Q1. Define force. Why is it called a push or a pull?

Answer: Force is an interaction which, when unbalanced, can change the state of motion of an object. It is called a push or a pull because whenever we apply force, we either push an object away from us or pull it towards us.

Q2. Explain with an example how interaction of two objects gives rise to a force.

Answer: When we kick a football, our foot and the ball come in contact and interact. This interaction produces a force that makes the ball move. Thus, force arises due to interaction between foot and ball.

Q3. Why is at least two objects necessary for force to act?

Answer: A single object cannot apply force on itself. Force always involves a source (object applying the force) and a receiver (object on which force is applied), e.g., hand and book. Therefore, at least two objects are needed for a force to act.

Q4. State two quantities needed to describe a force completely.

Answer: To describe a force completely, we must know:
(i) Its magnitude (how strong it is), and
(ii) Its direction (the line along which the force acts).

Q5. Why is force called a vector quantity?

Answer: Force is called a vector quantity because it has both magnitude and direction, and both are important to describe its effect. Changing either magnitude or direction changes the force.

Q6. What do you understand by the “state of motion” of an object?

Answer: The state of motion of an object tells whether it is at rest or in motion. For a moving object, it includes its speed and direction of motion. A change in speed or direction means change in state of motion.

Q7. Give the SI unit of force. What does 1 unit of this force represent?

Answer: The SI unit of force is newton (N). A force of 1 N is the force that produces an acceleration of 1 m/s² in a body of mass 1 kg.

Q8. Give two daily life activities where more than one force acts on an object at the same time.

Answer: Examples:
(i) A book kept on a table experiences the weight of the book (downwards) and the upward reaction of the table.
(ii) A person standing in a lift experiences gravitational force downwards and the normal reaction from the lift floor upwards.

Q9. List any four effects that a force can produce on an object.

Answer: A force can:
(i) Change the state of rest or motion of an object,
(ii) Increase or decrease the speed of an object,
(iii) Change the direction of motion,
(iv) Change the shape or size of an object.

Q10. Describe with an example how force can change the speed of a moving object.

Answer: When we pedal a bicycle, muscular force acts in the direction of motion. This increases the speed of the bicycle. Applying brakes produces frictional force opposite to motion and reduces the speed.

Q11. How can force change the direction of motion of an object? Explain with an example.

Answer: In cricket, a bowler throws the ball toward the batsman, and it is moving in one direction. When the batsman hits it with the bat, he applies force in a different direction and the ball changes its direction of motion.

Q12. Explain with an example how force can change the shape of an object.

Answer: When we stretch a rubber band, the muscular force applied by our hands changes its length and shape. Similarly, pressing a sponge or kneading dough changes their shape due to applied force.

Q13. What happens to the net force when two forces act in the same direction on an object?

Answer: When two forces act in the same direction, the net force is the sum of the two forces. The object experiences a greater force and thus a larger effect in that direction.

Q14. What is the net force on an object if equal forces act in opposite directions? What is the effect?

Answer: If equal forces act in opposite directions, the net force becomes zero. In this case, the state of motion of the object does not change (it remains at rest or continues to move uniformly).

Q15. What is a contact force? Give two examples.

Answer: A contact force is a force that acts only when objects are in physical contact. Examples: muscular force used to push a trolley, friction between the floor and a box being dragged on it.

Q16. Define muscular force and mention one use of it.

Answer: Muscular force is the force exerted by the muscles of our body. It is used in activities like walking, running, pushing, lifting or carrying objects such as school bags or buckets of water.

Q17. How do animals help us using muscular force? Give two examples.

Answer: Animals like bullocks, horses and camels use their muscular force to pull carts, plough fields, and transport people and goods. Example: bullocks pulling a plough; horses pulling a tonga or carriage.

Q18. What is friction? In which direction does it act?

Answer: Friction is the force that opposes the relative motion between two surfaces in contact. It always acts in a direction opposite to the direction of motion or the intended motion of the object.

