Physics Class 8th CH01: Pressure || By Pratap Sanjay Sir

Pressure Notes with Numerical Questions

Definition of Pressure

Pressure is the force exerted per unit area on the surface of an object.
It is a scalar quantity, meaning it only has magnitude and no direction.

Formula: Pressure (P) = Force (F) / Area (A)

P = F / A

Where:

• P is the pressure
• F is the force applied
• A is the area over which the force is applied

SI Unit: Pascal (Pa) where 1 Pascal = 1 Newton per square meter (N/m²).

Understanding Pressure

• If the same force is applied over a smaller area, the pressure increases.
• If the same force is applied over a larger area, the pressure decreases.

Examples of Pressure in Daily Life

Example 1: A sharp knife cuts better because it exerts more pressure on the object due to its smaller area of contact.

Example 2: Snowshoes have a large surface area to distribute the weight of a person over a larger area, reducing pressure and preventing sinking into the snow.

Types of Pressure

Atmospheric Pressure: The pressure exerted by the weight of the atmosphere above us. Standard atmospheric pressure at sea level is approximately 101,325 Pa or 1 atmosphere (atm). Applications: Weather forecasting, altimeters in aircraft.

Liquid Pressure: Pressure exerted by a liquid in all directions due to its weight. Depends on the height (depth) and density of the liquid.

Formula: P = h * ρ * g

Applications of Pressure

Hydraulic Systems: Use liquid pressure to transmit force. Examples: Car brakes, hydraulic lifts.

Barometers and Manometers: Devices used to measure atmospheric and liquid pressures, respectively.

Important Concepts

Pascal’s Law: States that pressure applied to a confined fluid is transmitted equally in all directions. Basis for hydraulic systems.

Buoyant Force and Archimedes’ Principle: Buoyant force is the upward force exerted by a fluid on an object placed in it. Archimedes’ Principle: The buoyant force is equal to the weight of the fluid displaced by the object.

Numerical Problems

Problem 1

A girl applies a force of 12 N on an area of 10 cm². What will be the pressure by her?

Solution:

Given:

• Force (F) = 12 N
• Area (A) = 10 cm² = 10 x 10^-4 m² = 0.001 m²

Pressure (P) is given by:

P = F / A

P = 12 N / 0.001 m² = 12000 Pa

Problem 2

Calculate the pressure exerted by a metallic block, which applies a force of 3 N, when it is placed on sand, (a) upright (b) on base. The dimensions of the metallic block are length 25 cm, height 10 cm, and width 5 cm.

(a) Upright

Solution:

Given:

• Force (F) = 3 N
• Length = 25 cm = 0.25 m
• Height = 10 cm = 0.10 m
• Width = 5 cm = 0.05 m

(a) When the block is upright:

• Area (A) = Width x Height = 0.05 m x 0.10 m = 0.005 m²

Pressure (P) is given by:

P = F / A

P = 3 N / 0.005 m² = 600 Pa

(b) On base

Solution:

(b) When the block is on its base:

• Area (A) = Length x Width = 0.25 m x 0.05 m = 0.0125 m²

Pressure (P) is given by:

P = F / A

P = 3 N / 0.0125 m² = 240 Pa

Problem 3

Calculate the pressure exerted on the floor by a boy standing on both feet if the weight of the boy is 40 kg. The area of contact of each shoe is 25 cm by 6 cm. (Take 1 kgwt = 10 N)

Solution:

Given:

• Weight (Force) of the boy (F) = 40 kg x 10 N/kg = 400 N
• Area of contact of each shoe = 25 cm x 6 cm = 150 cm² = 150 x 10^-4 m² = 0.015 m²
• Total area of contact (both shoes) = 2 x 0.015 m² = 0.03 m²

Pressure (P) is given by:

P = F / A

P = 400 N / 0.03 m² = 13333.33 Pa

Problem 4

A force of 200 N produces a pressure of 10000 Pa. Calculate the area of the surface on which the force acts.

Solution:

Given:

• Force (F) = 200 N
• Pressure (P) = 10000 Pa

Area (A) is given by:

A = F / P

A = 200 N / 10000 Pa = 0.02 m²