• Friction opposes motion until force exceeds its limit.
• Example: Pushing a wall (NCERT) also shows no displacement.

Real-world: A heavy cupboard requires more force to overcome friction.

• Rubbing charges the balloon, attracting it to the wall.
• Gravity acts always; electrostatic needs charging.

Example: Comb attracting paper bits (NCERT) shows similar behavior.

• Water weight above increases pressure with depth.
• Atmospheric pressure acts uniformly, unlike liquid pressure.

Example: Dam walls are thicker at the base (NCERT).

• More force increases acceleration if mass is constant.
• Example: Pushing a grocery cart (NCERT) shows this.

Real-world: Kicking a football harder makes it move faster.

A box resists motion until enough force overcomes friction.

• Static friction prevents motion until force exceeds it.
• Sliding friction acts once movement starts.

Our textbook shows friction opposes motion. The boy’s initial force balanced static friction. Increasing force overcame it, allowing motion as sliding friction took over.

Rubbed balloon sticks due to an invisible force.

• Electrostatic force attracts charged objects.
• Like NCERT’s comb-paper example.

Rubbing transfers charges, making the balloon attract the wall. We studied this as electrostatic force, similar to how a charged comb picks paper bits.

Water pressure increases with depth.

• Liquid pressure depends on depth (P = hρg).
• NCERT shows dams are thicker at the base.

Deeper water exerts more force per unit area. Our textbook explains this as liquid pressure, like dams resisting higher pressure at greater depths.

Rough surfaces slow motion more than smooth ones.

• Friction opposes motion, higher on rough surfaces.
• NCERT’s example of sliding a book on different surfaces.

Rough floors have more irregularities, increasing friction. We studied this as friction’s dependence on surface texture, like dragging objects on carpets versus tiles.

A box is pushed but doesn’t move due to balanced forces.

• Applied force by the boy.
• Frictional force opposes motion.

Our textbook shows that when forces are equal and opposite, the net force is zero. Here, friction balances the push, keeping the box stationary.

A charged balloon sticks to a wall due to electrostatic force.

• Electrostatic force attracts opposite charges.

We studied that rubbing transfers charges. The balloon gains a charge and induces an opposite charge on the wall, causing attraction, like in NCERT's comb-paper example.

Water pressure increases with depth, as felt by divers.

• Depth increases pressure due to water weight.

Our textbook shows deeper water exerts more pressure, like feeling ear pain while swimming deep. Similarly, dams are thicker at the base to handle higher pressure.

Gravitational and normal forces balance to keep the book stationary.

• Gravity pulls the book down.
• Normal force from the table balances it.

Like the moon orbits due to Earth’s gravity (NCERT example), the book stays put because forces are balanced. Both cases involve equilibrium.

The box moves more easily with wooden logs because rolling friction is much less than sliding friction. Rolling reduces the contact area between surfaces, making movement smoother.

The force opposing the motion of the box on the rough floor is called friction. Specifically, it is static friction when the box is stationary and kinetic friction when it starts moving.

Additional Insight: Using rollers or wheels is a practical application of reducing friction, commonly seen in transportation and machinery.

The plunger did not move because water is an incompressible fluid. Applying force increases the pressure inside, but since water cannot be compressed, the plunger stays in place.

This demonstrates Pascal's Law, which states that pressure applied to a confined fluid is transmitted equally in all directions. Since the nozzle was closed, the pressure couldn't escape, preventing movement.

Real-life Connection: Hydraulic systems (like car brakes) use this principle to transfer force efficiently without compression losses.

Frictional force is responsible for the difference in effort required to push the box on rough and smooth surfaces.

1. On a rough floor, the frictional force between the box and the surface is higher due to irregularities, making it harder to move.
2. The force opposing the motion is called friction.
3. Rahul can reduce the effort by:

• Using rollers or wheels under the box to convert sliding friction to rolling friction.
• Applying lubricants to decrease friction.
• Making the surface smoother.

Understanding friction helps in designing machines and everyday objects to minimize energy loss.

