Q1. Define force. Explain with suitable examples how force arises due to interaction between objects.

Answer:
A force is a push or pull acting on an object, which arises due to the interaction between two objects. Force can change the state of rest or motion of an object, change its speed, or even change its shape.

Force always involves two objects:

• One object that applies the force.
• Another object that experiences the force.

• When you push a table, your hands and the table interact. This interaction produces a force that makes the table move.
• When a player kicks a football, the player’s foot and the ball come in contact. Due to this interaction, a force acts on the ball and it moves forward.
• A magnet attracting an iron nail shows force due to interaction between the magnet and the nail, even though they may not touch initially.

Q2. Explain why at least two objects are needed for a force to come into play. Can an object apply force on itself?

Answer:
Force is the result of an interaction. Interaction always involves more than one object. One object acts as the source of force and the other is the receiver. Therefore, for a force to come into play, at least two objects must be present.

An object cannot apply a force on itself. For example:

• A book kept on a table experiences its weight (force due to gravity) and the reaction force from the table. Here the book and the Earth interact, and the book and the table interact.
• A person pulling a cart shows interaction between the person and the handle of the cart.

Q3. What do you understand by the term “state of motion” of an object? How does force change the state of motion?

Answer:
The state of motion of an object describes whether the object is at rest or in motion. For a moving object, it describes its speed and the direction in which it is moving.

A force can change the state of motion of an object in the following ways:

• It can set a stationary object into motion. For example, a football at rest starts moving when a player kicks it.
• It can bring a moving object to rest. For example, applying brakes on a bicycle produces friction and stops the moving bicycle.
• It can increase or decrease the speed of a moving object. Pedalling a bicycle increases speed, while using brakes decreases speed.
• It can change the direction of motion of an object, even if the speed remains the same. For example, in cricket, the direction of the ball changes when the batsman hits it.

Q4. Why is force called a vector quantity? What are the two things that you must know to describe a force completely?

Answer:
Force is called a vector quantity because it has both:

• Magnitude – how strong the force is.
• Direction – the line along which the force acts.

• Saying “a force of 10 N” is not complete. We must also say, “a force of 10 N acting towards the east” or “upwards”.
• If two forces of the same magnitude act in opposite directions, their effects can cancel each other, even though their magnitudes are equal.

Q5. Describe in detail the various effects that a force can produce on an object. Give at least two examples for each effect.

Answer:
A force can produce several kinds of effects on an object:

1. Change the state of rest or motion

• A stationary object can be set into motion. Example: A football at rest starts moving when kicked.
• A moving object can be brought to rest. Example: A moving bicycle stops when the brakes are applied.

• If a force acts in the direction of motion, it increases speed. Example: Pedalling a bicycle harder increases its speed.
• If a force acts opposite to the direction of motion, it decreases speed. Example: Air resistance and friction slow down a moving car.

• In cricket or football, when the player hits the moving ball, the direction of the ball changes.
• In athletics, when a runner takes a turn on the track, the frictional force between shoes and ground helps change direction.

• Stretching a rubber band changes its length and shape.
• Pressing a sponge, kneading dough, or hammering iron changes their shape and sometimes size.

Q6. Explain with examples what is meant by balanced and unbalanced forces. How do they affect the state of motion?

Answer:
When more than one force acts on an object simultaneously, we consider the effect of all forces together.

Balanced forces:

• Balanced forces are equal in magnitude but act in opposite directions.
• Their effects cancel each other and the net force becomes zero.
• Balanced forces do not change the state of rest or uniform motion of an object.

• Unbalanced forces are not equal in magnitude and/or not opposite.
• Their effects do not cancel and there is a net force on the object.
• Unbalanced forces can change the state of motion, speed, or direction of the object.

Q7. Explain with suitable examples how a force can change the speed as well as the direction of motion of an object.

Answer:
A single force or a combination of forces can change both the speed and direction of motion of an object.

Change in speed:

• When a cyclist pedals faster, the muscular force increases in the direction of motion and the bicycle speeds up.
• When brakes are applied, frictional force acts opposite to motion and reduces the speed until the bicycle stops.

