Pressure & Fluids
📚 Topic Overview: Pressure – Basics, Fluids & Everyday Applications
This module explains the idea of pressure as force acting on an area, how pressure behaves in
liquids and the atmosphere, and how it is used in simple devices like straws, syringes, dams
and brakes. Questions in competitive exams often test the relationship between force, area
and depth, plus reasoning on daily-life situations.
1. Pressure & Relationship Between Force and Area
Concept / Theory
When a force acts on a surface, it is important to know not only how large the force is,
but also on how large or small an area it is applied. The same force can have very different
effects if it is spread over a wide area or concentrated on a sharp tip.
Pressure is defined as the force acting per unit area
on a surface. It tells us how effectively a force is pressing on that surface.
Pressure increases when force increases, and also increases when the area becomes smaller.
For the same force, smaller area gives greater pressure; for the same area, larger force
gives greater pressure.
Analogy / Examples
- A sharp nail goes into wood easily, but a blunt nail requires much more effort.
- A lady wearing high-heel sandals exerts more pressure on the floor than the same person wearing flat slippers.
- Porters sometimes place a folded cloth on their head while carrying luggage to spread the force over a larger area and reduce pressure.
Conversions / Formulas
- Pressure (P) = Force (F) ÷ Area (A)
- SI unit of force: newton (N)
- SI unit of area: square metre (m²)
- SI unit of pressure: N/m², also called pascal (Pa)
- Greater force → greater pressure (if area is constant).
- Smaller area → greater pressure (if force is constant).
| Situation | Area of Contact | Pressure Effect |
|---|---|---|
| High-heel sandals | Small | High pressure on floor |
| Flat slippers | Larger | Lower pressure on floor |
| Knife with sharp edge | Very small | Cuts easily due to high pressure |
| English | Telugu |
|---|---|
| Pressure | పీడనం |
| Force | బలం |
| Area | విస్తీర్ణం |
| Pascal | పాస్కల్ (పీడన ఏకకం) |
In a village, a carpenter uses a hammer and nail to fix wooden planks. The force applied
by the hammer is concentrated on the very small tip of the nail, producing very high pressure,
so the nail enters the wood easily. If the same force were applied by pressing with a thumb,
the area would be larger, pressure would be less, and the nail would not enter the wood.
Whenever you see words like “sharp–blunt”, “high-heel–flat”, “wide–narrow tyre”, the
question is usually checking your understanding that pressure depends on area. Keep the
short rule in mind: same force + smaller area → higher pressure.
2. Fluid Pressure & Pressure in a Liquid Column
Concept / Theory
Fluids (liquids and gases) can flow and take the shape of their container. They also exert
pressure on the walls and bottom of the container. In liquids like water, pressure at a point
increases with depth below the free surface.
A liquid at rest exerts pressure in all directions: downwards on the bottom, sideways on the walls,
and even upwards on any surface placed in it. The deeper we go, the greater the pressure due to
the weight of the liquid column above that point.
For a liquid of density ρ, at depth h in a gravitational field g, pressure due to the liquid column
is often written as P = hρg. This shows that liquid pressure
increases with depth (h), density (ρ) and gravitational acceleration (g).
Analogy / Examples
- Three holes at different heights on a bottle wall: jet from the lowest hole goes farthest, showing maximum pressure at greater depth.
- Divers feel more pressure on their ears when they go deeper into a pond or pool.
- Walls of dams are thicker at the bottom than at the top because water pressure is higher at greater depth.
Conversions / Formulas
- Liquid pressure at depth h: P = hρg (qualitative use at this level).
- ρ (rho) = density of liquid (kg/m³).
- g ≈ 9.8 m/s², often taken as 10 m/s² in simple calculations.
- Greater depth (h) → greater liquid pressure.
- Greater density (ρ) → greater liquid pressure for same depth.
| Factor | Change | Effect on Liquid Pressure |
|---|---|---|
| Depth (h) | Increases | Pressure increases |
| Depth (h) | Decreases | Pressure decreases |
| Density (ρ) | Higher (e.g., oil vs water vs mercury) | More pressure at same depth |
| English | Telugu |
|---|---|
| Fluid | ద్రవం (లిక్విడ్ / గ్యాస్) |
| Depth | లోతు |
| Density | సాంద్రత |
| Liquid column | ద్రవ స్తంభం |
In a town, drinking water is stored in an overhead tank built on a tall tower. Because the
water level in the tank is high above the ground, the water at the taps in houses comes out
with enough pressure, even for upper floors. This is a direct use of the idea that the pressure
at the tap depends on the height (depth) of the water column above it.
Questions on “holes in a bottle”, “leaks near bottom of tank”, “thicker dam walls at bottom”
or “overhead tanks” are testing your understanding that liquid pressure increases with depth.
To answer quickly, first identify which point is deeper – that point will have greater pressure.
3. Atmospheric Pressure – Air as a Sea of Gas
Concept / Theory
The Earth is surrounded by a thick layer of air called the atmosphere. Air has mass, so it
exerts force on all objects on the Earth’s surface. The pressure exerted by this air is called
atmospheric pressure.
Atmospheric pressure acts in all directions – downwards, upwards and sideways. We do not normally
feel crushed because the pressure inside our body balances the external atmospheric pressure.
