Force and Laws of Motion: Newton's Laws, Momentum, Gravitation and Work-Energy for SSC
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Force
- 2.Newton’s Laws of Motion
- 3.Momentum and Impulse
- 4.Equations of Uniformly Accelerated Motion
- 5.Friction
- 6.Gravitation
- 7.Work, Energy and Power
- 8.Solved Examples
- 9.Quick Tips
- 10.Quick Revision Points
- 11.Practice Questions With Answers
- 12.How to Practise This Chapter
- 13.Frequently Asked Questions
PDF Notes: Force and Laws of Motion: Newton's Laws, Momentum, Gravitation and Work-Energy for SSC
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Force and motion is the base of school physics and a regular source of questions in the SSC general science section. The questions are mostly based on Newton’s three laws, units, simple formulas and daily-life examples. This note covers force, the three laws, momentum, equations of motion, friction, gravitation, work, energy and power with formulas, solved examples, practice questions and FAQs.
Force
A force is a push or a pull that can change the speed, direction or shape of an object. Its SI unit is the newton (N), where 1 N = 1 kg · m/s². Force is a vector quantity.
Newton’s Laws of Motion
| Law | Statement | Everyday example |
|---|---|---|
| First law (law of inertia) | An object stays at rest or in uniform motion in a straight line unless an external force acts on it. | Passengers lean forward when a bus stops suddenly; dust falls when a carpet is beaten |
| Second law | The rate of change of momentum is proportional to the applied force. F = ma. | A heavier cart needs a larger force for the same acceleration |
| Third law | To every action there is an equal and opposite reaction. | A rocket moves up when gas goes down; a gun recoils; swimming |
- Inertia is the tendency of a body to resist a change in its state. Mass is the measure of inertia.
- Types of inertia: inertia of rest, inertia of motion and inertia of direction.
- Action and reaction act on different bodies, so they do not cancel each other.
- Seat belts, headrests and airbags are safety devices that apply the first law.
Momentum and Impulse
| Quantity | Formula and unit |
|---|---|
| Momentum (p) | p = m × v; unit kg · m/s |
| Force (second law) | F = ma = change in momentum / time |
| Impulse | Force × time = change in momentum; unit N · s |
| Conservation of momentum | Total momentum before = total momentum after, when no external force acts |
Equations of Uniformly Accelerated Motion
| Equation | Use |
|---|---|
| v = u + at | Final velocity |
| s = ut + ½at² | Distance covered |
| v² = u² + 2as | Velocity without time |
Here u is the initial velocity, v the final velocity, a the acceleration, t the time and s the distance. For free fall, a = g.
Friction
- Friction is the force that opposes the relative motion between two surfaces in contact.
- Static friction > sliding (kinetic) friction > rolling friction. That is why wheels make moving heavy loads easier.
- Friction can be reduced by lubrication, ball bearings and streamlining.
- Friction is useful for walking, braking and writing.
Gravitation
| Fact | Value |
|---|---|
| Newton’s law of gravitation | F = G m1 m2 / r² |
| Universal gravitational constant G | 6.67 × 10−11 N m²/kg² |
| Acceleration due to gravity g (Earth) | About 9.8 m/s² |
| Weight | W = mg (a force, measured in newtons) |
| Escape velocity from Earth | About 11.2 km/s |
| Orbital velocity near Earth | About 7.9 km/s |
| g on the Moon | About one-sixth of g on Earth |
- Mass is constant everywhere; weight changes with the value of g.
- g is maximum at the poles and minimum at the equator.
- At the centre of the Earth, the value of g is zero.
- The centripetal force for circular motion is mv²/r.
Work, Energy and Power
| Quantity | Formula | SI unit |
|---|---|---|
| Work | W = F × s × cos θ | Joule (J) |
| Kinetic energy | ½ m v² | Joule |
| Potential energy | m g h | Joule |
| Power | Work / time | Watt (W) |
- 1 horse power (hp) = 746 watt.
- 1 kilowatt-hour (kWh) = 3.6 × 106 J (one unit of electricity).
- Work done is zero when the force is perpendicular to the displacement or when there is no displacement.
- The law of conservation of energy: energy can neither be created nor destroyed, only changed from one form to another.
Solved Examples
| Question | Solution |
|---|---|
| Force on a 5 kg body with acceleration 4 m/s² | F = ma = 5 × 4 = 20 N |
| Momentum of a 2 kg body moving at 10 m/s | p = mv = 20 kg m/s |
| Final velocity if u = 0, a = 2 m/s², t = 5 s | v = 0 + 2 × 5 = 10 m/s |
| Distance in the same case | s = ½ × 2 × 25 = 25 m |
| Weight of a 60 kg person on Earth (g = 9.8) | W = 60 × 9.8 = 588 N |
| Kinetic energy of a 4 kg body at 5 m/s | ½ × 4 × 25 = 50 J |
| Power of a machine doing 600 J of work in 10 s | 600/10 = 60 W |
Quick Tips
- Learn the units: force (newton), momentum (kg m/s), work and energy (joule), power (watt).
- Mass and weight are different; weight changes with g, mass does not.
- For examples of the third law, think of action on one body and reaction on the other.
- Write the formula, then put the values with units to avoid mistakes.
Quick Revision Points
- F = ma; 1 N = 1 kg m/s².
- First law is the law of inertia; mass measures inertia.
- Third law pairs act on different bodies.
- Momentum p = mv; impulse = change in momentum.
- g = 9.8 m/s² on Earth; escape velocity 11.2 km/s.
Practice Questions With Answers
- What is the SI unit of force?
Answer: Newton - Which law is also called the law of inertia?
Answer: Newton’s first law - Force on a 5 kg mass with acceleration 3 m/s²?
Answer: 15 N - What is the SI unit of momentum?
Answer: kg m/s - The recoil of a gun is based on which law?
Answer: Newton’s third law - What is the value of g on Earth?
Answer: About 9.8 m/s² - What is the SI unit of power?
Answer: Watt - 1 horse power equals how many watts?
Answer: 746 W - What is the escape velocity of Earth?
Answer: About 11.2 km/s - Which friction is the smallest: static, sliding or rolling?
Answer: Rolling friction
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Force and Laws of Motion notes page.
Frequently Asked Questions
What is force?
A push or a pull that can change the state of motion or shape of a body.
What is the first law of motion?
A body continues to be at rest or in uniform motion unless an external force acts on it.
What is inertia?
The property of a body to resist any change in its state of rest or motion.
What does the second law say?
Force equals mass times acceleration (F = ma).
What does the third law say?
Every action has an equal and opposite reaction.
What is momentum?
Momentum is the product of mass and velocity.
What is the difference between mass and weight?
Mass is the quantity of matter and is constant; weight is the force of gravity on the mass and changes with g.
Why does a passenger fall forward when a bus stops?
Because of inertia of motion; the body tries to keep moving.
What is the SI unit of work?
Joule.
What is the value of G?
6.67 × 10−11 N m²/kg².
Where is the value of g maximum?
At the poles.
What is the law of conservation of momentum?
If no external force acts, the total momentum of a system remains constant.
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Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.