an object is moving or when it is at rest. Figure shows some common examples of friction. (a) Friction is more between ball and grass, (b) Friction is less between sledge and ice hence ball stops Effects of frictional force 2 Suppose you are rowing a boat in water. What Since the force of friction arises due to happens when you stop rowing? contact between surfaces, Explanation it is also an example of a Once you stop rowing, friction between water and the contact force. boat brings it to a stop in some time. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure 3. Tension Tension is a force exerted by string, ropes, fibers, and cables when they are pulled. A spider web consists of numerous fine strands that pull on one another (see figure). We say that the strands are under tension. Thus, tension keeps the spider web together. (a) Tension keeps this spider web together (b) Two children pulling the rope, a tension force is exerted by the rope on them. Examples of tension force NCERT Basics : Class 8 Non contact forces When you jump up in the air, you are pulled back to the ground, even though nothing seems to be touching you. Forces can be exerted by one object on another even though they aren’t touching each other. The force pulling you down to Earth is the gravitational force exerted by Earth. This force is a noncontact force. A non contact force is a force that one object exerts on another when they are not touching. Non contact forces include the gravitational force, the electric force, and the magnetic force. 1. Magnetic force The force exerted by a magnet on a piece of iron or on another magnet is called magnetic force. Like (or similar) poles repel while unlike (or opposite) poles attract (see figure). S N N S Repulsive N S S N Repulsive S N S N Attractive Force between two magnets 7 4 Take a pair of bar magnets. Place one of the magnets over three round shaped pencils. Now bring one end of the other magnet near the end of the magnet placed on the pencils (see figure). Make sure that the two magnets do not touch each other. You may observe a repulsion or attraction. Suppose you observe a repulsion in first case. Now, bring the other end of the magnet near the same end of the magnet placed on the pencils. This time you Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure will observe an attraction. Conclusion : One magnet exerts a force on other magnet without actual physical contact with it. This force may be attractive or repulsive depending on the nature of poles. Like poles repel, unlike poles attract. Attraction between unlike poles Active physics 7   Physics 2. Electrostatic force The electrons in an atom move around the nucleus. What keeps these electrons close to the nucleus? The positively charged protons in the nucleus exert an attractive force on the negatively charged electrons. All charged objects exert a force on each other. This force is called electrostatic force or electric force. The force exerted by a charged body on another charged body or uncharged body is known as electrostatic force. The electrostatic force between two charges can be attractive or repulsive (see figure). Objects with the same type of charge repel one another and objects with opposite charges attract one another i.e., ‘like charges repel and unlike charges attract’. + + + – – – (a) Attraction between unlike charges (b) Repulsion between like charges Electrostatic force between two charges 3 You know that a coin or a pen falls to the ground when it slips off your hand. Is there any force acting on them? Explain. Explanation 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. This is not possible without a force acting on it. Yes, there is a force acting on them, ‘the force of gravity’. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure 3. Gravitational force The leaves or fruits fall to the ground when they get detached from the plant. The skydiver (see figure) is also being pulled downward, even though there seems to be nothing touching him. Have you ever wondered why is it so? Objects fall towards the Earth because it pulls them. This force is called the gravitational force or force of gravity, or just gravity. A skydiver is pulled downward by the gravitational force NCERT Basics : Class 8 2 State the direction of the named force acting on the object in italics. 1. The force of gravity exerts a force on a falling leaf. 2. A hockey stick experiences friction on rough ground as it slides northward. 3. The force of the wind is pushing against a cyclist who is cycling eastward. 