NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
CHAPTER 9
FORCE AND PRESSURE
KEYWORDS
Force: A push or a pull on the object.
Interact: Mutual or in between.
Magnitude : Expression for strength of force.
Change in state of motion : Change in either the speed or direction of an object.
Muscular force : Force resulting due to the action of muscles.
Contact force : Force applied an object when it is in contact.
Friction: Opposing force between two bodies when they are contact and both or either one is moving.
Electrostatic force : Force exerted by a charged body.
Force of gravity: Force exerted by earth.
Pressure: Force acting on a unit area of a surface.
Atmospheric pressure: The pressure exerted by air.
TEXT BOOK EXERCISES
Q.1. Give two examples each of situations in which you push or pull to change the state of motion of objects.
Ans.
Example of pull:
(1) Pulling of a suitcase,
(ii) Pulling of a cow by a man.
Example of push :
(1) Goalkeeper dives or jumps up to push a moving ball due to which he can stop or deflect the ball.
(ii) Pushing of a bicycle to move it.
Q.2 Give two examples of situations in which applied force causes a change in the shape of an object.
Ans. (i) On applying a force on an inflated balloon by pressing it between palms, we can change the shape of balloon.
(ii) When we stretch the rubber-band, then its shape is changed.
Q 3. Fill up the gap to complete 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) - shape;
(b) - attracts;
(c) - push;
(d) - repels.
Q.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 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) - gravity, friction.
Q.5. In the following situations identify the agent exerting a force and the object on which it acts. State the effect of force is observable in each case.
(a) Squeezing a piece of lemon between the fingers to extract it 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 taking a high jump to clear the bar at a certain height.
Ans. (a) Force is exerted by fingers on the lemon, due to which the shape of lemon changes and its juice expells.
(b) Force is exerts by fingers on toothpaste tube due to which toothpaste comes out from tube.
(c) Force is exerted by load on the spring due to which spring stretches.
(d) Force is exerted by body (muscles) on the earth, due to which athlete becomes able to clear the bar.
Q.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. Due to hammering, the shape of piece of iron changes.
Q.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. Name the force that might be responsible for attraction between the balloon and the wall.
Ans. The balloon sticks to the ball due to the electrostatic force because on rubbing the balloon with a piece of synthetic cloth, the balloon becomes charged and charged body attracts another uncharged body.
Q.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. Forces acting on a plastic bucket
(i) Force of gravity acting downwards.
(ii) Muscular force of arms acting upwards.
The two forces do not bring any change in state of motion of bucket because both forces are equal and act in the opposite direction and thus cancel each other's effect.
Q.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. (i) Gravitational force (downwards) (ii) Force of friction Besides, these propelling force of gases emitted by combustion of fuels also acting on it upwards.
Q.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
(1) pressure of water
(2) gravity of the earth
(3) shape of rubber bulb
(4) atmospheric pressure
Ans. (4) atmospheric pressure.
ADDITIONAL SHORT ANSWER TYPE
Q.1. Discuss the effect of force on speed of a moving body.
Ans. When a force is applied on an object, it may change its speed. If the applied force is in the direction of motion, the speed of the object increases. If the force is applied in the direction opposite to the motion, then it results in a decrease in the speed of the object.
Q.2. What is the net force of two forces acting in opposite direction?
Ans. 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.
Q.3. Define the state of motion.
Ans. 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 state of motion. Any change in the speed, or direction of motion, or both, implies a change in the state of motion of the object.
Q.4. Give two examples of contact forces?
Q.5. What do you understand by electrostatic force? What is non- contact force.
Q.6. Define force of gravity. Is it repulsive or attractive force?
Ans. Objects or things fall towards the surface because the earth pulls them towards its centre. This is the force of gravitation exerted by the earth. It is usually referred as the force of gravity. This is an attractive force. The force of gravity acts on all objects on or near the surface of the earth. The force of gravity acts on all of us all the time without our being aware of it.
Q.7. Define pressure.
Ans. The force acting on a unit area of a surface is called pressure. To find pressure, therefore, we need to divide force by the surface area on which it acts.
So, pressure=force/area on which it acts
Q.8. What is atmospheric pressure ?
