Samacheer Class 9 Science - Fluids

• (a) decreases
• (b) increases
• (c) remains same
• (d) may increase or decrease

Answer:
(b) increases

Pressure decreases upward, so the air bubble expands.

• (a) density
• (b) pressure
• (c) velocity
• (d) mass

Answer:
(a) density

• (a) increased pressure lowers the boiling point
• (b) increased pressure raises the boiling point
• (c) decreased pressure raises the boiling point
• (d) increased pressure lowers the melting point

Answer:
(b) increased pressure raises the boiling point

Answer: True. The weight of the fluid displaced determines the buoyant force. According to Archimedes' principle, the buoyant force acting on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. This principle applies to all fluids, whether liquids or gases. When an object is partially or completely immersed in a fluid, it displaces a volume of fluid equal to the volume of the object that is submerged. The weight of this displaced fluid equals the buoyant force acting upward on the object. Mathematically, buoyant force equals the weight of displaced fluid, which can be expressed as F = ρgV, where ρ is the density of the fluid, g is acceleration due to gravity, and V is the volume of fluid displaced.

Answer: False. The correct statement is that the density of an object determines whether it floats or sinks, not the shape of the object. An object will float in a fluid if its average density is less than the density of the fluid, and it will sink if its average density is greater than the density of the fluid. Shape does not directly determine floating or sinking behavior. However, shape can be important in practical applications because it affects the volume of the object and thus influences the weight of fluid displaced. For example, a piece of iron will sink in water because iron is denser than water, regardless of its shape. Conversely, a ship made of steel, which is denser than water, can float because its shape creates a large volume that displaces enough water to produce a buoyant force equal to its weight.

Answer: False. The correct statement is that wide foundations of high-rise buildings reduce pressure on the ground, not increase it. Pressure is defined as force per unit area. When the foundation area is increased while the weight of the building remains the same, the pressure exerted on the ground decreases according to the formula P = F/A, where P is pressure, F is the force (weight of the building), and A is the area of contact. By making foundations wide, the contact area increases, which distributes the weight of the building over a larger area, thereby reducing the pressure on the ground. This prevents the building from sinking into the soil and ensures structural stability.

Answer: True. Archimedes' principle applies to gases also. Archimedes' principle states that the buoyant force acting on an object immersed in a fluid is equal to the weight of the fluid displaced by the object. This principle is universal and applies to all fluids, including both liquids and gases. When an object is immersed in a gas, it displaces a volume of gas equal to the volume of the object. The buoyant force equals the weight of the displaced gas. This is why balloons filled with helium or hot air rise in the atmosphere: the weight of the displaced air is greater than the weight of the balloon and its contents, resulting in a net upward buoyant force. Therefore, Archimedes' principle is a fundamental law applicable to all fluids regardless of their state.

Answer: True. A hydraulic press is used for extracting oil from seeds. A hydraulic press operates on the principle of Pascal's law, which states that pressure applied to a confined fluid is transmitted undiminished throughout the fluid. In oil extraction, seeds are placed in the hydraulic press, and a large force is applied through a piston, creating high pressure that is transmitted to the seeds. This immense pressure crushes the seeds and forces out the oil. The hydraulic press is particularly effective for this purpose because it can generate very large forces from relatively small input forces due to the difference in piston areas. The extracted oil is then collected, while the remaining seed cake is removed. This method is widely used in industries for extracting oils from various seeds such as sunflower, mustard, coconut, and groundnut.

Answer:
Liquid pressure depends on:

• Depth of liquid
• Density of liquid
• Acceleration due to gravity

Helium is less dense than air, so the buoyant force acting on the balloon is greater than its weight. According to Archimedes' principle, the buoyant force on an object immersed in a fluid equals the weight of the fluid displaced. Since helium has a lower density than the surrounding air, the weight of air displaced by the balloon is greater than the weight of helium inside it plus the weight of the balloon material. This net upward force causes the balloon to float upward in the air.