7. Pressure What happens when you walk on soft snow or dry sand? Your feet sink into the snow or sand and walking can be difficult. If you ride a bicycle with narrow tires over these surfaces, the tires will sink even deeper than your feet. How deep you sink depends on your weight as well as the area over which you make contact with the sand or snow. For example, when you stand on snow with your normal shoes, you may sink deep into the soft snow [see figure (a)]. However, if you use special kinds of supports (or snow boards) having larger area than your shoes [see figure (b)], you can stand on the snow very easily. (a) (b) Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Understanding pressure In both cases, your weight exerts a downward force on the snow. What changed was the area of contact between you and the snow. By changing the area of contact, you changed the ‘pressure’ you exerted on the snow due to your weight. When you stood with supports (or snow boards), the area of contact increased, so that the same force was applied over a larger area. As a result, the pressure that was exerted on the snow decreased and you didn’t sink as deep.  Pressure is the force per unit area that is applied on the surface of an object Force Pressure = Area   Physics Unit of pressure Since pressure is force per unit area thus, unit of pressure is Newton per square meter and in short it is Pressure is inversely proportional to the called ‘pascal’ i.e., area of contact. So, the SI unit of pressure – Pascal ; smaller the area, the larger is the pressure C.G.S unit of pressure – dyne cm–2. on a surface for the same force. 1 Pascal = 1 Newton/(meter)2 or 1 Pa = 1 N/m2 = 1 N m–2  Take all the units in same system either in MKS or CGS system.  We know, 1 m = 100 cm Therefore, 1 m2 [1m × 1m] = 100 cm × 100 cm = 10000 cm2 = 104 cm2. 1 1 cm2 = m2 = 10–4 m2 104 1 m = 103 mm 1 m2 = 106 mm2 1 mm2 = 10–6 m2  Area is always in square units. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure r 22  Area of circle = r2   =  or 3.14   7   Area of rectangle = × b b   Area of square = side × side a = a × a = a2 a NCERT Basics : Class 8 2 1. A force of 20 N acts over a surface having an area of 4 m 2. Decode the problem What is the pressure on this surface? To find the pressure, Solution we should know the Given, force = 20 N ; area = 4 m2 ; pressure =? formula 20 force We know that, pressure = = 5 N/m2 pressure = 4 area 2. A pressure of 50 N/m2 acts on an area of 5 m2. Calculate the total force acting on the given area. Decode the problem Solution Identify the formula Given, pressure = 50 N/m2 ; force area = 5 m2 pressure = area ; force =? F=P×A or Force = 50 × 5 = 250 N 3. A force of 800 N exerts a pressure of 40 N/m2. What area is it acting on? Solution Decode the problem Given, pressure = 40 N/m2 ; Identify the formula force = 800 N ; area =? force pressure = 800 area or Area = 40 F  Area = = 20 m2 P 4. A person weighs 600 N. He is wearing shoes with a total area of 0.02 m 2. What Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure pressure do they exert on the floor? Decode the problem Solution Identify the formula Given, force = 600 N ; area = 0.02 m2 ; pressure =? force pressure = pressure = 600 = 30,000 N/m2 area 0.02 4 When you push a nail using a hammer into a wooden plank by its head, the nail cannot be inserted in the plank. When you push the nail by the pointed end, the nail can easily be inserted in the plank. Why?   Physics Explanation Hammer When you apply a force on the nail using a hammer with its head touching the wooden plank, the pressure exerted on the plank is quite small. This is because pressure is nail inversely proportional to area i.e., the larger the area, smaller the pressure on a surface for the same force. When you apply the same force on the nail with its Applying force on a nail with its pointed end touching the pointed end touching the plank, pressure exerted on the plank plank is sufficient to insert it into the plank (see figure). This is because the area of the pointed end of the nail is much smaller than that of its head.  Strong winds during a storm or a cyclone can blow away even the rooftops. The winds and cyclones are caused by the difference in air pressure. 2 1. Since, the force of gravity acts vertically downward on an object, here also, it acts vertically downward on a falling leaf. 2. A force of friction opposes the motion of an object. Since hockey stick is moving northwards, it experiences a force of friction in opposite direction i.e., southwards. 