• In football, when a moving ball is kicked from the side, the force changes its direction of motion, even if the speed remains almost the same.
• In a car taking a turn, friction between tyres and road provides the sideways force needed to change direction.

• When a fast-moving cricket ball is hit by a bat in the opposite direction, the ball not only changes its direction but also its speed, sometimes very quickly.
• A rocket engine provides thrust, changing both speed and direction of the rocket as it moves in space.

Q8. Describe with examples how force can change the shape and size of objects. Why is this property of force useful?

Answer:
Force can deform objects, that is, change their shape and sometimes their size. This property of force is very useful in various activities.

Examples of change in shape:

• When you knead dough to make chapatis, the muscular force of your hands changes the dough’s shape.
• Pressing a rubber ball or sponge changes its shape temporarily. When the force is removed, it often regains its original shape.

• Stretching a rubber band increases its length (size). When the force is removed, it may return to its original length.
• Hammering a hot piece of iron can increase its length and decrease its thickness, permanently changing its size and shape.

• This property allows us to shape metals into wires, sheets and tools, which is essential in construction and manufacturing.
• It helps in moulding clay into various shapes for pottery and bricks.

Q9. What are contact forces? Explain muscular force as a contact force with suitable daily life examples.

Answer:
Contact forces are forces that act only when two objects are in physical contact with each other. The force is transmitted through direct touching of the objects.

Muscular force is an important contact force produced by the muscles of our body. Whenever we do activities like pushing, pulling, lifting, throwing, or carrying objects, we use muscular force.

Examples:

• Picking up a school bag involves muscular force in the arms and shoulders.
• Pushing a stalled scooter or car uses muscular force in the legs and hands.
• A farmer ploughing a field with a bullock cart: the bullocks use muscular force to pull the plough.
• A swimmer uses muscular force of hands and legs to push water backward and move forward.

Q10. Define friction. Why is it called a contact force? Discuss its advantages and disadvantages in everyday life.

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

Friction is called a contact force because it arises only when two surfaces touch each other. Without contact, friction cannot act.

Advantages of friction:

• It helps us walk without slipping, as there is friction between our feet and the ground.
• It allows us to hold objects, such as a pen or book, firmly in our hands.
• Brakes in vehicles work because of friction between brake shoes and wheels, helping us to stop or slow down safely.

• It causes wear and tear of moving parts of machines, tyres, and shoe soles.
• It produces heat, leading to energy loss in machines like engines and mixers.

Q11. Describe with examples how friction can be increased when needed and reduced when not needed.

Answer:
We sometimes need more friction for better grip, and sometimes need to reduce friction to improve efficiency and reduce wear.

Ways to increase friction (with examples):

• Roughening surfaces: Treads on tyres and shoes are made rough to increase friction and prevent slipping on roads or floors.
• Using chalk on hands: Gymnasts and weightlifters rub chalk on their hands to increase friction and avoid slipping while gripping bars or weights.
• Carpet or mats: Placing mats on a slippery floor increases friction and prevents slipping.

• Using lubricants: Applying oil or grease between moving parts of a machine reduces friction and wear.
• Using ball bearings: Ball bearings in fan motors or vehicle wheels reduce friction by changing sliding friction into rolling friction.
• Smooth surfaces: Polishing surfaces like table tops makes them smooth and reduces friction, allowing objects to slide easily.

Q12. Why is friction called a necessary evil? Illustrate your answer with suitable reasons and examples.

Answer:
Friction is called a necessary evil because:

(A) It is necessary (useful):

• Without friction between our feet and the ground, we would slip and not be able to walk properly.
• We are able to hold objects like pens, books, and tools due to friction between our fingers and the objects.
• Brakes in vehicles work because of friction, helping us to control speed and stop safely.
• Friction between tyres and road provides grip, preventing vehicles from skidding.

• Friction causes wear and tear of moving parts in machines, needing frequent maintenance and replacement.
• It produces unwanted heat, wasting energy in engines and other machines.
• High friction reduces efficiency of machines and leads to fuel wastage in vehicles.

Q13. Describe an activity to show that friction always opposes the motion of an object. What conclusion do you draw from it?

Answer:
Activity:

• Take a small wooden block and place it on a table.
• Give it a gentle push so that it starts sliding.
• Observe that after some time, the block slows down and eventually comes to rest.

