1. Force-
ü A push or pull exerted by an object on another is a force.
ü Force arises due to the interaction between at least two objects.
ü Force has both Magnitude and Direction
ü Force applied on an object in the same direction add to one another.
ü If the two forces act in the opposite direction on an object, the net force acting on it is the difference between the two forces.
ü In general, more than one force may be acting on an object at any given point. However, a force acting on an object is always the mean net force acting on it.
2. Effect Of Force-
ü A Force can Change the State of Motion
ü A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
ü Force acting on an object may bring a change in its state of motion or a change in its shape.
ü To move a tyre faster it has to be pushed repeatedly.
3. Magnetic Force-
ü The force exerted by a magnet to pull/push a metallic object is called magnetic force.
ü A magnet can exert a force without being in contact with it. The force exerted by a magnet is an example of a non-contact force.
4. Thrust-
ü The force that acts on a surface in a direction perpendicular to it, is known as thrust.
5. Pressure-
ü The force acting on per unit area, applied to an object in the direction perpendicular to the surface is called pressure.
ü Thrust per unit area is called pressure. So, pressure = force/area on which it acts. Liquids exert pressure on the walls of the container in which they are kept.
ü Gases exert pressure in all directions.
6. Atmospheric Pressure-
ü The pressure exerted by atmospheric air around us is known as atmospheric pressure.
ü Atmospheric pressure is defined as the pressure exerted on a surface by the weight of air above that surface.
7. Contact Force-
ü A force that can be applied only when it is in contact with an object is called a contact force. For example, hammering a nail.
8. Electrostatic Force-
ü A force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
ü When a charged body, either having a (+) or (-) charge, exerts force on another charged or uncharged body, that force is known as electrostatic force.
9. Friction-
ü The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.
10. Gravitational Force-
ü The force exerted by the earth to pull the objects towards itself is called the force of gravity.
ü Every object in the universe, whether small or large, exerts a force on every other object. This force is called the force of gravity or just gravity.
NCERT PRACTICE QUESTIONS
1. Give two examples each of situations in which you push or pull to change the state of motion of objects
Ans.- Examples of situations in which you push or pull to change the state of motion of objects.
1. Pull-
ü In order to open a drawer, we have to pull it. This action changes the state of motion of the drawer.
ü To draw water from the well, the rope is pulled. This action changes the state of motion of the bucket.
2. Push-
ü A football is pushed by the foot of a player. This action changes the state of motion of the ball.
ü In order to change the place of the heavy box from one room to another, we have to push it. This action changes the motion of the box.
2. Give two examples of situations in which applied force causes a change in the shape of an object.
Ans.- The forces which change the shape of an object are as follows-
ü By pressing the clay between the hands, it deforms.
ü The shape of the plastic bottle changes by squeezing it.
3. 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 (b) attracts (c) pull or push (d) repels
4. 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. (b) muscular (c) contact (d) friction
5. 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.- a) We make a muscular force to extract the juice of the lemon by squeezing it. As a result, the shape of the lemon gets changed.
b) To take out paste from the toothpaste tube we use our muscular force. This muscular force acting on the toothpaste tube leads to a change of its shape.
c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.
d) The feet of the athlete exert the muscular force on the ground, which pushes the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.
6. 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.- A blacksmith uses his muscular force while hammering a hot piece of iron. The muscular force changes the shape of the iron so that it can be given the desired shape.
7. 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 rubbed with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body. When this charged balloon is pressed against a wall, it sticks to the wall. Thus, the electrostatic force acts between the charged balloon and the wall.
8. 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.- For holding the bucket of water above the ground, we use muscular force. This muscular force acts against the force of gravity that pulls the bucket towards the ground. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in the state of motion.
9. 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.- The force of gravity is the one which acts on the rocket to pull it towards the ground and the other one is the force of friction due to earth’s atmosphere, which opposes its motion.
10. 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.- a) Due to the atmospheric pressure, there is a rise of water in the dropper. When all the air escapes from the nozzle, the atmospheric pressure which is acting on the water forces the water to fill the nozzle of the dropper.
Junior Science Topic - Force And Pressure. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 11)
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