3. Here, the cyclist is moving eastwards and a force of wind is pushing against’ him. This means the force of wind is opposite to him i.e., westwards. Some common examples related to pressure (1) If you try to cut vegetables with a blunt knife, you will not succeed. But, if you use a Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure sharp knife, it is quite easy to cut the vegetables. This is because the area of sharp edge of the knife is quite small and thus, pressure is large enough to cut the vegetables. (2) The straps of shoulder bags are made wide (broad). This is because the wide straps has larger area so that the pressure exerted on the shoulder becomes quite small. Thus, it becomes quite comfortable to carry such shoulder bags. A porter carrying a heavy load (3) Porters place on their heads a round piece of cloth, when they have to carry heavy loads (see figure). By 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. NCERT Basics : Class 8 8. Pressure exerted by liquids (hydrostatic pressure) and gases Liquids and gases are fluids, which are materials that can flow and have no definite shape. Like solid objects, fluids can exert pressure (or forces). Fluid pressure is measured in pascal and also in mm of Hg. 8 Take a transparent hollow glass tube of length about 15 cm and diameter of about 2.5 cm. Take a good quality rubber balloon and tie it using thread on one end of the tube. Hold the tube from the middle to keep it in a vertical position [see figure (a)]. Ask one of your friends (a) (b) to pour some water in the tube. You will observe that the Active physics 8 rubber balloon bulges out [see figure (b)]. As you increase the height of the water column in the pipe, the bulge in the balloon will increase. Conclusion : Liquids exert pressure on the surfaces (objects) in contact. More the height of liquid column above a surface, more will be the pressure on that surface. 9 Take a big soft drink bottle. Take a cylindrical glass tube of length about 6 cm. Now, heat one end of the glass tube and then quickly insert it near the bottom of the bottle (see figure). Next, paste some adhesive material around the tube-bottle joint to avoid the Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure leakage of water from the joint. Cover the mouth of the glass tube with a good quality rubber balloon using a thread. Now fill the bottle up to half with water. You will observe that the rubber balloon fixed to the glass tube bulges. Pour some more water in Active physics 9 the bottle. You will observe that this bulge increases. Conclusion : Again, this activity shows that liquids can exert pressure on an object which is proportional to the height of liquid column above the object.  It is true for any liquid that ‘the pressure depends only on the height of the column of fluid above the surface where you measure the pressure’. It does not depends on the area of the surface in contact or the shape of the liquid column (see figure on next page). The greater the height of the column of fluid above a surface, the greater is the pressure exerted by the fluid on the surface.   Physics Pressure depends only on the height of the fluid above a surface, not on the shape of the vessel. Pressure at the bottom of each section of the vessel is same. Pressure and depth Figure shows how pressure changes with depth. At the top of the glass filled with water, the water pressure is zero because there is no column of water above that level. Pressure in the middle of the glass depends on the column of water from the top of the glass to the middle of the glass. Pressure at the bottom depends on the entire height of the water. Pressure increases with depth because the column of water pushing down becomes taller and heavier. Increasing pressure The pressure exerted by a fluid increases as the depth in the fluid increases. 10 Take a big soft drink bottle. Drill four holes all around near Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure the bottom of the bottle at the same height from the bottom (see figure). Now fill the bottle with water. You will observe that the water coming out of the holes falls at the same Active physics 10 distance from the bottle. Conclusion : The liquid exerts pressure on all side of the wall not just on the bottom. This pressure depends on the depth in the liquid. Pressure acts in all directions Figure shows that the fluid exerts pressure on each face of the cube or from all sides of a submarine, not just on their tops. This pressure is perpendicular to the surface, and the amount of pressure depends only on the depth in the fluid. NCERT Basics : Class 8 Submarine Cube Pressure Pressure The pressure on all objects in a fluid is exerted on all sides, perpendicular to the surface of the object, no matter what is its shape. Similar to liquids, gases also exert pressure on the walls of their containers. A force applied to a fluid (liquid or gas) creates pressure. Pressure acts in all directions, not just the direction of the applied force. When you inflate a car tyre, you are increasing the pressure in the tyre. This force acts up, down, and sideways, in all directions inside the tyre. 3 1. Why do the tools meant for cutting and piercing always have sharp edges? 