• The block should have continued moving forever if no opposing force acted on it.
• However, the block slows down and stops due to friction between the bottom surface of the block and the table.
• This frictional force acts in the direction opposite to the motion of the block.

Q14. What are non-contact forces? Explain magnetic force as a non-contact force with suitable examples.

Answer:
Non-contact forces are forces that can act between two objects even when they are not in physical contact. The objects may be separated by some distance.

Magnetic force is an important non-contact force. A magnet can attract certain materials like iron, nickel, and cobalt without touching them.

Examples:

• If you bring a bar magnet near some iron nails or paper clips, they move towards the magnet and stick to it, even before touching.
• Two magnets can either attract or repel each other when brought closer, depending on the poles facing each other. Opposite poles attract; like poles repel.
• In compasses, the magnetic needle aligns itself with Earth’s magnetic field without physical contact with the Earth’s core.

Q15. Explain electrostatic force with an activity. Why is it also considered a non-contact force?

Answer:
Electrostatic force is the force between electrically charged objects. Charged objects can attract or repel each other.

Activity:

• Take a plastic comb and rub it on dry hair several times.
• Bring the comb near small pieces of paper placed on a table.
• You will observe that the pieces of paper jump up and stick to the comb.

• Rubbing the comb on dry hair charges the comb (it gains electric charge).
• The charged comb exerts an attractive electrostatic force on the small, neutral pieces of paper.
• The pieces move towards the comb even when the comb is not touching them.

Q16. What is gravitational force? Explain how it acts between Earth and objects around us with examples.

Answer:
Gravitational force is the force of attraction between any two masses in the universe. Every object with mass attracts every other object.

The most important example of gravitational force is the attraction between the Earth and objects near its surface.

• When you drop a stone from a height, it falls downward due to Earth’s gravitational force pulling it towards the centre of the Earth.
• When you throw a ball upwards, it slows down, stops for a moment, and then falls back because gravity constantly pulls it down.
• The Moon is held in orbit around the Earth due to gravitational force between the Earth and the Moon.

Q17. Compare contact and non-contact forces by giving their definitions, examples, and main differences.

Answer:
Contact forces:

• Act only when two objects are in physical contact.
• Examples: muscular force, frictional force, normal reaction force.
• They arise due to direct touching of surfaces or objects.

• Act even when objects are separated by some distance and do not touch.
• Examples: gravitational force, magnetic force, electrostatic force.
• They arise due to fields (gravitational, magnetic, electric) around objects.

1. Contact is essential for contact forces but not required for non-contact forces.
2. Contact forces are often due to mechanical interactions of surfaces, whereas non-contact forces are due to invisible fields around objects.
3. Friction and muscular force slow or speed up objects; gravity and magnetism can act from far away.

Q18. A stone and a feather are dropped from the same height. In the absence of air, they reach the ground together. Explain this using the concept of gravitational force.

Answer:
Gravitational force depends on the masses of the objects and the distance between them. Near the Earth’s surface, all objects experience approximately the same acceleration due to gravity, regardless of their mass.

In the presence of air:

• A feather is light and has a large surface area, so it experiences more air resistance (air friction) compared to its weight.
• Thus, the feather falls slowly, while the stone, being heavier and less affected by air resistance, falls faster.

• There is no air resistance to oppose the motion of either the stone or the feather.
• Both objects are only under the influence of Earth’s gravitational force.
• Hence, they acquire the same acceleration due to gravity and fall together, reaching the ground at the same time.

Q19. Define pressure. Derive the formula for pressure and state its SI unit. Explain how pressure depends on force and area.

Answer:
Pressure is defined as the force acting per unit area of a surface.

If a force F acts normally on an area A, then:
Pressure (P) = Force (F) ÷ Area (A), or P = F / A.

SI unit:

• Force is measured in newton (N).
• Area is measured in square metre (m²).
• So, the SI unit of pressure is newton per square metre (N/m²), also called pascal (Pa).

• For a fixed area, if the force increases, pressure increases. (Directly proportional to force)
• For a fixed force, if the area decreases, pressure increases; if the area increases, pressure decreases. (Inversely proportional to area)

Q20. Explain with examples from daily life how changing the area of contact can increase or decrease pressure.

