Newton's Laws of Motion, Force, Inertia

Newton's Laws of Motion, Force, Inertia – Definition, Formula, Examples & Applications

Keywords: Newton's Laws of Motion, Force, Inertia, F = ma, Motion, SSC Maharashtra Science

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Newton's Laws of Motion

Newton's Laws of Motion are three fundamental laws introduced by Isaac Newton. They explain how objects move and how forces affect their motion. These laws form the foundation of classical mechanics and are widely used in science and engineering.

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What is Motion?

Motion is the change in the position of an object with respect to time.

Examples:

• A moving car
• A flying airplane
• A rolling football
• Earth revolving around the Sun

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What is Force?

A force is a push or pull that can change an object's state of rest or motion.

SI Unit: Newton (N)

Examples:

• Pushing a shopping cart
• Pulling a suitcase
• Kicking a football

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What is Inertia?

Inertia is the property of an object to resist any change in its state of rest or uniform motion.

Examples:

• A parked car does not move until pushed.
• Passengers move forward when a moving bus stops suddenly.

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First Law of Motion (Law of Inertia)

With no forces acting on it, the ball keeps moving at the same speed in the same direction

Definition

An object remains at rest or continues to move with constant velocity in a straight line unless acted upon by an external unbalanced force.

Animated Object Example

🚌 Bus Stops Suddenly

• Bus is moving.
• Driver applies brakes.
• Passengers lean forward because their bodies continue moving due to inertia.

More Examples

• Book resting on a table
• Coin falling into a glass after the card beneath it is flicked away
• Seat belts protecting passengers

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Second Law of Motion

Fnet​=ma

$𝑎=\frac{{𝐹}_{\mathrm{n}\mathrm{e}\mathrm{t}}}{𝑚}=\frac{\text{12}\mathrm{N}}{\text{1.5}\mathrm{k}\mathrm{g}}=\text{8}\mathrm{m}\mathrm{/}{\mathrm{s}}^2$

${𝐹}_{\mathrm{n}\mathrm{e}\mathrm{t}}$

N

${𝐹}_{\mathrm{n}\mathrm{e}\mathrm{t}}$

$𝑚$

kg

$𝑚$

Definition

The acceleration of an object depends directly on the applied force and inversely on its mass.

Formula

F = ma

Where:

• F = Force (Newton)
• m = Mass (kg)
• a = Acceleration (m/s²)

Example

A force of 20 N acts on a 4 kg object.

F = ma

20 = 4 × a

a = 5 m/s²

Animated Object Example

📦 Pushing Boxes

• Push an empty box → moves easily.
• Push a heavy box with the same force → moves slowly.

This shows heavier objects require more force to achieve the same acceleration.

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Third Law of Motion

Force of person on box and force of box on person are equal in magnitude, opposite in direction, and act on different objects

Definition

For every action, there is an equal and opposite reaction.

Animated Object Example

🎈 Balloon Rocket

• Air rushes backward.
• Balloon moves forward.

Other Examples

• Walking
• Swimming
• Rocket launch
• Bird flying

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Everyday Applications of Newton's Laws

Situation	Law Used
Seat belts in cars	First Law
Pushing a shopping trolley	Second Law
Rocket launch	Third Law
Walking	Third Law
Cricket bat hitting a ball	Second Law
Jumping from a boat	Third Law
Bus starting suddenly	First Law

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Summary Table

Law	Statement	Example
First Law	Objects resist changes in motion (inertia).	Passenger moves forward when a bus stops
Second Law	Force equals mass × acceleration.	Pushing a shopping cart
Third Law	Every action has an equal and opposite reaction.	Rocket launch

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Practice Questions

Multiple Choice Questions (MCQs)

1. Which law is known as the Law of Inertia?

A. First Law
B. Second Law
C. Third Law
D. Law of Gravitation

Answer: A

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2. The SI unit of force is:

A. Joule

B. Newton

C. Watt

D. Pascal

Answer: B

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3. The formula for force is:

A. F = mv

B. F = ma

C. P = mv

D. W = mg

Answer: B

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Short Answer Questions

1. Define inertia.
2. State Newton's First Law.
3. Write the formula of Newton's Second Law.
4. Explain Newton's Third Law with one example.
5. Why are seat belts important?

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Frequently Asked Questions (FAQs)

1. What are Newton's Laws of Motion?

They are three laws that explain the relationship between force and motion.

2. What is inertia?

Inertia is the tendency of an object to resist changes in its state of motion.

3. What is the formula of Newton's Second Law?

F = ma

4. Give one example of Newton's Third Law.

A rocket moves upward because hot gases are expelled downward.

5. Who discovered Newton's Laws of Motion?

They were formulated by Isaac Newton.