Sea water contains dissolved salts and has greater density than river water. According to Archimedes' principle, the buoyant force on a swimming body equals the weight of fluid displaced. Since sea water is denser than river water, it displaces more weight for the same volume of body submerged. Hence, sea water provides greater buoyant force compared to river water, making swimming easier as the body needs to displace less of itself to achieve the same buoyant force required to stay afloat.

The pressure exerted by the weight of the atmosphere on Earth's surface is called atmospheric pressure. The atmosphere is a layer of gases surrounding the Earth, and due to gravitational force, this entire mass of air exerts pressure on all objects at the surface. At sea level, the standard atmospheric pressure is approximately 101,325 Pa or 1 atm. This pressure acts in all directions and is responsible for various phenomena such as the functioning of barometers, the difficulty in separating two surfaces pressed together, and the behavior of fluids in containers.

Pascal's law states that pressure applied on an enclosed incompressible liquid is transmitted equally in all directions throughout the liquid. This means that when an external force is applied to a confined fluid, the resulting pressure increase is distributed uniformly throughout the entire volume of the fluid. The pressure acts perpendicular to the surfaces of the container at every point. This principle is the foundation for hydraulic machines such as hydraulic lifts, hydraulic presses, and hydraulic brakes, where a small force applied over a small area can produce a large force over a larger area.

Answer:

• If object density < liquid density → object floats
• If object density > liquid density → object sinks

Examples:

Construction

A hydrometer has:

• Cylindrical stem
• Spherical bulb at lower end
• Lead shots or mercury inside bulb
• Narrow graduated tube

Working

• Hydrometer is placed in liquid.
• It floats vertically.
• Reading at liquid surface gives relative density.

Laws:

• A floating body displaces liquid equal to its own weight.

• Centre of gravity and centre of buoyancy lie in the same vertical line.

Assertion: A floating body displaces liquid equal to its own weight. Reason: Then the body experiences no net downward force. Answer: (a) Both are true and reason correctly explains assertion. When a body floats in equilibrium, the buoyant force equals the weight of the body. By Archimedes' principle, the buoyant force equals the weight of liquid displaced. Therefore, the weight of liquid displaced equals the weight of the floating body. Since the buoyant force (upward) equals the weight (downward), the net force is zero, and the body remains in equilibrium without accelerating downward.

Assertion: Pascal's law is the principle behind hydraulic lift. Reason: Pressure is thrust per unit area. Answer: (b) Both are true but reason is not correct explanation. Pascal's law states that pressure applied to an enclosed incompressible fluid is transmitted equally in all directions. In a hydraulic lift, when pressure is applied to the fluid in a small cylinder, this pressure is transmitted equally to a larger cylinder, creating a larger force on the larger piston. While the reason statement that pressure is thrust per unit area is true, it does not explain why Pascal's law is the principle behind hydraulic lifts. The correct explanation is that Pascal's law enables the transmission of pressure equally throughout the fluid, allowing a small input force to produce a large output force.

Given:

• Weight of block = 200 g
• Volume = (300 cm^3)

Correct Concept:

For a floating body:

Therefore,

Correction made:
Original solution incorrectly used volume as upthrust.

Given:

Given:

Conversion:

Given:

Weight in air = 100 gwt

Since wood floats:

Thus,

Correction made:
Original answer was conceptually incorrect.

Given:

Using:

Answer:

Fish use swim bladders filled with gases.

• Increasing gas volume decreases density → fish rises.
• Releasing gas increases density → fish sinks.

Observation:

Reason:

Given:

• Initial weight = 600 g
• Mass of ball = 40 g
• Density = (0.80g/cm^3)

Volume of Ball:

Final Reading:

Correction made:
Original answer incorrectly added volume instead of mass.

Given:

• Weight in air = 60 N
• Weight in water = 40 N

Loss of weight:

Specific gravity:

Given:

Density:

Relative density:

Given:

Given:

Using: