Force and preesure
- 1. SELF LEARNING MATERIAL ON FORCE AND PREESURE MADE BY: PALLAVI GHOSH ID NO.14162087 B.ED(SCIENCE)
- 2. CONTENTS: Force are due to interaction Exploring force State of Motion Pressure KEYWORDS ATMOSPHERIC PRESSURE CONTACT FORCE ELECTROSTATIC FORCE FORCE MAGNETIC FORCE MUSCULAR FORCE NON-CONTACT FORCE PRESSURE PULL PUSH
- 3. As you already know how objects move in the previous year, ok you all know how a ball moving 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? How do fielders stop a ball hit by a batsman? A hockey player changes the direction of the moving ball with a flick of the stick (Fig. 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. Ok now tell me 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. Force means A Push or a Pull Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling are often used to describe certain tasks. Each of these actions usually results in some kind of change in the motion of an object. Fig.: (a) A goal keeper saving a goal (b) A hockey player flicking a ball (c) A fielder stopping a ball ACTIVITY: here is an activity which help you to understand the push or pull ,which one is called push & pull Identifying Actions as Push or Pull S.NO. Description of the situation Action can be grouped as a Push Pull
- 4. 1 Moving a book placed on a table Yes No Yes No 2 Opening or shutting a door Yes No Yes No 3 Drawing a bucket of water from a well Yes No Yes No 4 A football player taking a penalty kick Yes No Yes No 5 A cricket ball hit by a batsman Yes No Yes No 6 Moving a loaded cart Yes No Yes No 7 Opening a drawer Yes No Yes No have you notice that each of the actions can be grouped as a pull or a push or both this, that 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? We have learnt in Class VI 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? Forces are due to an Interaction Suppose a man is standing behind a stationary car in this fig. 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. Who is pushing whom Who is pulling whom ?
- 5. shows two situations that may be familiar to you. Can you decide who is pulling and who is pushing in these cases? both the girls appear to push each other while the pair of girls in are trying to pull each other. The girls in the two situations shown here are applying force on each other. The next one is Exploring Forces: Have you ever seen a game of tug-of war? In this game two teams pull at a rope in opposite directions see in the Fig. Members of both the teams try to pull the rope in their direction. Sometimes the rope simply does not move. Is it not similar to the situation shown in Fig 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 see in the next fig what happened when you and your friend pushed the heavy box in the same direction
- 6. In this fig we have seen two boys pushing the table 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 this fig when both of you were pushing the heavy box from opposite direction it mean that the net force on an object is zero if the two forces acting on it in opposite directions are equal So, we learn that a force could be larger or smaller than the 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. 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. Fill in the blanks: Force could be ………………………….. A force arises due to the ………………. between two objects. Force has ……………..as well as direction. . A Force can Change the State of Motion The another activity which shows 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?
- 7. 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 A ball at rest begins to move when a force is applied on it A ball set in motion by pushing it along a level surface and (b) the direction of motion of the ball after it strikes the ruler placed in its path 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. 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 speed. An object may be at rest or in motion; both are its states of motion 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. Force can Change the Shape of an Object Activity Some situations have been given in Column 1 of Table 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
- 8. 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 What do you conclude from the observations noted in Table 2? 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. may cause some or all of these effects. Table 2: Studying the Effect of Force on Object Description of situation How to apply force Diagram Action of force Change in state of motion Change in shape Yes/No Yes/No A lump of dough on a plate Pressing it down with your hands Yes No Yes No Spring fixed to the seat of a bicycle By sitting on the seat Yes No Yes No
- 9. A rubber band suspended from a hook/nail fixed on a wall By hanging a weight or by pulling its free end Yes No Yes No A plastic or metal scale placed between two bricks By putting a weight at the centre of the scale Yes No Yes No 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. Contact Forces:-Muscular Force 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 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. 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.
- 10. This is the example of muscular force. Muscular force of animals is used to carry out many difficult tasks Non-contact Forces Magnetic Force Take a pair of bar magnets. Place the longer side of one of the magnets over three round shaped pencils or wooden rollers as shown in Fig. 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. : Observing attraction and repulsion between two magnets
- 11. In your mind so many question was arise like 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? In class vi you learn 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. Electrostatic Force:- Activity:- 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 Fig. 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? 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.
- 12. The force exerted by a charged body on another charged or uncharged body is known as electrostatic force Pressure:- You have learnt in Class VII that strong winds during a storm or a cyclone can blow away even the roof-tops. You also learnt that winds and cyclones are caused by the differences in air pressure. Is there any relation between pressure and force? Let us find out. 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 (Fig. Could you do it this time? Fig. Pushing a nail into a wooden plank Try cutting vegetables with a blunt and a sharp knife. Which is easier? Do you get the feeling that the area over which the force is applied (for example, the pointed end of the nail) plays a role is making these tasks easier? The force acting on a unit area of a surface is called pressure. The force acting on a unit area of a surface is called pressure. pressure = force / area on which it acts 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. Pressure exerted by Liquids and Gases Activity Take a transparent glass tube or a plastic pipe. The length of the pipe/ tube should be about 15 cm and its diameter should be 5-7.5 cm. 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.
- 13. Hold the pipe at the middle, keeping it in a vertical position (Fig. 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? Fig. : Pressure exerted by water at the bottom of the container depends on the height of its column Activity 9 Take a plastic bottle. You can take a discarded water or soft drink bottle. Fix a cylindrical glass tube, a few cm long near its bottom as shown in Fig. 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 above Activity . 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? Fig. : A liquid exerts 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 . Activity
- 14. Take an empty plastic bottle or a cylindrical container. You can take a used tin of talcum powder or a plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom (Fig. Now fill the bottle with water. What do you observe? Does the water coming out of the holes falls at the same distance from the bottle? What does this indicate? Fig. : Liquids exert equal pressure at the same depth 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. Atmospheric Pressure 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 Otto von Guericke, a German scientist of 17th century, invented a pump to extract air out of the vessel. With the help of this pump, he demonstrated dramatically the force of the air pressure. He joined two 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.So great is the force of air pressure that the hemispheres could not be pulled apart. Horses pulling the hemispheres EXERCISE
- 15. Give two examples each of situations in which you push or pull to change the state of motion of objects. Give two examples of situations in which applied force causes a change in the shape of an object. Fill in the blanks in the following statements: o To draw water from a well we have to at the rope. o A charged body an uncharged body towards it. o To move a loaded trolley we have to it. o The north pole of a magnet the north pole of another magnet. 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 o To stretch the bow, the archer applies a force that causes a change in its . o The force applied by the archer to stretch the bow is an example of force. o The type of force responsible for a change in the state of motion of the arrow is an example of a force. o While the arrow moves towards its target, the forces acting on it are due to and that due to of air. 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. S.No Option 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.
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