NCERT Class 11 Physics Chapter 5 Notes Laws of Motion - Download PDF

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• Easy to understand
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The notes break down complex concepts like Newton’s laws, friction, and motion in a very clear and simple way. If you're short on time before an exam, these notes help you revise quickly without missing out on key points.

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Also, students can refer,

• NCERT Solutions for Class 11 Physics Chapter 4 Laws of Motion
• NCERT Exemplar for Class 11 Physics Solution Chapter 4 Laws of Motion

Forces in Touch and Non-Contact

Why Forces Matter in Motion?

Forces are super important in Physics because they help us understand how things start moving, stop, or change direction. All the action in motion is controlled by forces!

Types of Forces

1. Contact Forces

These happen when two objects actually touch.
Example: When a cricket bat hits a ball – the force is passed through direct contact.

2. Non-Contact Forces

These act without touching the object.

Example: A magnet attracts a coin even though they’re not touching.
This shows how forces can act from a distance – like gravity, magnetic force, and electric force.

A Rolling Ball Example

• According to Aristotle, a ball needs a constant push to keep moving.
• But actually, it stops because of friction, which slows it down.
  This shows that Aristotle’s theory was incorrect and that science has improved with better understanding over time.

Inertia – What Is It?

Inertia means:

An object doesn’t want to change its motion.

• If it's at rest, it wants to stay still.
• If it's moving, it wants to keep moving.

Some quick facts:

• Inertia is a property of mass – more mass = more inertia.
• It’s not a physical quantity, so no units or dimensions.
• Doesn’t depend on speed or direction—only on mass.

Galileo’s Inertia Law

Galileo explained that:

An object keeps moving unless something like friction stops it.

He proved Aristotle wrong and helped us better understand motion and the idea of inertia.

Newton’s First Law of Motion (Law of Inertia)

Statement:

If no external force acts on a body, it stays at rest or keeps moving in a straight line with the same speed.

This is called the Law of Inertia.

It explains:

• No force → no change in motion.
• If something changes (starts/stops/turns), a force must be acting.

Types of Inertia:

1. Inertia of Rest: An Object stays still unless something moves it.
2. Inertia of Motion: A Moving object keeps going unless stopped.
3. Inertia of Direction: The Object keeps moving in the same direction unless forced to turn.

Balanced and Unbalanced Forces

Balanced Forces:

• Two equal forces pulling/pushing in opposite directions
• Motion doesn’t change
  Example: In a tug of war, both teams pull equally → The rope doesn’t move.

Unbalanced Forces:

• Forces are not equal
• They cause objects to move, stop, or change shape
  Example: In tug of war, if one team pulls harder → the rope moves towards them.

Linear Momentum

• Linear momentum is a vector quantity (has both magnitude and direction).

• It’s the product of an object’s mass (m) and velocity (v).

$\overrightarrow{p}=m\overrightarrow{v}$

• SI Unit: kg·m/s

• CGS Unit: g·cm/s

• Dimension: [MLT^-1]

Note: If two objects have the same momentum but different masses, the lighter object will have a higher velocity.

Newton’s Second Law of Motion

The rate of change of momentum of a body is directly proportional to the applied external force, and this change happens in the direction of the force.

In Simple Words:

Force is what causes momentum to change.
More force → faster change in motion.

Mathematically:

Let a body of mass m move with velocity v.
Its momentum: p = m × v

If a force (F) is applied, then:

$\overrightarrow{F}=\frac{dp}{dt}=\frac{d}{dt}(m\overrightarrow{v})=m\frac{d\overrightarrow{v}}{dt}=m\overrightarrow{a}($ from $a=\frac{d\overrightarrow{v}}{dt})$

This formula tells us:
Force = mass × acceleration. It’s one of the most important equations in Physics!

Impulse

Impulse is the effect of a force applied to an object for a short time, causing a sudden change in its motion. It helps explain how quickly an object speeds up, slows down, or changes direction when hit or pushed. For example, when you kick a football or hit a cricket ball with a bat, you give it an impulse that changes its speed and direction instantly. Even airbags in cars use the concept of impulse—they increase the time of impact during a crash to reduce the force on passengers. In simple words, impulse shows how a quick force can create a big change in how something moves.

