Force And Pressure — for Class 8

What Force and Pressure Actually Mean

Push a door open. Kick a football. Squeeze a lemon. Every one of these actions involves force — and once you understand what force truly is, you start seeing it everywhere.

A force is a push or a pull acting on an object. That’s the complete definition. Forces can change an object’s speed, direction, or shape. They need an agent (something applying the force) and an object (something receiving it).

Now here’s where Class 8 gets interesting: two objects don’t always need to touch for a force to act between them. A magnet attracts an iron nail from a distance. The Earth pulls you downward without physically grabbing you. This distinction — contact vs. non-contact forces — is a favourite in CBSE board questions.

Pressure builds on force but adds something crucial: where the force is applied matters. The same force spread over a large area feels gentle; concentrated on a tiny area, it cuts through. A nurse uses this idea every time she uses a syringe needle instead of a thumb to inject medicine. Same applied force — completely different pressure.

These two concepts together explain why knives have sharp edges, why tractors have wide tyres, why dams are thick at the base, and why submarines need reinforced hulls. CBSE Class 8 has ~12–15 marks directly tied to this chapter, and once you genuinely understand the logic, those marks are yours.

Key Terms and Definitions

Force A push or a pull that can change the state of rest or motion of an object, or change its shape. Force is a vector — it has both magnitude and direction.

Contact Force Acts only when two objects are physically touching.

Examples: Muscular force (your muscles pushing/pulling), friction (surfaces opposing each other’s motion), spring force

Non-Contact Force Acts even without physical contact.

Examples: Gravitational force (Earth pulling every object toward its centre), magnetic force (magnets attracting iron), electrostatic force (a charged balloon attracting paper bits)

Pressure Force acting per unit area.

P = \frac{F}{A}

Where:

$P$ = Pressure (in Pascals, Pa)
$F$ = Force applied (in Newtons, N)
$A$ = Area over which force acts (in m²)

1 Pascal = 1 N/m²

Net Force (Resultant Force) When multiple forces act on an object, the net force is their combined effect. Forces in the same direction add up; forces in opposite directions subtract.

Balanced Forces When two or more forces acting on an object cancel each other out — net force is zero. The object stays at rest or keeps moving at constant speed.

Unbalanced Forces When the net force is not zero. The object accelerates, decelerates, or changes direction.

Fluid Pressure Pressure exerted by liquids and gases. A fluid exerts pressure in all directions — this is what makes water shoot out from a hole in a pipe equally regardless of which side the hole is on.

Methods and Concepts

How Forces Change Motion

Think of a football sitting still on the ground. It will stay there forever unless a force acts on it. Kick it (unbalanced force) and it moves. Two players kicking it from opposite sides with equal force? It stays put (balanced forces).

The key rule: unbalanced forces change the state of motion; balanced forces do not.

When we say “state of motion,” we mean:

A stationary object starts moving
A moving object speeds up, slows down, or changes direction
A moving object stops

How Forces Change Shape

A force doesn’t always produce visible motion. Press a lump of clay — it deforms. Stretch a rubber band — it elongates. Here, the force changes the shape of the object, not (necessarily) its position.

CBSE frequently asks whether a force is always needed to maintain motion. The answer for Class 8 is YES — because friction is always present in real life. In outer space (no friction), a moving object continues forever without any force. This is Newton’s First Law, which you’ll study in depth in Class 9.

Calculating Pressure: The Logic Behind the Formula

Why do we divide force by area? Because the same force spread over more surface distributes its effect. Imagine pressing your palm flat versus pressing just your fingertip. Your muscle applies roughly the same force in both cases, but your fingertip hurts much more because all that force is concentrated on a tiny area.

This is why:

Knives are sharpened — small area = high pressure = easier cutting
Nails are pointed — same logic
Snowshoes are wide — large area = low pressure = don’t sink into snow
Tractor tyres are broad — spread weight over large area so they don’t sink into soft soil
High heels damage floors — small area concentrates force into high pressure

Pressure in Liquids (Liquid Pressure)

Liquids exert pressure on everything submerged in them, and this pressure increases with depth. This is why:

Dams are built thicker at the base (more pressure at greater depth)
Deep-sea fish have special body structures
Your ears hurt when you swim to the pool’s deep end

P = h \times \rho \times g

Where:

$h$ = height/depth of liquid column (m)
$\rho$ = density of liquid (kg/m³)
$g$ = acceleration due to gravity (≈ 10 m/s²)

Note: This formula is for conceptual understanding at Class 8 level — CBSE won’t ask numerical calculations using this at Class 8.

Liquid pressure acts equally in all directions at a given depth. A hole at the side of a container at a certain depth will shoot water just as forcefully as a hole at the bottom at the same depth.

Atmospheric Pressure

The atmosphere (air around Earth) has weight, and this weight exerts pressure on everything below it. At sea level, atmospheric pressure is approximately 101,325 Pa (about 10⁵ Pa).

