You are here
๐ฌ Think about it
Megha and Pawan carry identical, equally heavy bags — yet Pawan's shoulders hurt and Megha's don't. The only difference? His straps are narrow, hers are broad. How can the same weight feel so different? The answer is one of the most useful ideas in science.
Why do broad straps feel more comfortable than narrow straps?
- Both bags have the same weight pulling down on the shoulders.
- Narrow straps press that weight onto a small area.
- Broad straps spread the same weight over a larger area.
- Spreading the force over more area reduces the pressure, so it feels easier.
Example:
A bucket with a broad handle is easier on the hand than one with a thin handle; a cloth ring under a head-load spreads the weight too.
Broad Straps
Narrow Straps
Weight acts on a
larger area
of the shoulder.
Weight acts on a
smaller area
of the shoulder.
Lower pressure
(the force is spread over more area).
Higher pressure
(the same force on less area).
More comfortable
to carry.
Hurts —
less comfortable
.
What is pressure?
- Pressure is the force acting per unit area.
- We consider the force acting perpendicular to the surface.
- The SI unit of pressure is newton per metre squared (N/m²).
- This unit is also called the pascal (Pa).
\( \text{Pressure} = \dfrac{\text{Force}}{\text{Area}} \)
How do we calculate pressure? (worked example)
- Use the formula pressure = force ÷ area.
- If a force of 100 N acts on an area of 2 m², divide them.
- The pressure works out to 50 N/m².
\( \text{Pressure} = \dfrac{100\ \text{N}}{2\ \text{m}^2} = 50\ \text{N/m}^2 \)
Example:
The same 100 N spread over 4 m² would give only 25 N/m² — double the area, half the pressure.
In this Activity, we will compare two ways of doing a task to see how the area of contact changes the pressure.
๐ก Safety first
- The activities in Table 6.1 should be done under the supervision of an adult.
Procedure & Observation
| Activity | Modes of action | Easy or difficult to perform? Give reasons. |
|---|---|---|
| Driving an iron nail | By the flat head of the nail | Difficult — large area, low pressure |
| Driving an iron nail | By the pointed end of the nail | Easy — small area, high pressure |
| Cutting an apple | Using the sharp edge of the knife | Easy — small area, high pressure |
| Cutting an apple | Using the blunt edge of the knife | Difficult — large area, low pressure |
Explanation
When the same force acts on a smaller area, the pressure is higher, making the task easier. That is why a pointed nail and a sharp knife edge work better.
โ Summary
- Same force, smaller area
- Higher pressure results
- Pointed or sharp = easier
Do liquids exert pressure?
- Yes — a column of water exerts pressure at the bottom of its vessel.
- The pressure depends on the height of the liquid column, not the vessel's width.
- Equal water heights give equal pressure, even in pipes of different diameters.
Why are overhead water tanks placed at a height?
- The taller the water column, the greater the pressure at the bottom.
- Placing the tank high gives a tall water column above the taps.
- This higher pressure gives a strong stream of water from the taps.
Example:
A friend on the first floor (more water column above them) gets a stronger tap stream than someone on the second floor.
In this Activity, we will check whether the pressure of a water column depends on the height or the width of the column.
Materials needed
Two transparent pipes of the same length (about 25 cm) but different diameters, two good rubber balloons, a stand, and water.
Procedure
1. Attach a balloon to one end of each pipe.
2. Clamp both pipes on a stand.
3. Fill both pipes with water to the same level, about halfway.
4. Observe the balloons. Do both bulge? Do they bulge equally?
2. Clamp both pipes on a stand.
3. Fill both pipes with water to the same level, about halfway.
4. Observe the balloons. Do both bulge? Do they bulge equally?
Observation
Both balloons bulge to the same extent, even though the pipes hold different weights of water.
Explanation
The bulge depends on the pressure of the water column, which depends on its height. Equal heights give equal pressure, so equal bulges — the width does not matter.
โ Summary
- Equal water heights
- Equal balloon bulges
- Pressure depends on height
Do liquids exert pressure on the sides of a container too?
- Yes — water spurts out of holes made in the sides of a bottle.
- So liquids press not only on the bottom but also on the sides.
- In fact, liquids exert pressure in all directions.
Example:
Water spurting like a fountain from a leak in a pipe shows liquid pressing on the pipe walls.
In this Activity, we will check whether a liquid presses on the side walls of its container.
Materials needed
A used plastic bottle (cap removed), a needle or nail, tape, and water.
Procedure
1. Make four small holes near the bottom, around the sides, all at the same height.
2. Seal the holes with tape and fill the bottle with water.
3. Remove the tape from all holes at the same time.
4. Observe what happens.
2. Seal the holes with tape and fill the bottle with water.
3. Remove the tape from all holes at the same time.
4. Observe what happens.
Observation
Water flows out sideways through all the holes.
Explanation
Water pushes out through the side holes, so liquids exert pressure on the sides of a container as well as the bottom — in all directions.
โ Summary
- Water jets out the sides
- Liquid presses sideways too
- Pressure acts all directions
โก Ever heard of ...
- The base of a dam is much broader than its top.
- Stored water presses horizontally on the side walls and vertically on the floor.
- The horizontal pressure is very large near the bottom.
- So the base is made broad to withstand this pressure.
โ
Test Yourself
-
Define pressure and give its SI unit.
View Answer
Pressure is force per unit area; its SI unit is N/m², also called the pascal (Pa). -
Why do broad straps feel more comfortable?
View Answer
They spread the weight over a larger area, reducing the pressure on the shoulders. -
A force of 100 N acts on 2 m². What is the pressure?
View Answer
\( \dfrac{100\ \text{N}}{2\ \text{m}^2} = 50\ \text{N/m}^2 \). -
On what does the pressure of a liquid column depend?
View Answer
On the height of the column (not the width of the vessel). -
In how many directions do liquids exert pressure?
View Answer
In all directions — on the bottom and the sides.
Important Definitions
- Pressure — The force acting per unit area, perpendicular to the surface (Pressure = Force ÷ Area).
- Pascal (Pa) — The SI unit of pressure, equal to one newton per metre squared (N/m²).
๐ NCERT Question 1 — Choose the correct statement
MCQs on liquid level in connected vessels, suckers on smooth/rough surfaces, water-tank height, and pressure/force at the bottom of two vessels.
View Answer →
๐ NCERT Question 3 — Fig. 6.23 a shows a boy
In which case does the boy sink more in loose sand — lying horizontally or standing vertically? Give reasons.
View Answer →
๐ NCERT Question 4 — An elephant stands on four feet
An elephant weighing 20000 N stands on four feet, each of area 0.25 m². Calculate the pressure it exerts on the ground.
View Answer →
๐ NCERT Question 5 — There are two boats, A and B
Boat A (7 m², 5 persons) and Boat B (3.5 m², 3 persons), each person 700 N. Which boat has more pressure on its base, and by how much?
View Answer →
๐ NCERT Question 7 — What will happen to the two
Two identical balloons A and B (Fig. 6.24): when water is filled to a height, will both bulge, and will they bulge equally? Explain.
View Answer →
