Chapter Three: Force
Inertia & Newton's First Law · Fundamental Forces · Balanced/Unbalanced Forces · Momentum · Collision & Conservation Laws · Newton's Second Law · Gravitational Force · Newton's Third Law · Frictional Force — Complete Guide.
📋 Chapter 3 — Complete Content Structure
3.1 Inertia and Concept of Force — Newton's First Law
Inertia is the property of matter that resists changes in motion. Understanding inertia is the foundation for Newton's First Law and the concept of force.
🔍 3.1.1 Inertia — Definition & Types
Definition: Inertia is the property of a body by virtue of which it tends to preserve its state of rest or uniform motion in a straight line.
Mass is a measure of inertia: Greater mass ⇒ Greater inertia.
🪑 Inertia of Rest
The tendency of a body at rest to remain at rest.
Example: When a bus suddenly starts, passengers tend to fall backward.
🏃 Inertia of Motion
The tendency of a moving body to continue moving.
Example: When a moving bus suddenly stops, passengers tend to fall forward.
🧭 Inertia of Direction
The tendency of a moving body to maintain its direction of motion.
Example: Passengers lean sideways when a moving bus takes a sharp turn.
📜 Newton's First Law of Motion (Law of Inertia)
Every object continues in its state of rest or uniform motion in a straight line unless an external force acts on it.
Key Insight: This law defines force as an external cause that changes or tends to change the state of rest or motion.
⚡ Concept of Force
Force is an external cause that changes or tends to change the state of rest or motion of a body.
Force can: Start motion, Stop motion, Change velocity, Change direction, Change shape.
SI Unit: Newton (N)
💡 Tips & Tricks — Inertia & Force
- When a bus starts → passengers fall backward (Inertia of Rest)
- When a bus stops → passengers fall forward (Inertia of Motion)
- When a bus turns → passengers lean sideways (Inertia of Direction)
- Mass = measure of inertia. More mass → harder to change motion.
3.2 Nature of Fundamental Forces
There are four fundamental forces in nature. They govern all physical interactions in the universe.
🌌 Gravitational Force
Attractive force between any two masses. Always attractive, long-range, weakest fundamental force. Examples: Earth attracts objects, Sun attracts planets.
⚡ Electromagnetic Force
Force associated with electric charges and magnetic effects. Can be attractive or repulsive. Includes electric force and magnetic force.
☢️ Weak Nuclear Force
Short-range force involved in radioactive processes like beta decay. Very short range, stronger than gravity at nuclear scales.
🔬 Strong Nuclear Force
The strongest fundamental force. Binds quarks and holds protons/neutrons together in the nucleus. Extremely strong, very short range.
| Force | Nature | Relative Strength | Range |
|---|---|---|---|
| Gravitational | Attractive | Weakest | Long |
| Electromagnetic | Attractive/repulsive | Strong | Long |
| Weak nuclear | Nuclear | Stronger than gravity | Very short |
| Strong nuclear | Nuclear | Strongest | Very short |
3.3 Balanced and Unbalanced Forces
The net force acting on an object determines whether its motion will change.
✅ Balanced Force
When net force is zero (F_net = 0). Object remains at rest or continues with uniform velocity. Example: book resting on a table.
⚠️ Unbalanced Force
When net force is not zero (F_net ≠ 0). Object accelerates, decelerates, or changes direction. Example: pushing a car from rest.
💡 Key Insight
Balanced force does NOT mean the object is stationary. A body can move with constant velocity while forces are balanced (e.g., car cruising at constant speed).
3.4 Momentum
Momentum is the product of mass and velocity. It is a vector quantity that describes the quantity of motion.
SI Unit: kg·m/s | Dimension: [MLT⁻¹]
📌 Key Points
Momentum is a vector (direction = direction of velocity). Change in momentum = Impulse (J = FΔt = Δp).
3.5 Collision & Conservation of Momentum
Collisions involve short-duration interactions where momentum is conserved (in isolated systems).
(Total momentum before = Total momentum after)
✅ Elastic Collision
Both momentum AND kinetic energy conserved. Example: billiard balls.
🔄 Inelastic Collision
Momentum conserved, but kinetic energy is NOT conserved. Example: car crash, objects sticking together.
🚗 Safe Journey — Seat Belt & Airbag
Seat Belt: Provides force to reduce forward motion (inertia of motion). Airbag: Increases time Δt over which momentum changes, reducing average force (F = Δp/Δt).
3.6 Newton's Second Law — F = ma
Newton's second law quantifies the relationship between force, mass, and acceleration.
F = Δp / Δt = (mv - mu) / t
1 N = 1 kg·m/s²
💡 Tips for F = ma Problems
Always identify net force first. If multiple forces act, find the resultant. Remember unit conversions: 1 km/h = 5/18 m/s.
3.7 Gravitational Force & Weight
Gravitational force is the attraction between masses. Weight is the gravitational force on an object near Earth's surface.
g ≈ 9.8 m/s²
⭐ Mass vs Weight
Mass is amount of matter (scalar, kg, constant). Weight is gravitational force (vector, N, changes with g). At Earth's centre, g = 0 → W = 0, but mass ≠ 0.
3.8 Newton's Third Law — Action & Reaction
For every action, there is an equal and opposite reaction. Forces act on different objects.
📌 Examples
Walking (foot pushes ground backward, ground pushes foot forward), Swimming (push water backward, water pushes forward), Rocket (expel gases downward, rocket moves upward), Jumping from boat (boat moves backward).
3.9 Frictional Force
Friction opposes relative motion or tendency of motion between surfaces. It is both useful and harmful.
Static Friction
Acts when no relative motion. Example: pushing a heavy box that doesn't move.
Sliding Friction
Acts when surfaces slide over each other. Example: box sliding on floor.
Rolling Friction
Acts when object rolls. Generally less than sliding. Example: wheel on road.
💡 Order of Friction
F_rolling < F_sliding < F_limiting. Ball bearings convert sliding to rolling friction to reduce wear.
⭐ Friction: A Necessary Evil
Useful: walking, braking, holding objects. Harmful: wear, heat, energy loss. Called "necessary evil" because it is both helpful and harmful.
Interactive MCQ Bank — 30 Questions
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Board Question Analysis (2016–2025)
Force is one of the most heavily tested chapters in SSC Physics.
🏆 Highest Priority Topics
| Topic | Priority |
|---|---|
| Newton's First Law | ⭐⭐⭐⭐⭐ |
| Types of inertia | ⭐⭐⭐⭐⭐ |
| Momentum (p=mv) | ⭐⭐⭐⭐⭐ |
| Conservation of momentum | ⭐⭐⭐⭐⭐ |
| Newton's Second Law (F=ma) | ⭐⭐⭐⭐⭐ |
| Newton's Third Law | ⭐⭐⭐⭐⭐ |
| Friction types & applications | ⭐⭐⭐⭐⭐ |
| Safe journey (seat belt/airbag) | ⭐⭐⭐⭐ |
| Mass vs weight | ⭐⭐⭐⭐⭐ |
Mathematical Problems with Solutions
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Important Short Questions (SQ)
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Creative Questions (CQ) with Solutions
Key CQs for board preparation.
Chapter 3 Formula Sheet
Force: F = ma
Impulse: J = FΔt = Δp = mv - mu
Weight: W = mg
Newton's 3rd: F_AB = -F_BA
Momentum conservation: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
1 N = 1 kg·m/s²
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Md. Mominul Islam