Answer:
According to the formula P = F / A, pressure increases when area decreases (for the same force), and pressure decreases when area increases.

Examples of increasing pressure by reducing area:

• Knife and blade edges are made very sharp and thin. The same cutting force acts on a small area, producing high pressure and making cutting easier.
• Nail tips and needle points are pointed (small area) so that they can easily penetrate wood, cloth, or skin with little applied force.

• Porters place a folded cloth pad on their heads while carrying loads. The cloth increases the area of contact, reducing pressure on the head, so it is more comfortable.
• Camels have broad, wide feet that increase the area in contact with sand. This reduces pressure, preventing them from sinking too much into the sand.
• Tractors have large, wide tyres to increase area and reduce pressure on soft soil, preventing the vehicle from sinking.

Q21. A person stands on the ground first on one foot and then on both feet together. In which case is the pressure more and why? Explain in detail.

Answer:
The weight of the person (force) remains the same in both cases. Only the area of contact between the feet and the ground changes.

Case 1: Standing on one foot

• The entire weight of the person acts on one foot only.
• The area of contact is equal to the area of one foot.
• So, the pressure on the ground is P₁ = Weight / Area of one foot.

• The same weight is now shared by both feet.
• The area of contact becomes roughly double (area of two feet).
• So, the pressure is P₂ = Weight / (Area of two feet), which is less than P₁.

Q22. Explain with reasons: (a) It is easier to cut vegetables with a sharp knife than with a blunt knife. (b) Why do army tanks have broad caterpillar tracks instead of wheels?

Answer:
(a) Sharp knife vs blunt knife:
Pressure = Force / Area.

• A sharp knife has a very thin and sharp edge (small area of contact).
• For the same cutting force applied by our hand, the pressure produced by the sharp knife is high.
• This high pressure allows the knife to cut vegetables easily.
• A blunt knife has a larger area at its edge, so the pressure is less. Therefore, cutting with it is more difficult.

• Army tanks are very heavy vehicles. If normal wheels were used, the entire weight would act on a small area of the ground.
• This would produce very high pressure on the ground, causing the tank to sink into soft soil or mud.
• Caterpillar tracks are broad and long, increasing the area of contact with the ground.
• For the same weight, increased area produces lower pressure, allowing the tank to move smoothly even on soft ground without sinking.

Q23. Explain how high-heeled shoes can damage soft floors while flat shoes usually do not. Relate your answer to the concept of pressure.

Answer:
When a person stands on a floor, the floor experiences a force equal to the person’s weight. The effect of this force depends on the area over which it acts.

High-heeled shoes:

• The heel has a very small area of contact with the floor.
• For the same body weight (force), pressure = Force / Area becomes very high because the area is small.
• This high pressure can cause damage to soft floors, such as leaving dents in wooden flooring or tearing vinyl floor coverings.

• Flat shoes have a comparatively large area of contact with the floor.
• For the same weight, the pressure on the floor is much lower.
• As a result, flat shoes are less likely to damage the floor.

Q24. What are fluids? Do fluids exert pressure? Explain with examples how liquids exert pressure in different directions.

Answer:
Substances like liquids and gases which can flow are called fluids.

Yes, fluids do exert pressure on the walls and bottom of the container and on objects placed in them.

Liquids exert pressure in all directions as shown by the following examples:

• Take a plastic bottle and make small holes on its side at the same height all around. Fill the bottle with water. Water flows out from all holes, showing that liquid exerts pressure sideways in all directions.
• When you immerse your hand in water, you feel pressure from all sides – from the sides, top, and bottom, because water pushes on every part of the hand.
• Water also exerts pressure on the bottom of the container. This can be observed as the weight felt by the container due to water.

Q25. How does pressure in a liquid vary with depth? Explain with an activity and give one application of this concept in real life.

Answer:
Pressure in a liquid increases with depth. The deeper you go, the greater the pressure.

Activity:

• Take a plastic bottle and make three holes on one side at different heights: near the top, middle and bottom.
• Cover the holes with tape and fill the bottle with water.
• Remove the tape simultaneously from all three holes.