Q19. How is friction useful to us in daily life? Give any two examples.

Answer: Friction is useful because:
(i) It helps us walk without slipping, as there is friction between our feet and the ground.
(ii) It allows us to hold objects firmly, as friction acts between our fingers and the object.

Q20. Mention two disadvantages of friction.

Answer: Disadvantages:
(i) Friction causes wear and tear of machine parts, tyres, and shoe soles.
(ii) It produces heat, leading to loss of energy, for example in vehicle engines and moving machinery.

Q21. State two ways to reduce friction between moving parts of machines.

Answer: Friction can be reduced by:
(i) Using lubricants like oil or grease between moving surfaces.
(ii) Using ball bearings to convert sliding friction into rolling friction.

Q22. Why is friction called a necessary evil?

Answer: Friction is called a necessary evil because it has both advantages and disadvantages. We need friction for walking, writing, and gripping objects, but it also causes wear and tear and wastes energy as heat.

Q23. What is a non-contact force? Give two examples.

Answer: A non-contact force is a force that can act between objects even when they are not in physical contact. Examples: gravitational force of the Earth, magnetic force between two magnets, electrostatic force between charged bodies.

Q24. Define magnetic force and give one example.

Answer: Magnetic force is the force exerted by a magnet on another magnet or on magnetic materials like iron. Example: a bar magnet attracting iron nails or paper clips kept near it.

Q25. What is electrostatic force? Describe a simple activity to observe it.

Answer: Electrostatic force is the force between electrically charged objects. Activity: Rub a plastic scale or comb on dry hair and bring it near small pieces of paper. The pieces of paper get attracted, showing electrostatic force in action.

Q26. Why is electrostatic force called a non-contact force?

Answer: In electrostatic force, charged objects can attract or repel each other even when separated by some distance and not touching. Hence, electrostatic force acts without direct contact and is called a non-contact force.

Q27. What is gravitational force? Name one heavenly body that exerts this force on us.

Answer: Gravitational force is the force of attraction between any two masses. The Earth exerts gravitational force on all objects near its surface, pulling them towards its centre.

Q28. How does gravitational force help us in our daily life?

Answer: Gravitational force keeps us firmly on the ground, causes objects to fall when dropped, and helps maintain the atmosphere around the Earth. It also keeps planets and satellites in their orbits.

Q29. Compare contact and non-contact forces with one example each.

Answer: A contact force needs physical contact, e.g., friction between shoe and ground. A non-contact force acts from a distance, e.g., gravitational force of the Earth on a falling stone. Both can change motion of objects.

Q30. Why does a piece of paper get attracted towards a charged comb but fall down when comb is taken away?

Answer: The charged comb exerts electrostatic force on the paper and attracts it. When the comb is taken away, this non-contact force disappears and only gravitational force acts on the paper, so it falls down.

Q31. Define pressure. Write its formula.

Answer: Pressure is defined as the force acting per unit area of a surface. The formula is: Pressure = Force / Area or P = F / A.

Q32. Name the SI unit of pressure. How is it related to its basic units?

Answer: The SI unit of pressure is the pascal (Pa). One pascal is equal to a force of 1 newton acting on an area of 1 square metre (1 Pa = 1 N / m²).

Q33. How does pressure change when the same force is applied on a smaller area? Explain with an example.

Answer: When the same force acts on a smaller area, pressure increases. For example, a sharp knife has a thin edge with smaller area, so the same cutting force produces greater pressure and cuts easily.

Q34. Why do nail tips and needle points have a very small area?

Answer: Nail tips and needle points have very small areas so that the applied force produces high pressure. This high pressure helps them pierce wood, cloth, or skin easily with little force.

Q35. Why is it easier to walk on soft sand with flat shoes rather than high-heeled shoes?

Answer: Flat shoes have larger area of contact with the sand, so pressure on sand is less and the feet do not sink much. High-heeled shoes have smaller area, increasing pressure and causing the heel to sink deeper into the sand, making walking difficult.