Ans. Our earth is surrounded by an air column. This air column like liquid columns exerts pressure on all the objects on earth. This
pressure exerted by air of the atmosphere is called atmospheric pressure.
Q.9. One atmospheric pressure is equivalent to how much kilogram?
Ans. The atmospheric pressure is equivalent to a weight of 103000 kg on each square metre near the surface of the earth.
Q.10. Atmospheric pressure is so great why are we not crushed by it ?
Ans. Our body and the bodies of other organisms are made up of cells which have fluids that exert pressure from within. This pressure exerted from the inside of the cell is equal to the atmospheric pressure and prevents us from being crushed.
Q.11. Write some properties of force.
Ans. The properties of a force as under :
A force
(i) may make an object move from rest.
(ii) may change the speed of an object, if it is moving
(iii) may change the direction of motion on an object.
(iv) may bring about a change in the shape of an object.
(v) may cause some or all of these effects.
Q.12. What is muscular force? Why is it called contact force ?
Ans. The force resulting due to the action of muscles is known as the muscular force. Muscular force is called contact force because it comes in action on when two bodies come in contact of each other.
Q.13. How do we feel force in our daily life?
Ans. Many big or small actions make us feel the force. We have to push or pull many objects daily. A moving ball stops on its own, the ball changes the direction of its motion, when hits with a bat. We churn curd to make Lassi and many other actions.
Q.14.On what principle does a fountain pen with a pump work?
Ans. Principle: Atmospheric pressure
Explanation: On pressing the rubber tube there is an empty space i.e., gases are released. As the atmospheric pressure acts on each and every object, due to that the ink is pushed up to fill the Space when the rubber tube is released slowly.
Q.15. How gases exerts pressure on the walls of the container?
Ans. When we fill air in a balloon, the air exerts pressure on the inner walls of the balloon. The pressure of the air inside the balloon stretches it. This shows that the gases exert pressure on the walls of the container.
LONG ANSWER TYPE QUESTIONS
Q.16. What are contact forces? State different contact forces.
Ans. The forces which come in play only when two objects come in contact with each other are called contact forces.
Some contact forces are:
(i) Muscular force: The forces resulting due to the action of muscles are known as muscular forces. Muscular force is a contact force because it comes in play when two bodies come in contact with each other. for example: We apply muscular force to pull or push an objects.
(ii) Force of friction: When a body is moving, then a force of equal and opposite direction of motion is exerted on that moving body. This force is called force of friction. It is also a contact force because it exerts when two surfaces come in the contact with each other. for example: A marble rolled on ground stops after sometime due to the frictional force.
Q. 17. What are non-contact forces'? Explain different types of non- contact forces.
Ans. The forces, which can be exerted from a distance, without establishing a contact are called non-contact forces. Some non- contact forces are:
(i) Magnetic force: The force exerted by a magnet on other magnet or some other magnetic substances, like; iron is called magnetic force. Like; poles of a magnet repel each other and unlike poles of a magnet attract each other without contact. So, it is called non-contact force. for example: A magnet attracts iron or can move small iron pins.
(ii) Electrostatic force: The force exerted by a charged body on other charged or uncharged body is called electrostatic force. Electrostatic force also acts without making a direct contact with other charged or uncharged body. So, it is also a non-contact force. for example: When an inflated balloon rubbed with a piece of synthetic cloth, it sticks to the wall.
(iii) Force of gravity : Earth pulls everything or body towards it. The force of alternation exerted by earth on any objects is called force of gravity. This is also a non-contact force as it acts from a distance. for example: An object dropped from a certain height falls on the earth due to gravity.
Q.19.Show experimentally that pressure increases with the depth.
Ans. Take a deep vessel having atleast three holes as shown in Fig. Fill it with water. We shall find that the jet of water from the lowest hole reaches the farthest. This proves that at the lowest hole, the force or pressure is maximum.
Q.20. Explain that liquids exert equal pressure at the same depth.
Ans. Take an empty plastics bottle. Drill four holes all around near the bottom of the bottle. Make sure that all the holes are at the same height from the bottom. Now fill the bottle with water. We observe that different streams of water coming out of the holes fall at the same distance from the bottle. This observation indicates that liquids exert equal pressure at the same depth.
0 Comments