Thrust, Pressure, and Buoyancy

Thrust is the force acting perpendicular (normal) to a surface.

• Thrust is a force → measured in Newtons (N)
• Direction: always at 90° to the surface it acts on
• Same force can produce different effects depending on area

Pressure = Force per unit area (how concentrated the force is)

P = F / A

• P = Pressure (Pa = pascals = N/m²)
• F = Force / Thrust (Newtons)
• A = Area (m²)

1 Pascal (Pa) = 1 N/m²

Why area matters:

• Same force, smaller area → MORE pressure
• Sharp knife: small area → high pressure → cuts easily
• Wide base of dam: large area → low pressure on ground
• Camel's foot: wide, padded → spreads weight → low pressure on sand

Worked Example:

A box of mass 10 kg rests on a table. Base area = 0.5 m².

Weight (thrust) = 10 × 10 = 100 N Pressure = 100 / 0.5 = 200 Pa

Fluids exert pressure in all directions (unlike solids — only downward).

Liquid Pressure at Depth:

P = hρg

• h = depth below surface (m)
• ρ = density of liquid (kg/m³)
• g = gravitational field strength (10 N/kg)

Key facts:

• Pressure increases with depth
• Pressure depends on density, not volume of liquid
• Pressure at same depth is equal in all directions (Pascal's principle)

Atmospheric Pressure:

• Weight of air column above us → ~101,325 Pa (≈ 1 atm)
• At higher altitude: less air above → less atmospheric pressure
• Why blood vessels don't burst: internal pressure balanced by atmospheric pressure

Buoyant force (upthrust) = upward force exerted by a fluid on a submerged object.

Archimedes' Principle: The buoyant force on an object = weight of fluid displaced by the object.

Upthrust = Volume of object submerged × density of fluid × g

Floating and Sinking:

Why ships float despite being made of steel?

A ship is hollow — its average density (steel + air inside) is less than water's density → it displaces enough water to equal its own weight.

• Thrust = perpendicular force on surface (N)
• Pressure = Thrust/Area; unit = Pascal (Pa) = N/m²
• Liquid pressure = hρg; increases with depth
• Archimedes' Principle: upthrust = weight of displaced fluid
• Floating condition: object's weight = weight of fluid displaced (average density ≤ fluid density)

1. Why is it painful to step on a nail but not on a broad flat surface with the same force?
2. A wooden block weighing 50 N is placed on water. If upthrust = 50 N, will it float or sink?
3. Calculate the pressure: force = 200 N, area = 0.4 m².
4. Why does a submarine sink when it fills its tanks with water?
5. Stretch: A diver goes from 5 m depth to 10 m depth. How does the water pressure on the diver change, and by how much? (ρwater = 1000 kg/m³, g = 10 N/kg)