 # Chapter 11 Force and Pressure

• Force is simple called as push or pull of the body.
• Interaction between two surfaces is must for force to come into play.
• Magnitude is called as the strength of the force.
• If two forces applied on the same direction to the body then the force are ready.
• If two forces applied in the different directions then the net force acting on the body is the difference between two forces.
• Force brings change in the state of motion of an object.
• Force brings change in the shape of an object.
• Force come in play when two bodies comes in contact with each other called as contact force.
• Muscular force may be defined as the force which is due to the action of muscles.
• Friction is the opposing force which can change in the state of the motion of the body.
• Force exert by the magnet is the example of non- contact force.
• Force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
• Earth attracts things towards itself is known as gravity.
• Gravitational force is the force exerted by the every small things on the earth.
• Force per unit area is known as pressure.
• Gases, liquid exerts pressure on the walls of their container.
• Pressure exerted by the air is known as atmospheric pressure.

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### Exercise 1 ( Page No. : 144 )

•  Q1 Give two examples each of situations in which you push or pull to change the state of motion of objects. Ans: Force is simple called as push or pull. Examples of push: Pushing a car. Inserting a plus in socket. Close the door Pushing a trolley. Kicking a ball. Examples of pull force are as follows: Opening of door. Horse pulling the cart. Drawing a bucket of water from the well. Plucking the string of the guitar. Dog pulls its piece of the meat towards itself. Q2 Give two examples of situations in which applied force causes a change in the shape of an object. Ans: By applying force, shape of the object got changed. Some examples are as follows: Change in the shape of dough occurs by applying force. By using muscular force, we can change the shape of a plastic ball by squeezing it. Deformation of clay by pressing it in between the hands. Elongation and contraction of a rubber band when force is applied on it. Q3 Fill in the blanks in the following statements: (a) To draw water from a well we have to ——— at the rope. (b) A charged body ———— an uncharged body towards it. (c) To move a loaded trolley we have to ———— it. (d) The north pole of a magnet ———— the north pole of another magnet. Ans: (a) Pull Pull or push is simply called as force. To draw water from a well, we have to pull whereas we have to apply push force on bucket to throw it in the well. (b) Attracts When a charged body attracts another charged or uncharged body towards it, is called as the electrostatic force of attraction. (c) Push If we want to change in the direction of trolley, we have to apply either push force or pull force on it. In the pull force, the direction changes in object is same to the applying force. (d) Repels Magnet have two poles: North and South. Same pole repels each other whereas opposite poles of magnet attracts each other. Q4 An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms: muscular, contact, non-contact, gravity, friction, shape, attraction (a) To stretch the bow, the archer applies a force that causes a change in its ——. (b) The force applied by the archer to stretch the bow is an example of ——— force. (c) The type of force responsible for a change in the state of motion of the arrow is an example of a ———— force. (d) While the arrow moves towards its target, the forces acting on it are due to ———— and that due to ———— of air. Ans: (a) Shape Applied force can change the shape and the direction of the object. So, when we stretch a bow, pull force is applied on it, so its shape is changed. (b) Muscular Stretching of bow is the example of a contact force. In this, muscular force is applied on it. With the help of muscular force, we stretch the bow. (c) Contact Contact force are the forces that act between two objects that are physically touching each other. Here, we touch bow physically, this is an example of contact forces. (d) Gravity, friction The friction force is applied by the air is known as the drag force. When arrow moves towards the target, earth attracts the arrow towards itself due to gravity and the frictional force of air also come into play. Q5 In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case. (a) Squeezing a piece of lemon between the fingers to extract its juice. (b) Taking out paste from a toothpaste tube. (c) A load suspended from a spring while its other end is on a hook fixed to a wall. (d) An athlete making a high jump to clear the bar at a certain height. Ans: In squeezing a piece of lemon, the muscular force is used to extract juice from it. It is an example of contact force. Our muscles apply force on lemon, then due to squeezing its shape got changed. In taking out paste from the toothpaste tube, we use muscular force here. Hence its shape got changed due to applied muscular force. Here, push force is applied by the load on the spring. Due to it, spring goes downward. Hence, the spring is stretched and the shape of the spring is change due to the applied push force. Here, athlete applies push force on the ground with his feet. By doing this, his muscular feet feels some muscular force on the ground. Due to this, he jumps over the bar. Change in the motion of the athlete occurs by applying push and muscular force. Q6 A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron? Ans: When a blacksmith hammers a hot piece of iron, he uses its muscular force on the hot piece of iron. This is the example of contact force also. By applying muscular force, blacksmith is able to change in the shape of the hot iron piece. Q7 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? Ans: When an inflated balloon is rubbed with a piece of synthetic cloth, then it becomes charged. The balloon sticks to the wall because of the attractional force present between them. When a charged body attracts an charged or uncharged body towards itself then it is known as electrostatic force of attraction. Hence, the balloon sticks on the wall due to the presence of electrostatic force of attraction between the balloon and the wall. Q8 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. Ans: Here, muscular force is acting on the plastic bucket containing water held above the ground level. Muscular force is applied in the opposite direction of gravity; here which pulls the bucket towards the ground. Here both muscular and gravity are equal in magnitude. So, both the forces cancel out each other. Here the net force on the bucket is zero. Hence there is no change in the state of the motion of the bucket. Q9 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. Ans: When a rocket is fired upwards to launch a satellite in its orbit, then the force of gravity is acting on it. The force of gravity pulls the rocket towards the earth but the force of friction which is due to the earth’s atmosphere, opposes its motion. Q10 When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to (a) pressure of water (b) gravity of the earth (c) shape of rubber bulb (d) atmospheric pressure Ans: (d) An atmospheric pressure It is because, 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. So, rise of the water in the dropper is due to the atmospheric pressure. When all the air escapes from the nozzle, then the atmospheric pressure, act on water, forces to fill the dropper.