We don’t feel this pressure crushing us because the fluids inside our body push outward with equal pressure.

CBSE Class 8 loves application questions on atmospheric pressure: Why does a straw work? (You reduce air pressure inside the straw by sucking; atmospheric pressure on the liquid’s surface pushes it up.) Why do sealed packets puff up in the mountains? (Lower atmospheric pressure outside; the higher pressure inside expands the packet.)

Solved Examples

Example 1 — Easy (CBSE Board Level)

Q: A force of 200 N acts on an area of 4 m². Find the pressure.

$P = \dfrac{F}{A} = \dfrac{200}{4} = 50 \text{ Pa}$

The pressure is 50 Pascals.

Example 2 — Easy (CBSE Board Level)

Q: A man weighing 600 N stands on one foot. The area of his foot is 0.03 m². What pressure does he exert on the ground?

$P = \dfrac{F}{A} = \dfrac{600}{0.03} = 20{,}000 \text{ Pa}$

The man exerts 20,000 Pa on the ground.

Don’t forget to halve the weight when the person stands on both feet. The 600 N gets distributed across 0.06 m² total area. If the question asks for pressure with both feet, use $P = 600 \div 0.06 = 10{,}000$ Pa.

Example 3 — Medium (CBSE Board Level)

Q: The same force is applied on two different surfaces — one with area 10 cm² and another with area 100 cm². In which case is the pressure more, and by how much?

Let force = $F$ (same for both).

For surface 1: $P_1 = \dfrac{F}{10}$

For surface 2: $P_2 = \dfrac{F}{100}$

Ratio: $\dfrac{P_1}{P_2} = \dfrac{F/10}{F/100} = \dfrac{100}{10} = 10$

Surface 1 has 10 times more pressure than Surface 2.

This confirms: smaller area → higher pressure (with same force).

Example 4 — Medium-Hard (CBSE + Concept Application)

Q: A brick of dimensions 20 cm × 10 cm × 5 cm and weight 15 N can be placed in three different orientations. Find the pressure in each case and identify which orientation exerts maximum and minimum pressure.

Orientation 1 (20 × 10 face down): $A = 200 \text{ cm}^2 = 0.02 \text{ m}^2$ ; $P = \frac{15}{0.02} = 750 \text{ Pa}$
Orientation 2 (20 × 5 face down): $A = 100 \text{ cm}^2 = 0.01 \text{ m}^2$ ; $P = \frac{15}{0.01} = 1500 \text{ Pa}$
Orientation 3 (10 × 5 face down): $A = 50 \text{ cm}^2 = 0.005 \text{ m}^2$ ; $P = \frac{15}{0.005} = 3000 \text{ Pa}$

Maximum pressure: Orientation 3 (smallest face down) — 3000 Pa Minimum pressure: Orientation 1 (largest face down) — 750 Pa

This is a classic CBSE application question — the concept tests whether you understand that weight (force) stays constant, only area changes.

Exam-Specific Tips

CBSE Class 8 Marking Scheme

1-mark questions: Usually definition-based or identify-the-type-of-force. Know all force types cold.
3-mark questions: Typically a numerical (show formula, substitute, calculate) + a real-life application. Show every step — partial marks are given.
5-mark questions: Rare for this chapter, but if asked, expect compare-and-contrast (contact vs. non-contact) or multi-part numericals.

The question “Give two examples of non-contact forces” appears in nearly every school examination. The three you must know: gravitational force, magnetic force, electrostatic force. Two is usually enough, but knowing three prevents blanks.

Unit Awareness

Pressure in Pascals (Pa) when force is in Newtons and area in m².

If area is given in cm², convert first: divide by 10,000 to get m² (since 1 m² = 10,000 cm²).

Not converting units is the single biggest source of wrong answers in numericals.

Diagram Questions

CBSE sometimes asks you to draw the direction of forces in a tug-of-war or a book resting on a table. Practice drawing force arrows with clear labels — direction matters.

Common Mistakes to Avoid

Mistake 1: Confusing force and pressure. Students write “applying more force = more pressure” — this is only true if area stays constant. You can apply a huge force over a large area and have very low pressure. Always check whether the area is changing.

Mistake 2: Forgetting unit conversion for area. Area is almost always given in cm² in problems. Convert to m² before substituting in $P = F/A$ . Missing this step gives an answer 10,000 times too large.

Mistake 3: Treating atmospheric pressure as negligible. When asked why a suction cup works or why a straw draws liquid, students say “because of suction force” — there is no such thing as suction force. The correct answer is always “atmospheric pressure pushes the object.” Examiners specifically mark for this.

Mistake 4: Saying balanced forces mean no forces act. “No net force” is not the same as “no forces acting.” A book on a table has gravity pulling it down AND the table’s normal force pushing it up. Two real forces, both present, both cancelling — the book stays still. Big difference from saying “no forces act on the book.”