2. In figure, two identical bricks are placed on a table in two different ways (A and B). In which case, the pressure on the table is more and why? Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure Check your concepts 3(2) 3. What happens when you push down on an inflated balloon? 9. Atmospheric pressure Though you don’t feel it, you are surrounded by a fluid that exerts pressure on you constantly. That fluid is air or atmosphere surrounding the Earth. The envelop of air surrounding the Earth is known as the atmosphere. The atmosphere at Earth’s surface is only about 1/1000 times as dense as water. But, the atmospheric air extends up to many kilometres above the surface of the earth which is large enough to exert a large pressure on the objects at Earth’s surface.   Physics The pressure exerted by the air present in the atmosphere is known as atmospheric pressure. Atmospheric pressure on our body is a result of the weight of the atmosphere exerting force on our body. Atmospheric pressure is approximately 100,000 N/m2 at sea level. A manometer is used to measure pressure. A barometer is used to measure the atmospheric pressure. Suppose the area of your head is 10 cm × 10 cm which is equal to (10/100) m × (10/100) m = 0.01 m2. Therefore, the weight (force) of air on your head is 100,000 Pressure on your head N/m2 × 0.01 m2 = 1000 N. This means weight (force) of air in a column of the height of the atmosphere and area 10 cm × 10 cm (see figure) is as large as 1000 N. 5 The force pushing down on your body due to atmospheric pressure is as large as the weight of a luxury car. Then, why doesn’t this pressure cause you to be crushed? Explanation Your body is filled with fluids such as blood that also exert pressure. The pressure exerted outward by the fluids inside your body is equal to the atmospheric pressure. This pressure inside your body cancels the pressure exerted from outside by the atmosphere. Thus, you are not crushed under this large force due to the atmospheric pressure. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure You don’t feel the atmospheric pressure because there is an equal, internal pressure pushing out from the inside of your body. This internal pressure balances the external pressure exerted on you by the atmosphere. 6 What happens to the atmospheric pressure if you go at higher altitudes or walk up on a mountain? Explanation If you start at the base of a mountain and walk up, the height of the column of air above you decreases. This means, with increasing elevation, the amount of air above you decreases, and thus, the atmospheric pressure also decreases. As you go higher in the atmosphere, atmospheric pressure decreases. NCERT Basics : Class 8 3 1. Tools meant for cutting and piercing always have sharp edges because their edges have small areas which create a large pressure on the surface to make the cutting or piercing quite easy. 2. In the case B, the pressure on the table is more because the area of contact of the brick with the table is small (smaller the area of contact, larger will be the pressure on the surface). 3. The downward force you apply creates forces that act sideways as well as down (see figure). As a result, the shape of balloon changes from all sides. This means a downward push on the balloon creates pressure in all directions. Applied force Fluid (gas) Pressure Check your answers 3 (3) 11 Take a good quality rubber sucker. It looks like a small rubber cup [see figure (a)]. Press it hard on a smooth plane surface. You will observe that the sucker stick to the surface [see figure (b)]. This is because 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. Now try to pull it off the surface. You have to apply a large force to Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure overcome the atmospheric pressure. Conclusion : The magnitude of atmospheric pressure is very large. In fact, it would not be possible for any human being to pull the sucker off the surface if there was no air at all