Solving Problems in Mechanics NEET Notes | Free Body Diagram & Newton's Laws

 - Dr.Sanjaykumar pawar 

Understanding Free Body Diagrams is the first step toward solving mechanics problems in NEET Physics.

Solving Problems in Mechanics Class 11 Physics Notes for NEET

4.11 SOLVING PROBLEMS IN MECHANICS

│

├── Foundation

│   ├── Newton's First Law

│   ├── Newton's Second Law

│   └── Newton's Third Law

│

├── Mechanics Problems

│   ├── Single Body Problems

│   └── Multiple Body Systems

│       ├── Bodies interact with each other

│       ├── Gravity acts on each body

│       └── Forces act between bodies

│

├── Important Concepts

│   │

│   ├── System

│   │   ├── Chosen part for analysis

│   │   └── Newton's laws applied here

│   │

│   └── Environment

│       ├── Remaining bodies

│       ├── Surroundings

│       └── External force agencies

│

├── Problem Solving Steps

│   │

│   ├── Step 1: Draw Complete Diagram

│   │   ├── Bodies

│   │   ├── Strings

│   │   ├── Supports

│   │   └── Pulleys

│   │

│   ├── Step 2: Choose System

│   │   ├── Single body

│   │   └── Group of bodies

│   │

│   ├── Step 3: Draw Free Body Diagram (FBD)

│   │   ├── Show selected system only

│   │   ├── Show all external forces

│   │   └── Exclude surroundings

│   │

│   ├── Step 4: Include Known Information

│   │   ├── Force magnitudes

│   │   ├── Directions

│   │   ├── Tension direction

│   │   └── Unknowns as variables

│   │

│   └── Step 5: Repeat for Other Systems

│       └── Use Newton's Third Law

│

├── Free Body Diagram (FBD)

│   │

│   ├── Definition

│   │   └── Diagram showing all external forces

│   │

│   ├── Include

│   │   ├── Weight (mg)

│   │   ├── Normal Reaction (N)

│   │   ├── Tension (T)

│   │   ├── Friction (f)

│   │   └── Applied Forces

│   │

│   └── Do Not Include

│       ├── Surrounding objects

│       └── Forces exerted by system on environment

│

├── Newton's Third Law in FBD

│   │

│   ├── Force on A due to B = F

│   ├── Force on B due to A = -F

│   ├── Equal Magnitude

│   ├── Opposite Direction

│   └── Act on Different Bodies

│

├── Common Forces

│   │

│   ├── Weight

│   │   └── W = mg

│   │

│   ├── Normal Reaction

│   │   └── Perpendicular to surface

│   │

│   ├── Tension

│   │   └── Along string

│   │

│   └── Friction

│       └── Opposes motion

│

├── Newton's Laws Equations

│   │

│   ├── Equilibrium

│   │   └── ΣF = 0

│   │

│   ├── Motion

│   │   └── ΣF = ma

│   │

│   └── Weight

│       └── W = mg

│

├── NEET Tips

│   ├── Draw FBD first

│   ├── Identify all forces

│   ├── Choose convenient system

│   ├── Apply Newton's laws

│   └── Solve equations

│

└── Golden Rule

    └── "No FBD = No Proper Solution"

Class 11 Physics (CBSE)

Chapter: Laws of Motion

Topic: 4.11 Solving Problems in Mechanics

A. Multiple Choice Questions (MCQs)

1. The selected part of an assembly on which Newton's laws are applied is called:

(a) Environment (b) Force (c) System (d) Reaction

Answer: (c) System

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2. A Free Body Diagram (FBD) shows:

(a) Only the body (b) Only the surroundings (c) The body and all external forces acting on it (d) Internal forces only

Answer: (c)

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3. Which force must always be included in the FBD of an object near Earth?

(a) Tension (b) Friction (c) Weight (d) Spring force

Answer: (c)

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4. The force exerted by a string on a body is called:

(a) Normal reaction (b) Tension (c) Friction (d) Weight

Answer: (b)

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5. Newton's Third Law states:

(a) F = ma (b) ΣF = 0 (c) Every action has an equal and opposite reaction (d) Momentum is conserved

Answer: (c)

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6. Normal reaction acts:

(a) Along the surface (b) Opposite to velocity (c) Perpendicular to the surface (d) Vertically upward always

Answer: (c)

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7. In a Free Body Diagram, we should show:

(a) Forces acting on surroundings (b) Forces acting on the chosen body (c) Internal details of the body (d) Shape of surroundings

Answer: (b)

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8. Friction acts:

(a) Along motion (b) Opposite to tendency of motion (c) Vertically upward (d) Downward

Answer: (b)

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9. The first step in solving a mechanics problem is:

(a) Apply F = ma (b) Draw complete diagram (c) Calculate acceleration (d) Draw graph

Answer: (b)

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10. Weight of a body is:

(a) N (b) mg (c) ma (d) mv

Answer: (b)

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B. Very Short Answer Questions (1 Mark)

1. What is a system?

Answer: The selected object or group of objects chosen for analysis is called a system.

