8th Sci. Chapter 3: Force and Pressure—Notes 

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                  👉.   माहिती विज्ञानाची 🎷

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❶. Types of Forces

👊 Contact Force: A force applied through direct contact between two bodies or via an intermediate object.

Examples: Muscular force (pushing, pulling, lifting) and frictional force (slows down moving objects).

👐Non-Contact Force: A force applied between two objects even when they are not in physical contact.

Examples: Magnetic force, gravitational force, and electrostatic force.

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❷. Balanced and Unbalanced Forces

Balanced Forces: Equal forces acting on an object in opposite directions. The effective net force is zero, resulting in no change in motion.

Unbalanced Forces: Unequal forces acting on an object. They cause a stationary object to move or change its speed and direction.

⛓️‍💥 Rules of Net Force:

• Forces in the same direction add together.
• Forces in opposite directions subtract from each other.
• Force has magnitude and direction (vector quantity).

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❸. Inertia

Definition: The inherent tendency of an object to remain in its existing state of rest or uniform motion in the absence of an external force.

Types:

1. Inertia of Rest: Inability to change state of rest (e.g., coin falling in glass when card is flicked).
2. Inertia of Motion: Inability to change state of motion (e.g., a fan continuing to rotate after power is switched off).
3. Directional Inertia: Inability to change direction of motion on its own (e.g., passengers leaning outward when a vehicle takes a sharp turn).

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❹. Pressure

Definition: The force exerted perpendicularly on a unit surface area.

Formula: 

Pressure = Force/Area

Units: The SI unit is N / m.²

or Pascal (Pa). 

In atmospheric science, 1 bar = 10⁵ Pa.

Key Relationship: Pressure is inversely proportional to area. Smaller surface area creates higher pressure (e.g., a sharp knife cutting vegetables). 

whereas a larger surface area reduces pressure (e.g., the broad feet of a camel).

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❺. Pressure in Liquids, Gases, and Atmosphere

• Fluid Pressure: Liquids and gases are fluids. Enclosed fluids exert equal pressure in all directions on the walls of the container.
• Liquid Pressure: Liquid pressure increases with depth. At any given horizontal level, liquid pressure is equal.
• Atmospheric Pressure: Pressure created by the weight of the air column above Earth.

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Air pressure at sea level ≈ 101 × 10³ Pa = 1 bar = 1000 mbar.

Atmospheric pressure decreases with increasing altitude/height.

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❻. Buoyant Force & Archimedes Principle

Buoyant Force: The upward force exerted by a fluid on an object immersed in it depends on the volume of the object (larger volume = greater force) and the density of the liquid (higher density = greater force).

Flotation Rules:

Floats: Buoyant force > Weight.

Sinks: Buoyant force < Weight.

Floats inside liquid: Buoyant force = Weight.

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👍Archimedes' Principle: When an object is partially or fully immersed in a fluid, an upward buoyant force acts on it equal to the weight of the fluid displaced by the object.

Applications: 

• Used in designing ships, 
• submarines, 
• lactometers, and 
• hygrometers.

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❼. Density 

Density: 

Density = Mass / Volume

SI Unit: kg/m³.

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❽. Relative Density (Specific Gravity):

Relative Density} = Density of substance/ Density of water. 

Relative density has no units, as it is a ratio of two identical quantities.

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Example . The area of the bottom of a tiffin box is 0.25 m², and the weight is 50 N. Calculate the pressure exerted by the box on the shelf.

Given: Area = 0.25 m², 

weight of the box = 75 N, 

Pressure = ?

Pressure = force / area

= 75 N / 0.25 m²

Pressure = 300 N/m²

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A piece of marble tile weighs 100 g in air. If its density is 2.5 g/cc, what will be its displaced volume of water?

Weight = 100 g

Density = 2.5 g/cc

Volume = ? cc

Density = Mass / Volume 

∴ volume = Mass / Density 

= 100 ÷ 2.5

volume = 40 cc

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