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FORCE AND LAWS, OF ‘MOTION, , hen we want to open a-door, we have to push the door handle. And when we want to close the, Wse= we have to pull the door handle with our hand. This means that to move a body (or an, object), it has.either to be pushed or pulled. A push or pull on a body is called force. The, direction in which a body is pushed or pulled is called the direction of force. We open or close a door by, applying force. Now, when we push the door to open it, we apply a force on the door in a direction away, from us. And when we pull the door to close it, then we exert a force on the door in a direction towards us., , , , , , , , Figure 1. A push or pull.on an object is called force., , Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing., For example, a force is used when we push (kick) a football; a force is used when we pull the drawer of a, table ;.a force is used when we lift a box from the floor ; a force is used when we stretch.a rubber band ; a, force is used when we twist a wet cloth to squeeze out water ; and a force is used when we press the brake, pedal of a car. The fallen leaves of trees fly away with wind because the force of wind pushes them away., Even the roofs of some huts fly away during a storm because the force of strong winds pushes them away., And when we fly a kite, we can actually feel the force (or push) of the wind on it. We will now describe the, effects of force.

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46 SCIENCE FOR NINTH CLASS : PHYSICS, , , , (a) Weightlifter pulling the weights (6) Weightlifter pushing the weights, Figure 2. This weightlifter first exerts a ‘pull’ on the weights and then a ‘push’ on the weights (so as to lift, them up). A push or pull on a body is called force. So, this weightlifter is exerting force on the weights., Effects of Force, , A force cannot be seen. A force can be judged only by the effects which it can produce in various, bodies (or objects) around us. A force can produce the following effects :, , , , 1. A force can move a stationary body., , 2. A force can stop a moving body., , 3. A force can change the speed of a moving body., , 4. A force can change the direction of a moying body., 5. A force can change the shape (and size) of a body., , We will now give examples of all these effects produced by a force when it acts on a body (or an, object)., , If we kick a football kept on the ground with our foot, then the, football starts moving (see Figure 3). In this case, the force of our, foot moves a stationary football. Similarly, the force of engine can, movea stationary car. From these examples we conclude that a, force can make a stationary body move. It is a common observation, that a football moving on the ground stops after some time. In this, case, the force of friction of ground stops the moving football., Similarly, the force of brakes can stop a moving car. From these, examples we conclude that a force can make a moving body stop., , Suppose we are moving on a bicycle at a certain speed. Now, if, someone pushes the moving bicycle from behind, then the speed, of bicycle increases and it will move faster. On the other hand, if Figure 3. When a player kicks the football,, someone pulls the moving bicycle from behind, then the speed of his foot exerts a force on the football. This, bicycle decreases and it will move slower. Thus, a push or pull can force causes the football to move:, change the speed of a moving bicycle. But a push or pull is called force. So, we'can say that a force can, change the speed of a moving bicycle (or any other moving body). If the force is applied in the direction, of motion of a body, its speed increases. On the other hand, if the force is applied in the direction opposite

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FORCE AND LAWS OF MOTION 47, , to the direction of motion of a body, then its speed decreases. Let us take another example. When a ball is, dropped from a height, its speed goes on increasing. The speed of a falling ball (or any other falling, body) increases because the earth applies .a pulling force on it which is called the force of gravity. It is, the force of gravity of the earth which pulls a falling ball towards its centre and increases its speed. On the, other hand, when a ball is thrown upwards, then its speed goes on decreasing. This is because the earth, applies a pulling force of gravity on the ball in the downward direction (opposite to the motion of the ball)., , In a tennis match, when a moving tennis ball is hit by a, racket, then the direction of tennis ball changes and it goes, in a different direction (see Figure 4). In this case, the force, exerted by the tennis player’s racket changes the direction, of a moving tennis ball. Similarly, in a cricket match, when, a moving cricket ball is hit by a bat, then the direction of, cricket ball changes and it goes in another direction. In this, case, the force exerted by the cricket player’s bat changes, the direction of a moving cricket ball. In the game of carrom,, when we take a rebound, then the direction of striker, changes. This is because the edge of the carrom board exerts, a force on the striker. If we blow air from our mouth on the Figure 4. The force from a tennis player’s racket can, smoke rising up from a burning incense stick (agarbatti), then Change the direction of motion of the tennis ball, the direction of motion of smoke changes. In this case, the force exerted by the blowing air changes the, direction of moving smoke. From these examples we conclude that a force can change the direction of, motion of a moving body., , , , , , If we take a light spring and pull it at both the ends with our hands, then the shape-and size of the, , spring changes (see Figure 5). The turns of the spring become farther apart and its length increases. In this, case, the force of our hands changes the shape and size, of the spring. Here are some more examples in which AMM», a force changes the shape (and size) of an object. The, shape of dough (kneaded flour) changes on pressing, with a rolling pin (belan) to make chapatis. When we, press the dough with a rolling pin, we apply force. So,, we can say that the shape of dough-changes on, applying force. The shape of kneaded wet clay (geeli, mitti) changes when a potter converts it into pots of, different shapes and sizes. This happens because the, potter applies force on the kneaded wet clay. The shape, of a tooth paste tube (or an ointment tube) changes when we squeeze it because we apply force while, squeezing it. Similarly, the shape of a sponge, tomato, balloon, rubber ball or tennis ball changes on pressing., And the shape and size of a rubber band changes.on stretching. From all.these examples we conclude that, a force can change the shape and size of a body (or object)., , (a) Original shape and size of spring, , , , (4) Force (of pulling) changes the shape and size of the, spring, Figure 5., , We can now define force as follows : A force is an influence which tends to set a stationary body in, motion or stop a moving body ; or which tends to change the speed and direction of a moving body ; or, which tends to change the shape (and size) of a body. We will now discuss the various types of forces., , BALANCED AND UNBALANCED FORCES, , Forces are of two types : Balanced forces and Unbalanced forces. We will now discuss balanced and, unbalanced forces in detail, one by one. Let us start with balanced forces:

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hy, , 48 SCIENCE FOR NINTH CLASS : PHYSICS, , Balanced Forces, (0, , If the resultant of all the forces acting on a body is zero, the Force of reaction, (Exerted by ground), , forces are called balanced forces. A body under the action of, balanced forces does not change its position of rest (or of uniform, motion) and it appears as if no force is acting on it. This point will, become more clear from the following example., , Force of, our push, , Suppose a heavy box is lying on the ground (Figure 6). Let us, push this box with our hands. We find that the box does not move, (and remains in its state-of rest) though as many as four forces are, acting on it. The four forces acting on the box are:, , F Force of gravity, (i) Force of our push (Weight of box), , Figure 6. When balanced forces act on a, body (here a heayy box), they do not, (iii) Force of gravity (which pulls the box downwards) produce any motion in it., , , , (ii) Force of friction (which opposes the push and does not allow, the box to move), , (iv) Force of reaction (exerted-by the ground on the box upwards which balances the force of gravity), , Now, though the box is at rest, four forces are acting on it. Since the box does not move at all, we, conclude that the resultant of all the forces acting on it is zero. The box, therefore, behaves as if no force is, acting on it. The forces acting on this stationary box are an example of balanced forces. Please note that the, force of our push on.the box is balanced by the force of friction, and the force of gravity is balanced by the, force of reaction of the ground. Similarly, when we hold a suitcase steady at some height from the ground,, the resultant force acting on the suitcase is zero and it does not change its position. Again, ina tug of war,, that is, in rope pulling between two teams, if the resultant of forces applied by the two.teamis is zero, the, rope does not move in either direction. The forces exerted by the two teams are balanced. From this, discussion we conclude that if a number of balanced forces act on a stationary body, the body continues, to remain in its stationary position. Similarly, if amumber of balanced forces act on a body in uniform, motion, the body continues to be in its state of uniform motion., , , , Figure 7, In a tug of war (or rope pulling), when the Figure 8. When a balloon is pressed between hands, then, forces exerted by the two teams on the rope are balanced balanced. forces (equal and opposite forces) act on balloon, , (equal and opposite), then the rope does not move in que to which the shape of balloon changes., either direction., , Though balanced forces cannot produce motion in a stationary body or stop a moving body, they, can, however, change the shape of the body. An example of the balanced forces changing the shape of a, body is in the squeezing of a rubber ball or balloon. When we press a rubber ball or a balloon between our, two hands, the shape of rubber ball or balloon changes from spherical to oblong. In this case we apply two

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FORCE AND LAWS OF MOTION 49 }, equal and opposite forces (balanced forces). with our hands. Though the ball or balloon does not move, its, shape changes. We will now discuss the case of unbalanced forces., , Unbalanced'Forces (Ey, , If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces., When unbalanced forces act on a body, they produce a change in its state of rest or of uniform motion. That, is, unbalanced forces can move a stationary body or they can stop a moving body. In other words,, unbalanced forces acting on a body can change its speed or direction of motion. This point will become, more clear from the following example., , Force of reaction, (Exerted by ground), , Suppose a toy car is lying on the ground (see Figure 9). Let us, push this car with our hand. We find that the toy car starts moving., Now, in this.case-also four forces are acting on the toy car. These are :, , , , (i) Force of our push Force of Force of, , (ii) Force of friction, (iii) Force of gravity, , (iv) Force of reaction of ground Force of gravity, , (Weight of toy car), Figure 9. An unbalanced force of our, push produces motion in the toy car., , In this case also, the force of gravity on the car acting downwards, and the force of reaction of ground acting upwards are equal and, opposite, so they balance each other. Now, due to the wheels of the, toy car, the opposing ‘force of friction’ is much less here. The force of our push is, therefore, greater than, the force of friction in this case, so they cannot balance each other. Thus, the resultant of all the forces, acting on the toy car is not zero. There is a net unbalanced force acting on the toy car which makes the car, move from its position of rest. The toy car moves in the direction of greater force (which is the direction of, our push). Thus, to move a stationary object, we have to push it with a force greater than the opposing, force of friction., , Please note that even the heavy box shown in Figure 6 can be moved if pushed with a very strong force, (by more than one person). This is because in that case the force of push will become greater than the, opposing force of friction. An unbalanced force will then act on the heavy box and make it move., , When we are holding a suitcase above the ground, then the force of gravity acting on the suitcase is, balanced by the upward force of our-hands. Now, if we release the suitcase from our hand, then the, unbalanced force of gravity acts on it and the suitcase falls to the ground. In this case the force of gravity, , , , Figure 10. Ina tug of war (or rope pulling), if one Figure 11, If there were no unbalanced force of friction, of the teams suddenly releases the rope, then an and air resistance, then a-moving bicycle would go on, unbalanced force acts on the other team due to moving for ever (without stopping)., , which it falls backwards.