2. What is an environment?

Answer: Everything outside the system that exerts force on it is called the environment.

3. What is FBD?

Answer: A Free Body Diagram is a diagram showing a body and all external forces acting on it.

4. Write the formula for weight.

Answer: W = mg

5. Which force acts perpendicular to a surface?

Answer: Normal reaction.

6. What is the SI unit of force?

Answer: Newton (N)

7. Along which direction does tension act?

Answer: Along the string.

8. Which law gives F = ma?

Answer: Newton's Second Law.

9. What is the direction of weight?

Answer: Vertically downward.

10. Which force opposes motion?

Answer: Friction.

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C. Short Answer Questions (2–3 Marks)

1. Define a Free Body Diagram.

Answer: A Free Body Diagram (FBD) is a diagram of a selected body showing all external forces acting on it. It helps in applying Newton's laws correctly.

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2. Why is FBD important?

Answer:

1. It identifies all forces acting on a body.
2. It simplifies complex problems.
3. It helps apply Newton's laws accurately.

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3. State Newton's Third Law.

Answer: For every action, there is an equal and opposite reaction. The two forces act on different bodies and are equal in magnitude but opposite in direction.

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4. Differentiate between system and environment.

System	Environment
Chosen for study	Surroundings
Newton's laws applied	Exerts forces on system
Main object of analysis	External agencies

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5. What forces act on a block resting on a table?

Answer:

1. Weight (mg) downward.
2. Normal reaction (N) upward.

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D. Long Answer Questions (5 Marks)

1. Explain the steps involved in solving mechanics problems.

Answer:

The following steps should be followed:

1. Draw the complete diagram.
2. Choose a convenient system.
3. Draw the Free Body Diagram.
4. Include all known forces and directions.
5. Treat unknown forces as variables.
6. Apply Newton's laws.
7. Solve the equations obtained.

These steps help solve mechanics problems systematically.

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2. Explain Free Body Diagram with an example.

Answer:

A Free Body Diagram is a diagram showing a body isolated from its surroundings along with all external forces acting on it.

Example: For a block resting on a table:

Forces acting:

• Weight (mg) downward
• Normal reaction (N) upward

Using equilibrium:

N = mg

The FBD helps in identifying forces and solving the problem correctly.

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E. Assertion and Reason Questions

1.

Assertion (A): A Free Body Diagram contains all external forces acting on a body.

Reason (R): FBD helps in applying Newton's laws.

Answer: Both A and R are true and R is the correct explanation of A.

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2.

Assertion (A): Weight acts vertically downward.

Reason (R): Weight is the gravitational force exerted by Earth.

Answer: Both A and R are true and R correctly explains A.

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3.

Assertion (A): Action and reaction act on the same body.

Reason (R): Newton's Third Law states forces are equal and opposite.

Answer: Assertion is false but Reason is true.

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4.

Assertion (A): Friction opposes relative motion.

Reason (R): Friction acts along the direction of motion.

Answer: Assertion is true but Reason is false.

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5.

Assertion (A): Normal reaction acts perpendicular to a surface.

Reason (R): It is a contact force.

Answer: Both A and R are true but R is not the correct explanation.

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F. Fill in the Blanks

1. The selected body for analysis is called a _______. Answer: System

2. The force due to gravity is called _______. Answer: Weight

3. The force exerted by a string is called _______. Answer: Tension

4. A diagram showing all external forces is called _______. Answer: Free Body Diagram

5. Weight of a body is equal to _______. Answer: mg

6. Newton's Second Law is represented by _______. Answer: F = ma

7. Friction opposes _______. Answer: Motion

8. Normal reaction acts _______ to the surface. Answer: Perpendicular

9. Newton's Third Law involves action and _______. Answer: Reaction

10. The SI unit of force is _______. Answer: Newton

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G. Statement-Based Questions

Statement I:

A Free Body Diagram contains only the chosen body.

Statement II:

All external forces acting on the body must be shown.

(a) Both statements are true. (b) Both are false. (c) Statement I true, II false. (d) Statement I false, II true.

Answer: (a)

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Statement I:

Action and reaction act on the same body.

Statement II:

Action and reaction are equal and opposite.

Answer: Statement I is false and Statement II is true.

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H. Match the Columns

Column A	Column B
A. Weight	1. Along string
B. Tension	2. Opposes motion
C. Friction	3. mg
D. Normal Reaction	4. Perpendicular to surface

Answer:

A → 3

B → 1

C → 2

D → 4

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I. Case Study Questions

Case Study

A block rests on a horizontal table. The block experiences its weight downward and the normal reaction upward. The block remains at rest.

Q1. Which force acts downward?

Answer: Weight (mg)

Q2. Which force acts upward?

Answer: Normal reaction

Q3. Why does the block remain at rest?

Answer: Net force is zero.