$\overrightarrow{I}={\int }_{t_1}^{t_2}\overrightarrow{F}dt$

• Impulse is a vector quantity, so it has both magnitude and direction.
• Dimension of impulse: [MLT^-1]

• SI unit- Newton-second or Kg-m-s^-1 and CGS unit- Dyne-second or gm-cm-s^-1

Newton’s Third Law of Motion

Newton's Third Law of Motion says that for every action, there is an equal and opposite reaction. This means that if one object applies a force on another, the second object pushes back with the same amount of force but in the opposite direction. If we say ${\overrightarrow{F}}_{AB}$​ is the force that object B applies on object A (action), and ${\overrightarrow{F}}_{BA}$​ is the force that object A applies on object B (reaction), then these forces are equal in size but opposite in direction, written as:

${\overrightarrow{F}}_{AB}=-{\overrightarrow{F}}_{BA}$

This law helps explain things like how a swimmer pushes water backward and moves forward, or how a rocket launches by pushing gas downward and lifting upward.

Laws of Conservation of Momentum

According to the Law of Conservation of Linear Momentum, when no external forces act on an isolated system, its total linear momentum remains constant. In other words, the total momentum of a closed system of interacting objects remains constant over time, assuming no external forces influence it.

According to this law, $\overrightarrow{F}=\frac{d\overrightarrow{p}}{dt}$

In the absence of external forces, If F equals 0, then p is constant.

i.e, $P_{\text{system }}=P_1+P_2+P_3+P_4+\dots \dots \dots ..=$ constant

Friction

Friction is a force that occurs at the point of contact between two surfaces and works to oppose relative or approaching motion between them.

Friction can be categorized into three main types:

• Static Friction: Acts on stationary objects, preventing them from initiating motion. The force of static friction increases with the applied force until motion is initiated.
  $f_{\mathrm{s}}\le {\mu }_{\mathrm{s}}N$
• The maximum static frictional force ${\left({\mathrm{f}}_{\mathrm{s}}\right)}_{max}={\mu }_{\mathrm{s}}N$
• Kinetic friction: Kinetic friction is the force that arises between two surfaces when they are moving relative to one another.

  The kinetic frictional force ($f_{\mathrm{k}}$) is proportional to the normal force ($N$) and can be expressed as $f_{\mathrm{k}}={\mu }_{\mathrm{k}}N$

• Rolling friction: Rolling friction is the force that opposes the motion of an object as it rolls across a surface. It occurs when there is relative motion between the surface and the part of the object that makes contact with it. it is similar to kinetic friction.

Circular Motion

• Angular Displacement(θ): The angular displacement of an object moving around a circular path is defined as the angle traced out by the radius vector at the center of the circular path over time. This is a vector quantity.

θ= s/r, where s is the arc length and r is the radius.

• Angular Velocity (ω): The angular velocity of an object in circular motion is defined as the rate at which its angular displacement changes over time.

ω= Δθ/Δt, where Δθ is the change in angular displacement, and Δt is the change in time.

• Angular Acceleration (α): The angular acceleration of an object in circular motion is defined as the rate at which its angular velocity changes over time.

α= Δt/Δω, where Δω is the change in angular velocity, and Δt is the change in time

• Uniform Circular Motion: Uniform circular motion occurs when a point object moves along a circular path at a constant speed.

  In this type of motion, the speed remains constant while the direction changes continuously, resulting in circular motion.

a_c=v²/r, where r is the radius

Key Features of Physics Class 11 Chapter 4 Notes PDF

• Accessibility and Portability:
  The notes are available in digital format and can be easily accessed on laptops, tablets, or smartphones, enabling students to study anytime, anywhere.

• Structured Content:
  Chapter 4 notes in PDF format are well-organized, following the sequence of topics as per the NCERT textbook, which aids in systematic learning.

• Visual Aids and Diagrams:
  Notes include clear diagrams and illustrations that enhance understanding of physics concepts, especially when dealing with forces, motion, and free-body diagrams.

• Printable Format:
  Students can print the notes for offline use, which is ideal for those who prefer reading from hard copies or highlighting important points.

• Consistency and Accuracy:
  The PDF notes maintain consistent formatting and accurate content across all devices, ensuring a uniform learning experience.

Significance of NCERT Class 11 Physics Chapter 4 Notes

NCERT Class 11 Physics Chapter 4 is “Laws of Motion”, and it's one of the most important chapters in the Class 11 Physics syllabus as well as for competitive exams like JEE Main, NEET, and other entrance tests.

Here’s the significance of Chapter 4 Notes and why they matter:

• Builds a strong foundation in Newton’s Laws of Motion, the core of classical mechanics.

• Helps in understanding real-life concepts like friction, tension, normal force, and pseudo forces.

• Essential for solving numerical problems involving force and motion.

• Frequently asked in JEE, NEET, and other competitive exams.

• Supports quick and effective revision before tests.

• Forms the basis for advanced chapters like Work, Energy & Power, Rotational Motion, and Gravitation.

• Simplifies complex topics using clear diagrams, examples, and key formulas.

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