At sea level, the average value of atmospheric pressure is about 1.01 × 10⁵ Pa, which is called
1 atmosphere (1 atm). As we go to higher altitudes (hills, mountains, aeroplanes), atmospheric
pressure decreases because the thickness of air column above us becomes less.
Analogy / Examples
- A rubber sucker or suction cup sticks to a smooth wall when pressed – outside air pressure holds it there.
- A packet sometimes collapses inwards when air inside is removed – outside atmospheric pressure is greater.
- In high mountains, people sometimes feel breathing difficulty due to lower atmospheric pressure and less oxygen.
Conversions / Formulas
- Standard atmospheric pressure at sea level ≈ 1.01 × 10⁵ Pa.
- 1 atmosphere (1 atm) ≈ 76 cm of mercury column (in barometer).
- Altitude ↑ (go higher) → atmospheric pressure ↓.
- Altitude ↓ (go towards sea level) → atmospheric pressure ↑.
| Location | Air Column Above | Approximate Atmospheric Pressure |
|---|---|---|
| Sea level | Maximum | About 1 atm |
| Hill station | Less | Less than 1 atm |
| High mountain | Much less | Significantly lower than 1 atm |
| English | Telugu |
|---|---|
| Atmosphere | వాయుమండలం |
| Atmospheric pressure | వాయుమండల పీడనం |
| Barometer | పీడనమాపక (బ్యారోమీటర్) |
When you press a rubber suction hook against a bathroom tile and release it, some air from
inside the cup is pushed out. The air pressure inside becomes lower than the outside atmospheric
pressure. The greater pressure outside presses the cup against the tile, so it sticks and can
hold light objects like towels. This simple device uses atmospheric pressure in daily life.
If the question involves words like “suction”, “sucked out air”, “vacuum created”, “barometer”,
or “straw, syringe, dropper”, it usually tests understanding of atmospheric pressure and
lower pressure region inside the device. Look for which region has lower pressure and which
region has higher pressure pushing fluid or objects.
4. Applications of Pressure (Straw, Syringe, Dams, Brakes) & Exam Tips
Concept / Theory
Many simple tools and structures around us work on the ideas of pressure in solids, liquids and
gases. In most reasoning questions, we are asked to explain “why” something happens – which usually
comes down to understanding where pressure is high, where it is low and how fluids or objects move
from high pressure region to low pressure region.
Analogy / Examples
- Straw and syringe: create low pressure inside, so liquid is pushed in by atmospheric pressure outside.
- Dams: water pressure is greater at the bottom, so walls are made thicker there.
- Brakes (friction + pressure): pressing brake pedal applies force on small area of pads, increasing pressure and friction to stop wheels.
Conversions / Formulas
- Pressure basics: P = F ÷ A (used in brake systems, sharp tools, heels, etc.).
- Liquid pressure: P ∝ h (depth) and P ∝ ρ (density) – used in dams and tanks.
- Pressure difference: Fluids move from region of higher pressure to region of lower pressure.
- For liquids in containers, high level → more pressure at outlets below.
- For gases, removing air inside device creates lower pressure region.
| Application | Principle of Pressure | Key Idea for Exams |
|---|---|---|
| Drinking straw | Sucking reduces pressure inside straw; outside air pressure pushes liquid up. | Liquid rises from high pressure region (outside) to low pressure region (inside straw). |
| Syringe | Pulling plunger decreases pressure inside barrel; atmospheric pressure pushes liquid in. | Used in injections, sampling etc., based on pressure difference. |
| Dams | Water pressure increases with depth; bottom experiences maximum pressure. | Dam wall is thicker at bottom and thinner at top to withstand greater pressure. |
| Brakes in vehicles | Force applied on brake pedal transmitted to brake pads; high pressure on small contact area increases friction. | Friction + pressure together help stop rotating wheels safely. |
| English | Telugu |
|---|---|
| Straw | స్ట్రా (ద్రవం పీచే గొట్టం) |
| Syringe | సిరింజ్ |
| Dam | అణకట్ట |
| Brake | బ్రేక్ (ఆపే పరికరం) |
Children enjoying sugarcane juice at a roadside stall use a straw to drink from a steel tumbler.
They suck air from the straw, reducing the air pressure inside it. The atmospheric pressure on
the surface of the juice in the tumbler is now greater than the pressure inside the straw, so it
pushes the juice up through the straw into the mouth. This simple action beautifully shows how
fluids move from high pressure region to low pressure region.
Use these quick memory lines and tricks while answering pressure-based questions.
- Formula key: P = F ÷ A; liquid P ∝ hρ; gases flow from high P to low P.
- Sharp vs Blunt: same force, small area → high pressure → cutting, piercing is easy.
- Depth rule: deeper in water → greater pressure → stronger walls needed.
- Devices rule: straw, syringe, dropper, suction cup → always think “low pressure inside, higher atmospheric pressure outside”.
- One-line memory: “High P to low P, liquid will go; depth and density make pressure grow.”
For one-mark conceptual questions, look for words like “depth”, “height”, “sharp”, “suck”, “vacuum”.
These indicate whether the question is about solid pressure (force/area), liquid pressure (depth),
or atmospheric pressure (air pushing in).