Quick Summary
This chapter explains how force changes the state of motion of a body. It connects everyday motion with Newton's three laws, momentum, impulse, and conservation of momentum. For board exams, you need both theory (laws + examples) and numericals (F = ma, momentum, collisions).
Key Definitions
- Force: A push or pull that changes or tries to change the state of rest or uniform motion of a body.
- Inertia: The property of a body to resist any change in its state of rest or uniform motion. Greater mass means greater inertia.
- Momentum (p): The product of mass and velocity.
p = mv. SI unit: kg·m/s or N·s. - Impulse (J): The change in momentum produced by a force acting for a short time.
J = F × t = Δp. - Unbalanced force: A net force that is not zero — it causes acceleration.
Formulae Reference
| Concept | Formula | SI Unit |
|---|---|---|
| Momentum | p = mv | kg·m/s |
| Force | F = ma | N (newton) |
| Impulse | J = F × t = Δp | N·s or kg·m/s |
| Weight | W = mg | N |
| Conservation of momentum | m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ | — |
Newton's Laws — Explained with Real Examples
First Law: Law of Inertia
Statement: A body continues to be at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.
Inertia of Rest — why a passenger jolts forward when a bus brakes: When a bus is moving, the passenger is also moving at the same speed. When the bus brakes suddenly, the lower body (in contact with the seat) stops along with the bus. But the upper body has inertia of motion — it tries to continue moving forward. This is why the passenger lurches forward. The bus provides a stopping force to the lower body, but no such force acts on the upper body quickly enough.
Inertia of Motion — the coin and cardboard trick: Place a coin on a cardboard that rests on top of a glass. When you pull the cardboard sharply, the coin does not move with it — instead, it falls straight into the glass. This happens because the coin has inertia of rest and tends to stay where it is. The force of pulling acts on the cardboard, not the coin.
Second Law: F = ma — Derived from Momentum
Full Derivation:
This means: a larger force produces greater acceleration, and a heavier body needs more force to achieve the same acceleration.
Third Law: Action and Reaction
Statement: For every action force, there is an equal and opposite reaction force, acting on two different bodies.
Key point: Action and reaction forces are equal in magnitude and opposite in direction, but they act on different objects, so they do not cancel each other.
Examples:
the rocket forward. There is no ground or air needed — this is why rockets work in space.
(reaction). This forward reaction is what moves you.
forward (reaction).
- Rocket propulsion: Burning fuel expels hot gases backward (action). The reaction force pushes
- Walking: Your foot pushes the ground backward (action). The ground pushes your foot forward
- Swimming: Swimmer pushes water backward with their arms (action). Water pushes the swimmer
- Recoil of a gun: When a bullet is fired forward (action), the gun kicks backward (reaction).
Worked Numerical Examples
Example 1 — Finding Acceleration
Problem: A net force of 20 N acts on an object of mass 5 kg. Find the acceleration.
Example 2 — Finding Force from Change in Momentum
Problem: A cricket ball of mass 0.5 kg moving at 10 m/s is stopped by a fielder in 0.05 seconds. Find the force exerted by the fielder.
Example 3 — Conservation of Momentum (Collision)
Problem: Object A (mass 2 kg) moves at 5 m/s and object B (mass 3 kg) moves at 2 m/s in the same direction on a frictionless surface. They collide and stick together. Find their common velocity after collision.
Example 4 — Impulse Calculation
Problem: A force of 50 N acts on a body for 0.2 seconds. Find the impulse and the change in momentum.
Board Exam Q&A — Model Answers
One Mark Questions
Q: Define inertia. A: Inertia is the property of a body to resist any change in its state of rest or uniform motion. It depends on the mass of the body — greater mass means greater inertia.
Q: What is the SI unit of momentum? A: The SI unit of momentum is kg·m/s (also written as N·s).
Q: State Newton's third law of motion. A: For every action force, there is an equal and opposite reaction force. These forces act on two different bodies and never cancel each other.
Q: What is the relationship between inertia and mass? A: Inertia is directly proportional to mass. A heavier body has greater inertia and is harder to start moving or to stop.
Q: Write the SI unit of force. A: The SI unit of force is newton (N). 1 N = 1 kg·m/s².
Two Mark Questions
Q: Differentiate between mass and weight.
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter in a body | Gravitational force on a body |
| Changes with location? | No — constant everywhere | Yes — varies with g |
| SI unit | kilogram (kg) | newton (N) |
| Formula | — | W = mg |
Q: A moving bus suddenly stops — why does a passenger jolt forward? A: The passenger's body has inertia of motion. When the bus stops suddenly, the lower body (in contact with the seat) stops along with the bus. But the upper body, due to inertia, continues to move forward. This makes the passenger lurch forward. This is an application of Newton's first law.
Q: Explain how a rocket works using Newton's third law. A: In a rocket, burning fuel produces hot gases that are expelled backward at very high speed. This is the action force. By Newton's third law, an equal and opposite reaction force acts on the rocket in the forward direction. This reaction force propels the rocket forward. Since no ground or air is needed for the reaction, rockets can work in the vacuum of space.
Q: Define impulse. Write its unit and formula. A: Impulse is the product of force and the time for which it acts. It equals the change in momentum of the body. Formula: J = F × t = Δp. SI unit: N·s (newton-second) or kg·m/s.
Five Mark Questions
Q: State and explain Newton's three laws of motion with one example each.
Newton's First Law (Law of Inertia): A body remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. Example: A book on a table stays at rest because no unbalanced horizontal force acts on it. If you push it (applying an unbalanced force), it moves.
Newton's Second Law (F = ma): The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force. For constant mass, F = ma. Example: Pushing a heavy cart needs more force than pushing a lighter one to get the same acceleration.
Newton's Third Law (Action and Reaction): For every action force, there is an equal and opposite reaction force acting on a different body. Example: When you walk, your foot pushes the ground backward (action); the ground pushes your foot forward (reaction) — this reaction moves you forward.
Q: Define momentum. Derive the mathematical form of Newton's second law from it.
Momentum: Momentum is the product of the mass and velocity of a body. p = mv. It is a vector quantity. SI unit: kg·m/s.
Derivation of F = ma: Consider a body of mass m with initial velocity u. A force F acts on it for time t, changing its velocity to v.
SI unit of force: 1 N = 1 kg × 1 m/s² = 1 kg·m/s².
Common Board Exam Mistakes
- Not writing "unbalanced force" in Newton's first law — examiners deduct marks for just writing "force."
- Using mass in grams — always convert to kilograms before substituting in F = ma.
- Skipping units in intermediate steps — write the unit with every quantity.
- Confusing action-reaction pairs — they always act on two different bodies, never on the same body.
- Omitting the law before the example — in 2-mark and 5-mark answers, always state the law first.
Tamil Nadu Board Exam Pattern
conservation of momentum with a numerical.
(mass vs weight) or short explanations (why passenger jolts forward).
- Chapter weightage: 5–7 marks (varies by year)
- Five-mark questions: Usually 1 five-mark on Newton's laws OR derivation of F = ma OR
- Two-mark questions: 1–2 questions on definitions (inertia, impulse, momentum) or differences
- One-mark questions: 1–2 questions from key definitions or SI units.
- Numericals: At least one numerical is expected — practice Examples 1, 2, and 3 above.
Revision Checklist
- Newton's three laws stated correctly with "unbalanced force" in First Law
F = maderivation written from memoryp = mv, impulse formula, and conservation of momentum formula memorised- Units of force (N), momentum (kg·m/s), and acceleration (m/s²) confirmed
- Examples ready for all three laws
- All four worked numericals above solved independently
- Mass-weight difference table prepared