Q4. Which law explains equilibrium?

Answer: Newton's First Law.

Q5. What is the relation between N and mg?

Answer: N = mg

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J. Competency-Based Questions

1.

A student forgets to include weight while drawing an FBD. How will this affect the solution?

Answer: The force analysis becomes incorrect and wrong answers may be obtained.

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2.

Why should action and reaction never be shown on the same FBD?

Answer: Because they act on different bodies and belong to different free body diagrams.

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Important CBSE Exam Tip

Always draw a neat Free Body Diagram before applying Newton's laws. Most mistakes in mechanics occur because of incorrect or incomplete FBDs.

 Internal Links

Newton's First Law of Motion – Complete Notes

Newton's Second Law of Motion – Formula and Examples

Newton's Third Law of Motion Explained

Force and Its Types in Physics

Free Body Diagram (FBD) Explained with Examples

Friction – Static and Kinetic Friction Notes

Tension in Strings and Pulley Problems

Laws of Motion Class 11 Notes

Work, Energy and Power NEET Notes

Important NEET Mechanics Questions and Solutions

Circular Motion NEET Notes

Centre of Mass and Momentum Notes

Gravitation Complete NEET Guide

Physics Formula Sheet for NEET

Class 11 Physics Chapter-wise Revision Notes

4.11 Solving Problems in Mechanics (NEET Notes)

Introduction

The three laws of motion are the foundation of mechanics. In many problems, more than one object is involved. These objects interact with each other through different forces such as tension, friction, normal reaction, and gravity.

To solve such problems systematically, we divide the arrangement into:

• System → The part chosen for study.
• Environment → Everything outside the system that exerts force on it.

Key Idea: Newton's laws can be applied to any chosen system if all external forces acting on it are considered.

1. System

The object or group of objects selected for analysis is called the system.

Example

• A single block on a table.
• Two connected blocks together.
• A pulley-block arrangement.

2. Environment

The surroundings that interact with the system are called the environment.

Example

• For a block on a table:
• System = Block
• Environment = Table, Earth, String, etc.

Free Body Diagram (FBD)

Definition: A Free Body Diagram (FBD) is a diagram showing only the chosen system and all external forces acting on it.

An FBD does not mean that the body is free from forces. It simply means the body is shown separately from its surroundings.

Steps for Solving Mechanics Problems

Step 1: Draw the Complete Diagram

• Draw all bodies.
• Show strings and pulleys.
• Show supports and surfaces.
• Understand the complete arrangement.

Step 2: Choose the System

Select a convenient object or group of objects for analysis.

Step 3: Draw the Free Body Diagram (FBD)

Separate the chosen system and show all external forces acting on it.

Step 4: Include Known Information

• Given forces
• Directions of forces
• Tension direction in strings
• Unknown forces represented by symbols

Step 5: Apply Newton's Laws

Use Newton's laws to determine unknown quantities.

Newton's First Law

ΣF = 0

If the net force acting on a body is zero, the body remains at rest or moves with constant velocity.

Newton's Second Law

ΣF = ma

The acceleration of a body is directly proportional to the net force acting on it.

Newton's Third Law

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

If object A exerts force F on object B, then object B exerts force -F on object A.

Using Newton's Third Law in FBD

Suppose two blocks A and B are in contact.

• Force by B on A = F
• Force by A on B = -F

These forces:

• Are equal in magnitude.
• Are opposite in direction.
• Act on different bodies.

Rules for Drawing Free Body Diagrams

Do's ✔

• Draw only the selected body.
• Show all external forces.
• Mark directions clearly.
• Label unknown forces.

Don'ts ✘

• Do not draw surrounding objects.
• Do not omit weight (mg).
• Do not omit normal reaction.
• Do not show action-reaction pairs on the same body.

Common Forces in Mechanics

Force	Symbol	Direction
Weight	W = mg	Vertically Downward
Normal Reaction	N	Perpendicular to Surface
Tension	T	Along the String
Friction	f	Opposite to Motion
Applied Force	F	Specified Direction

Example: Block on a Table

A block is resting on a horizontal table.

Forces acting on the block:

• Weight (mg) downward
• Normal reaction (N) upward

N ↑

[ BLOCK ]

mg ↓

Since the block is at rest:

N = mg

NEET Quick Tips

Always Follow:
1. Draw Diagram
2. Choose System
3. Draw FBD
4. Identify Forces
5. Apply Newton's Laws
6. Solve Equations

Common Mistakes in NEET

❌ Forgetting Weight (mg)
❌ Wrong Friction Direction
❌ Missing Normal Force
❌ Drawing Action-Reaction Pair on Same Body
❌ Solving Without FBD

Formula Box

W = mg

ΣF = 0

ΣF = ma

One-Line Summary

To solve any mechanics problem, first choose a system, draw its Free Body Diagram (FBD), identify all external forces, and then apply Newton's Laws of Motion.

⭐ Golden Rule for NEET: "No FBD = No Proper Solution."