Pressure: Summary Note For MCQ

Bridge Course for NEB 11 Science (Post-SEE)

Pressure: Summary

Here is a clear and concise summary note for the lesson “Pressure”, suitable for NEB Grades 8–11, Bridge Course, or entrance exam preparation.

Pressure

🔎 Definition of Pressure

Pressure is defined as the force applied per unit area of a surface.

SI Unit: Pascal (Pa)

$1 Pa = 1 N/m^{2}$
Pressure is a scalar quantity (has magnitude but no direction).

🔑 Key Concepts

1. Factors Affecting Pressure

Directly proportional to force: more force → more pressure.
Inversely proportional to area: smaller area → higher pressure.

2. Pressure in Solids

Solids apply pressure by exerting force on contact surfaces.
Sharp objects exert more pressure due to a smaller area (e.g., knife edge, nail).

3. Pressure in Liquids (Hydrostatic Pressure)

Liquids exert pressure in all directions.
Pressure increases with depth and density.

$P = h ρ g$

Where:
- $h$ = height (depth)
- $ρ$ = density of fluid
- $g$ = gravitational acceleration

4. Atmospheric Pressure

Caused by the weight of the air above us.
Standard atmospheric pressure at sea level is:

$1 atm = 1.013 \times 1 0^{5} Pa$
Measured using a barometer.

5. Pascal’s Law

“Pressure applied to a fluid in an enclosed container is transmitted equally in all directions.”

Basis for hydraulic machines like:
- Hydraulic press
- Car brakes
- Lifts

6. Applications of Pressure

Wide shoes reduce pressure on snow.
Syringes, air pumps, and hydraulic systems.
Dam walls are thicker at the bottom due to higher water pressure.

📌 Important Units

Quantity	SI Unit	Symbol
Force	Newton	N
Area	m²	A
Pressure	Pascal	Pa

🧠 Remember:

More force + less area = higher pressure
Pressure in a fluid increases with depth.
Pascal’s Law explains how pressure is distributed in fluids.

⚓ Law of Floatation

The Law of Floatation states:

“An object will float in a fluid if the weight of the fluid displaced by the object is equal to the weight of the object itself.”

📚 Explanation

When an object is placed in a fluid (like water or oil), it experiences:

Downward force: weight of the object
Upward force: buoyant force or upthrust, caused by displaced fluid

🧪 Conditions for Floatation

Floating (object stays on the surface or partially submerged):

$Density of object < Density of fluid$
Sinking (object goes completely under fluid):

$Weight of object > Buoyant force, and$
Suspension (neutral float) (object stays fully submerged but doesn’t sink):

⚙️ Applications

Ships and boats float despite being heavy (they displace a large volume of water).
Submarines adjust their buoyancy to dive or rise.
Hydrometers float at different levels in different liquids based on density.

🔑 Important Formulas

Buoyant Force:
Relative density using flotation:

🧠 Remember:

A lighter object (by density) floats more in a denser fluid.
Floatation is governed by Archimedes’ Principle and the balance of forces.