• Water from the lowest hole (greatest depth) flows out the farthest.
• Water from the middle hole flows less far.
• Water from the highest hole (least depth) flows the least distance.

• Dams are made thicker at the bottom because the pressure of water is much higher at greater depths. The thick walls at the bottom can withstand this high pressure and prevent damage.

Q26. Explain how gas exerts pressure. Give examples to show that air (a gas) exerts pressure in our daily life.

Answer:
Gases like air consist of tiny molecules moving randomly in all directions. These molecules constantly collide with the walls of their container. Each collision exerts a small force on the wall. The total force per unit area of these collisions is called gas pressure.

Examples of air pressure in daily life:

• Inflated tyres: Air pumped into bicycle or car tyres exerts pressure on the tyre walls, making them firm and able to support the weight of the vehicle and rider.
• Football and balloons: When air is blown into a balloon or football, the air pressure inside stretches the rubber or leather, inflating it.
• Spray bottles: In perfume or insect spray cans, compressed gas exerts pressure on the liquid inside. When the nozzle is pressed, liquid comes out as a fine spray due to this pressure.

Q27. Why are deep-sea divers and submarines specially designed to withstand high water pressure? Explain using the idea of liquid pressure.

Answer:
As we go deeper in the sea, the pressure of water increases because the weight of the water column above increases with depth. This very high water pressure can crush ordinary objects.

Deep-sea divers:

• At great depths, water exerts enormous pressure on the diver’s body.
• To protect them, divers wear specially designed diving suits that can withstand high pressure without collapsing.
• These suits prevent the high external pressure from directly acting on the diver’s body and maintain safe internal pressure.

• Submarines are built with strong, thick metal shells (hulls) that can bear very high external water pressure.
• If a submarine were not strong enough, it would be crushed by water pressure at great depths.
• The inside of the submarine is maintained at normal atmospheric pressure for people to live and work comfortably.

Q28. What is atmospheric pressure? Explain why we do not feel this huge pressure acting on our body.

Answer:
The layer of air surrounding the Earth is called the atmosphere. This air has weight and exerts pressure on the Earth’s surface and on all objects. This pressure is called atmospheric pressure.

Although atmospheric pressure is very large (it can be considered equivalent to the weight of a 10-metre high water column), we do not feel crushed under it.

Reasons:

• The pressure inside our body (due to blood and other body fluids) is approximately equal to the atmospheric pressure outside.
• Because the pressures inside and outside are balanced, we do not feel any net force on our body.
• Our body has evolved to withstand this pressure naturally and we do not usually notice it.

Q29. Describe in detail the working of a drinking straw using the concept of atmospheric pressure and pressure difference.

Answer:
A drinking straw works on the principle of pressure difference created by sucking air and the action of atmospheric pressure.

Stepwise explanation:

• When the straw is placed in a liquid and you have not sucked, the air pressure inside the straw and the atmospheric pressure on the liquid surface are equal. The liquid does not rise in the straw.
• When you suck air out of the straw, you reduce the air pressure inside the straw.
• The liquid in the glass is still under normal atmospheric pressure. Therefore, the pressure on the surface of the liquid in the glass becomes greater than the pressure inside the straw.
• This pressure difference pushes the liquid up into the straw and then into your mouth.

Q30. Explain the working of a syringe or suction cup with reference to atmospheric pressure. How is low pressure created inside them?

Answer:
Both syringes and suction cups work based on the principle of creating low pressure inside and allowing atmospheric pressure to push liquid or objects.

(a) Syringe:

• A syringe consists of a cylindrical tube and a movable plunger.
• When the plunger is pulled back with the nozzle dipped in a liquid, the space inside the tube increases.
• This reduces the air pressure inside the syringe compared to the atmospheric pressure outside.
• The liquid outside, under higher atmospheric pressure, is pushed into the syringe through the nozzle to fill the low-pressure region.

• When a suction cup is pressed firmly against a smooth wall, most of the air between the cup and the wall is pushed out.
• The air pressure inside the cup becomes lower than the atmospheric pressure outside.
• The higher atmospheric pressure outside presses the cup strongly against the wall, holding it in place.
• To remove it, you must pull hard to let air in and equalise the pressure on both sides.