CBSE Class 9 Science Chapter 11: Work and Energy NCERT Solutions

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NCERT Solutions PDF Class 9 PDF

This chapter delves into the fundamental concepts of Work and Energy for CBSE Class 9 Science. It explains the conditions under which work is done, defining work as the product of force and displacement when they are in the same direction. The solutions provide a clear understanding of the formula for work done (W = F x S) and introduce the SI unit of work, the Joule (J). It also covers the definition of 1 Joule of work. These NCERT Solutions are designed to help students grasp these core physics principles, solve related numerical problems accurately, and prepare effectively for their examinations by reinforcing their understanding of work and energy.

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BoardCBSE
ClassClass 9
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 11: Work and Energy

Chapter summary

Chapter 11 of the CBSE Class 9 Science textbook focuses on Work and Energy. The NCERT Solutions provided here clarify the definition of work done, emphasizing the necessity of both a force acting on an object and a displacement in the direction of that force. It presents the mathematical expression for work done and defines the standard unit of work, the Joule. These solutions aim to build a strong foundational understanding of these physics concepts.

Learning outcomes

  • Understand the conditions required for work to be done.
  • Define work done in physics.
  • Apply the formula for calculating work done when force and displacement are in the same direction.
  • Define and understand the SI unit of work (Joule).
  • Calculate the work done given force and displacement.

Topics covered

Paper topics

  • Work Done
  • Force
  • Displacement
  • Direction of Force and Displacement
  • Formula for Work Done
  • SI Unit of Work
  • Joule (J)

Important topics

  • Definition of Work Done
  • Conditions for Work Done
  • Formula: W = F x S
  • Understanding 1 Joule

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Questions and Solutions

Question 1

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Solution:

Work is done when a force causes a displacement. The formula for work done (W) when the force (F) and displacement (S) are in the same direction is given by:

W = F \times S

Given:

  • Force, F = 7 \text{ N}
  • Displacement, S = 8 \text{ m}

Substituting the given values into the formula:

W = 7 \text{ N} \times 8 \text{ m}

W = 56 \text{ Nm}

Since 1 Nm is equal to 1 Joule (J), the work done is:

W = 56 \text{ J}

Therefore, the work done in this case is 56 Joules.

Question 1

When do we say that work is done?
Solution:

In physics, work is considered to be done on an object when two conditions are met:

  1. A force must be applied to the object.
  2. The object must undergo a displacement, and this displacement must occur in the direction of the applied force.

If either of these conditions is not met (e.g., a force is applied but there is no displacement, or there is displacement but no force causing it), then no work is done.

Question 2

Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Solution:

When a force acts on an object and causes it to move through a displacement in the same direction as the force, the work done (W) is calculated by multiplying the magnitude of the force (F) by the magnitude of the displacement (S). The expression for work done is:

W = F \times S

Here, W represents the work done, F is the applied force, and S is the displacement in the direction of the force.

Question 3

Define 1 J of work.
Solution:

One Joule (1 J) is defined as the amount of work done when a force of 1 Newton (1 N) acts on an object and causes it to be displaced by a distance of 1 meter (1 m) in the direction of the applied force.

Mathematically, if F = 1 \text{ N} and S = 1 \text{ m} in the same direction, then the work done W = 1 \text{ N} \times 1 \text{ m} = 1 \text{ J}.

Common mistakes

  • Confusing work done with the mere application of force without displacement.
  • Incorrectly applying the work formula when force and displacement are not in the same direction (though not explicitly covered in these specific solutions, it's a common extension).
  • Errors in unit conversions or calculations.

Revision tips

  • Memorize the definition of work and the conditions under which it is done.
  • Practice the formula W = F x S with the given numerical examples.
  • Ensure you understand the definition of 1 Joule.
  • Review the relationship between force, displacement, and work done.

Practice MCQs

Q1. Under which condition is work considered to be done in physics?

Q2. What is the SI unit of work?

Q3. If a force of 10 N acts on an object and displaces it by 5 m in the direction of the force, what is the work done?

Q4. What does 1 Joule of work represent?

Frequently asked questions

What is the definition of work done in physics for Class 9?

Work is done when a force acts on an object and causes a displacement in the direction of the applied force. It is calculated as the product of the force and the displacement in the direction of the force.

What are the conditions for work to be done?

Two conditions must be met: 1. A force must act on the object. 2. The object must be displaced, and this displacement must have a component in the direction of the force.

How is work done calculated if the force and displacement are in the same direction?

When the force (F) and displacement (S) are in the same direction, the work done (W) is calculated using the formula: W = F × S.

What is the SI unit of work and what does 1 Joule mean?

The SI unit of work is the Joule (J). 1 Joule is the amount of work done when a force of 1 Newton displaces an object by 1 meter in the direction of the force.

How do these NCERT Solutions help with Chapter 11?

These solutions provide clear explanations and rewritten answers for all questions in Chapter 11, helping students understand the concepts of work and energy and how to solve related problems accurately for exams.

Important links

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