between the sucker and the surface. (a) A rubber sucker (b) Pressing the sucker on a smooth surface Active physics 11   Physics Did you know? Otto von Guericke, a German scientist of the 17th century, inverted 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 shown below. So great is the force of air pressure that the hemispheres could not be pulled apart. Horses pulling the hemispheres Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure NCERT Basics : Class 8 Concept Map Normal Muscular Gravitational Electrostatic Magnetic force force Friction Tension force force force Contact force Non-Contact The sum of all Types of forces forces acting Net force on a body Net force = 0 Balanced force Force Pressure And Pressure Unbalanced Net force 0 force Atmospheric The equilibrium pressure Net force acting on system is zero rule Mercury barometer Effect of force Can change Can move a Can stop Can change Can change Can make stationary moving direction of shape of an an object Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure state of motion object object object object rotate   Physics Some Basic Terms 1. Frequently – Often, Regularly 2. Interaction – When two or more objects have an effect up on one another. 3. Tug-of war – A game in which two teams pull and opposite and of a rope until one drag the other. 4. Magnitude – It is a number that characterize the size of something. 5. Scalar – Quantities which have only magnitude. 6. Vector – Quantities which have both magnitude and direction. 7. Axis – A line we imagine through the middle of an object around which the object turns. 8. Compressing – To make something fill less space than usual. 9. Gradually – Little by little. 10. Resists – Try to stop something from doing something. 11. Plank – A long flat piece of wood. 12. Directly proportional – Related so that one becomes larger or smaller when the other becomes larger or smaller. 13. Inversely proportional – Two quantity behave opposite in nature. One becomes larger when other becomes smaller. 14. Fluid – A substance that can flow. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure NCERT Basics : Class 8 SOLVED EXAMPLES 1. Find the net force on the object due to Solution forces acting on the object according to Given : FA = 2FB and FB =2FC, FB = 30N diagram shown  FA = 2 × 30 = 60N 10N 30 20 kg FC = = 15N 20 N 5N 2  Net force on the block is Solution FN = 60 – (30 + 15) = 60 – 45 = 15 N Force 10 N and 20 N act in same direction on the object and 5N acts in the opposite towards right. direction. 4. Three confused sledge dogs are trying to Net force acting on box pull a sled across the Alaskan snow. Alutia Fn = 10 N + 20 N – 5 N = 25 N. pulls west with a force of 35 N, Seward 2. A block of weight 5 N is placed on a horizontal table. A person pushes the also pulls west but with a force of 42 N, block from top by exerting a downward and big Kodiak pulls east with a force of force of 3 N on it. Find the force exerted 53 N. What is the net force on the sledge? by block on the table. Solution 35 N 42 N 53 N The block exerts a downward force equal to its weight of the block and a pushing force of 3N downward. 3N Solution Since the forces are in opposite direction. Publishing\PNCF\2024-25\LIVE Module\SET-1\NCERT\8th\Physics\1_Force and Pressure  Net force on the sledge is Fn = (35 N + 42 N) – 53 N = 24N 5N 5. If an object exerts a force of 45 N over a Fn = 3N + 5 N = 8 N in downward direction. surface of dimension 15 m × 20 m, what is 3. Three forces A, B and C are acting on a the pressure exerted by the liquid? heavy box as shown in figure Solution B Given, Force = 45 N A C Area = 15m × 20m = 300 m2 Force A is twice of B while B is twice as Force 45N 0.45 Pressure= = 2 = = 0.15 Pa. compare to that of C. If force B is 30 N, Area 300m 3 then find the resultant force on the box.   Physics 6. The given figure shows a statue. It exerts a pressure of 340 Pa on the floor. What is 4m the force exerted by the statue if base area 1m of its one foot is 17 cm2 ? 2m Solution Given; Force = 20 N Block dimensions = 2 m × 1 m × 4 m Pmin. =? Pmax. Solution To find the maximum pressure, the area Given; pressure = 340 Pa should be minimum and vice versa. Base area = 17 cm2 = 17 × 10– 4 m2. A1 = 2m × 1m = 2m2 Weight is a force that statue exerts A2 = 1m × 4m = 4m2 downward. A3 = 4m × 2m = 8m2 Force F = 20 N. So, Pressure = Area Pmin. F A =  min. Pmax. A max. F Force 340 Pa = A min. A 2 1 17  10−4 m2 = = 1 = = = 1 : 4. A max. A3 8 4 340  17 F= = 0.578 N 9. Calculate the pressure produced by a 10000 force of 800 N acting on a rectangular