Q36. Explain why porters use a thick folded cloth on their head while carrying heavy loads.

Answer: The thick folded cloth increases the area of contact between the load and the head. For the same weight (force), more area means less pressure on the head, making it more comfortable to carry heavy loads.

Q37. A boy stands first on one foot and then on two feet. In which case is the pressure on the floor more and why?

Answer: Pressure is more when he stands on one foot because the same weight acts on a smaller area. When he stands on two feet, the area of contact doubles, so pressure on the floor reduces to half.

Q38. How can you show with an activity that pressure depends on area?

Answer: Place a sharp pencil and the blunt end of the same pencil on a clay block and press with equal force. The sharp end sinks more due to smaller area and higher pressure, showing that pressure increases when area decreases.

Q39. What are fluids? Do fluids exert pressure?

Answer: Substances that can flow, such as liquids and gases, are called fluids. Yes, fluids exert pressure on the walls and bottom of their containers and on objects placed in them.

Q40. How does the pressure of a liquid vary with depth? Explain briefly.

Answer: Pressure of a liquid increases with depth. The deeper we go in a liquid, the greater the weight of the liquid column above that point, so the pressure at that depth is higher than near the surface.

Q41. Describe an activity to show that liquids exert pressure on the walls of the container.

Answer: Take a plastic bottle and make small holes at the same level on its side. Fill it with water. Water flows out from all holes, showing that water exerts pressure on the walls. This pressure pushes water out through the holes.

Q42. Why are the walls of dams thicker at the bottom than at the top?

Answer: The pressure of water increases with depth, so the bottom part of the dam experiences greater pressure. To withstand this high pressure and prevent damage, the walls of dams are built much thicker at the bottom than at the top.

Q43. How is gas pressure produced in a container?

Answer: Gas consists of tiny molecules moving randomly in all directions. When these molecules collide with the walls of the container, they exert force on the walls. The total force per unit area due to these collisions produces gas pressure.

Q44. Give two examples from daily life where gas pressure is used.

Answer: Examples:
(i) Air pressure inside bicycle and car tyres keeps them firm and allows them to support the weight of the vehicle.
(ii) Aerosol or perfume spray bottles work due to gas pressure pushing the liquid out in the form of a fine spray.

Q45. What is atmosphere? How does it exert pressure on us?

Answer: The atmosphere is the layer of air surrounding the Earth. The air in the atmosphere has weight and presses on the surface of the Earth and all objects, thus exerting atmospheric pressure on them.

Q46. Define atmospheric pressure. Why don’t we get crushed under it?

Answer: Atmospheric pressure is the pressure exerted by the air column of the atmosphere on the Earth’s surface. We do not get crushed because the pressure inside our body (due to blood and body fluids) is equal to the atmospheric pressure outside, so the forces balance each other.

Q47. Explain how a drinking straw works using the concept of atmospheric pressure.

Answer: When we suck air from the straw, air pressure inside the straw decreases. The liquid in the glass is still under normal atmospheric pressure. The greater atmospheric pressure on the liquid surface pushes the liquid up into the straw and into our mouth.

Q48. How does a syringe fill with liquid when the plunger is pulled back?

Answer: Pulling back the plunger increases the volume inside the syringe and reduces the air pressure inside. The liquid outside, under atmospheric pressure, is pushed into the syringe through the needle to occupy the low-pressure space.

Q49. Explain the working of a suction cup or suction pad using atmospheric pressure.

Answer: When a suction cup is pressed against a smooth surface, most of the air inside is forced out. The pressure inside becomes lower than atmospheric pressure outside. The higher atmospheric pressure outside presses the cup strongly against the surface, holding it in place.

Q50. Why is it difficult to remove a rubber sucker fixed tightly on a smooth wall?

Answer: The rubber sucker has low pressure inside and normal atmospheric pressure outside. The large atmospheric pressure on the outer surface presses it firmly against the wall, so a large force is required to pull it away and allow air to enter.