Force and pressure — types of forces, pressure in fluids, atmospheric pressure

Question

A block of weight 100 N rests on a table. Its base area is 0.5 m $^2$ . Calculate the pressure exerted on the table. Why does a sharp nail pierce easily while a blunt one does not?

(CBSE Class 8 — Force and Pressure)

Force Classification

flowchart TD
    A["Types of Forces"] --> B["Contact Forces"]
    A --> C["Non-Contact Forces"]
    B --> B1["Muscular Force"]
    B --> B2["Friction"]
    B --> B3["Normal Force"]
    B --> B4["Tension"]
    C --> C1["Gravitational Force"]
    C --> C2["Magnetic Force"]
    C --> C3["Electrostatic Force"]
    A --> D["Pressure = Force / Area"]
    D --> D1["Same force, less area = more pressure"]
    D --> D2["Same force, more area = less pressure"]

Solution — Step by Step

Pressure is defined as force per unit area:

P = \frac{F}{A}

Given: $F = 100$ N (weight of block), $A = 0.5$ m $^2$

P = \frac{100}{0.5} = \mathbf{200 \text{ N/m}^2 = 200 \text{ Pa}}

The unit N/m $^2$ is also called Pascal (Pa).

A sharp nail has a very small tip area (say 0.001 cm $^2$ ), while a blunt nail has a larger tip area (say 0.1 cm $^2$ ). When we hammer with the same force:

Sharp nail: $P = F / 0.001$ = very high pressure
Blunt nail: $P = F / 0.1$ = much lower pressure

The sharp nail exerts 100 times more pressure than the blunt one with the same force. This concentrated pressure is enough to pierce the material.

Fluids (liquids and gases) exert pressure in all directions at any point. This is why:

A dam is thicker at the bottom (pressure increases with depth)
A diver feels more pressure deeper underwater

Atmospheric pressure is the weight of the air column above us. At sea level, it is about 101,325 Pa (1 atm). We do not feel crushed because the pressure inside our body equals the pressure outside.

Why This Works

Pressure depends on two things: the magnitude of the force and the area over which it acts. This inverse relationship between area and pressure explains many everyday observations: heavy trucks have wide tyres (to reduce ground pressure), knives have thin edges (to increase cutting pressure), and snowshoes spread weight over a large area (to avoid sinking).

Alternative Method — Real-Life Examples Table

Situation	Force	Area	Pressure	Effect
Elephant standing	Very large	Large feet	Moderate	Does not sink much
High heels	Body weight	Tiny heel	Very high	Damages soft floors
Tank treads	Very large	Huge track area	Low	Moves on soft ground
Needle tip	Small push	Extremely tiny	Enormous	Pierces skin easily

CBSE almost always asks one numerical on pressure (like the block problem) and one conceptual question (like sharp vs blunt nail). Make sure you state the formula, substitute values with units, and write the final answer with the correct unit (Pa or N/m $^2$ ). Missing units costs marks.

Common Mistake

Students write the formula as $P = A/F$ instead of $P = F/A$ . Remember: Pressure increases when area decreases (for the same force). If your formula gives the opposite relationship, you have it inverted. A quick sanity check: a needle (small area) should give high pressure.