Mistake 5: Assuming liquids only push downward. Liquid pressure acts in ALL directions. The classic proof: punch a hole on the side of a water-filled bottle — water spurts out sideways. If pressure only acted downward, this wouldn’t happen. CBSE has asked this concept directly.

Practice Questions

Q1. Define force. What are the two effects of force on an object?

Force is a push or a pull acting on an object. The two effects of force: (1) it can change the state of motion of an object (make it start, stop, speed up, slow down, or change direction), and (2) it can change the shape of an object.

Q2. Classify the following as contact or non-contact forces: friction, gravitational pull, magnetic attraction, muscular force, electrostatic force.

Contact forces: Friction, muscular force. Non-contact forces: Gravitational pull, magnetic attraction, electrostatic force.

Q3. A force of 500 N acts on a surface of area 0.25 m². Calculate the pressure exerted.

$P = \dfrac{F}{A} = \dfrac{500}{0.25} = 2000 \text{ Pa}$

The pressure exerted is 2000 Pascals.

Q4. Why are the tyres of tractors and army tanks very wide?

Wide tyres increase the area over which the vehicle’s weight (force) acts on the ground. By the formula $P = F/A$ , a larger area means lower pressure. This prevents the vehicle from sinking into soft or muddy ground.

Q5. A girl weighing 400 N is wearing stiletto heels. Each heel has an area of 1 cm². She stands on both heels. What pressure does she exert on the floor through her heels?

Total heel area = 2 × 1 cm² = 2 cm² = 2 × 10⁻⁴ m²

$P = \dfrac{400}{2 \times 10^{-4}} = 20,00,000 \text{ Pa} = 2 \times 10^6 \text{ Pa}$

That’s 20 lakh Pascals — enough to dent wooden flooring! This is why high heels are often banned in old buildings.

Q6. Two students pull a rope from opposite ends with equal force of 80 N each. What is the net force on the rope? Will the rope move?

The two forces are equal in magnitude (80 N) but opposite in direction. They form balanced forces, so the net force = 0. The rope will not move — it will remain stationary.

Q7. Why does a dam have a wider and thicker base compared to its top?

Water pressure increases with depth. The water at the base of a dam exerts much greater pressure than water near the top. The dam’s base must be thick enough to withstand this higher pressure without breaking. If the base were as thin as the top, the dam would collapse under the pressure of deep water.

Q8. Explain why it is easier to walk on snow with snowshoes than without them.

Without snowshoes, your body weight (force) is distributed over the small area of your boots — creating high pressure that causes you to sink into the snow. Snowshoes have a much larger area, so the same body weight creates much lower pressure on the snow’s surface, which is not enough to compress and break the snow — you float on top instead of sinking.

Frequently Asked Questions

Q: What is the difference between force and pressure?

Force is simply the push or pull — it’s measured in Newtons. Pressure is force per unit area — it’s measured in Pascals. Force depends only on how hard you push; pressure depends on both how hard you push AND how small an area you push on. A nail and a thumb can be pushed with the same force, but the nail creates enormously more pressure because its tip is tiny.

Q: Can pressure exist without force?

No. Pressure is defined as force divided by area — if there is no force, there is no pressure. Even atmospheric pressure exists because air molecules exert a gravitational force (they have mass and Earth pulls them down, creating weight).

Q: Why does a knife cut better when it is sharp?

A sharp knife has a very thin edge — tiny contact area. When you apply force on the knife, that force concentrates on this tiny area, creating very high pressure. High pressure is what actually cuts through materials. A blunt knife has a wider edge, so the same force spreads over more area, creating lower pressure — hence it doesn’t cut well.

Q: Is friction a contact force or non-contact force?

Friction is a contact force. It arises only when two surfaces are in contact and one moves (or tends to move) relative to the other. No contact = no friction.

Q: What happens to pressure if force is doubled but area remains the same?

From $P = F/A$ , if $F$ doubles and $A$ stays constant, then $P$ also doubles. Pressure is directly proportional to force when area is constant.

Q: Why do liquids exert pressure in all directions?

Unlike solids, liquid molecules can move freely in all directions. When you submerge an object in a liquid, molecules bombard it from all sides — above, below, left, right. This is why liquid pressure acts equally in all directions at a given depth (called Pascal’s Law at higher levels).

Q: What is 1 Pascal equal to?

1 Pascal = 1 Newton per square metre (1 N/m²). It is a relatively small unit — atmospheric pressure is about 1,00,000 Pa, which is why we sometimes use kilopascal (kPa) for larger values.

Q: Why do our eardrums hurt when we swim underwater?

The deeper we go, the higher the water pressure (pressure increases with depth). At greater depths, this pressure pushes against the eardrum from outside, while the air pressure inside the ear canal remains normal. This pressure difference causes pain. The solution is to equalise — let air flow through the Eustachian tube to balance pressures.