1. Water is an incompressible fluid because its molecules are tightly packed, resisting volume change.
2. Air is a compressible fluid as gas molecules have large gaps between them, allowing compression.
3. The property demonstrated is compressibility, which varies between liquids and gases.
4. A real-life application is in hydraulic brakes, where incompressible fluids transmit force efficiently.

This principle is crucial in designing systems like shock absorbers and pneumatic tools.

The force responsible for the balloon's motion is the reaction force exerted by the escaping air.

When air rushes out of the balloon's neck, it applies an action force backward (as per Newton's Third Law of Motion). The balloon experiences an equal and opposite reaction force, propelling it forward.

The scientific principle involved is Newton's Third Law of Motion, which states: "For every action, there is an equal and opposite reaction."

Additional Insight: This principle is also used in rocket propulsion, where exhaust gases pushed downward create an upward thrust.

The force causing this sensation is muscular force, specifically the effort exerted by Priya's muscles to counteract gravity.

The bag feels heavier over time due to muscle fatigue. As muscles work continuously, they tire and require more effort to sustain the same force, making the bag feel heavier even though its weight (gravitational force) remains unchanged.

To reduce this effect, Priya could:

• Use a backpack with wide, padded straps to distribute the pressure over a larger area of her shoulders.

Concept Connection: This demonstrates how pressure (force per unit area) affects comfort—wider straps reduce pressure on the shoulders.

(a) The box does not move easily on the rough floor because of friction, which is an opposing force acting between the surfaces in contact. The rough surface increases the frictional force, making it harder to move the box.

(b) The round wooden logs act as rollers, reducing the contact area between the box and the floor. This converts sliding friction into rolling friction, which is much smaller. Rolling friction makes it easier to move the box because less force is needed to overcome it.

Additional Insight: This principle is used in ancient times to move heavy stones for construction, demonstrating how science simplifies real-world problems.

(a) When the nozzle was sealed, the water inside the syringe could not escape, making it a closed system. Pushing the plunger compresses the water, increasing its pressure. Since liquids are nearly incompressible, the pressure resists the force applied, making it difficult to push.

(b) When the seal is removed, the plunger moves easily because the water can now flow out of the nozzle. The pressure is no longer confined, allowing the liquid to escape, and thus, the force applied does not face resistance.

Application: This demonstrates Pascal's Law, which states that pressure applied to an enclosed fluid is transmitted equally in all directions. It explains hydraulic systems like brakes in vehicles.

(a) The box does not move easily on the rough floor because of friction, which is a force that opposes the motion between two surfaces in contact. The rough surface increases the frictional force, making it harder to move the box.

(b) Rollers help in moving the box smoothly by reducing the friction between the box and the floor. They convert sliding friction into rolling friction, which is significantly lesser. This makes it easier to move heavy objects with less effort.

Additionally, rollers distribute the weight of the box over a larger area, minimizing the resistance caused by surface irregularities.

(a) Priya couldn't push the plunger easily because the water inside the syringe is incompressible. When the nozzle is sealed, the water resists being compressed, creating a pressure that opposes the force applied by the plunger.

(b) This activity demonstrates the principle of Pascal's Law, which states that pressure applied to a confined fluid is transmitted equally in all directions. Since water cannot be compressed, the force applied by the plunger increases the pressure inside the syringe uniformly.

This principle is also the basis for hydraulic systems, where fluids are used to transmit force efficiently.

(a) The force acting against the motion of the box is friction. It opposes the relative motion between the box and the surface.

(b) The nature of the surface affects friction because rough surfaces have more irregularities, which increase the interlocking between the surfaces. This results in higher frictional force. Smooth surfaces have fewer irregularities, reducing friction.

(c) One way to reduce the effort required is by using rollers or wheels under the box. This converts sliding friction into rolling friction, which is significantly lesser.

(a) The property of water that makes it difficult to compress is incompressibility. Liquids, like water, have very little empty space between their molecules, making them resistant to compression.

(b) When force is applied to an incompressible fluid, it gets transmitted equally in all directions, creating pressure. This is explained by Pascal's Law, which states that pressure applied to a confined fluid is transmitted undiminished in all directions.

(c) A real-life application is hydraulic brakes in vehicles. When the brake pedal is pressed, the incompressible brake fluid transmits the pressure equally to all wheels, ensuring effective braking.