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Work and Energy — Question 18

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Question 18

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. NCERT Class 9 Science CBSE Solutions.
Answer

Case I

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. NCERT Class 9 Science CBSE Solutions.

The direction of force acting on the block is perpendicular to the displacement. Hence, work done by force on the block will be zero.

Case II

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. NCERT Class 9 Science CBSE Solutions.

The direction of force acting on the block is in the direction of displacement. Hence, work done by force on the block will be positive.

Case III

In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero. NCERT Class 9 Science CBSE Solutions.

The direction of force acting on the block is opposite to the direction of displacement. Hence, work done by force on the block will be negative.

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Science | Chapter 10: Work and EnergyWeb Content — Quick Reference

Chapter 10: Work and Energy — Quick Reference

work kinetic energy potential energy conservation of energy power kWh

Quick Revision Points

  • Work W = Fs cosθ (joule); W = 0 when F ⊥ s or displacement = 0
  • KE = ½mv²; PE = mgh; Work-energy theorem: W = ΔKE
  • Conservation: energy transforms but total remains constant; PE + KE = constant in free fall
  • Power P = W/t (watt = J/s); 1 HP = 746 W
  • 1 kWh = 3.6 × 10⁶ J = 1 unit of electricity
Exam Tips for Chapter 10
  • Read the detailed chapter notes for complete coverage of all NCERT topics.
  • Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
  • For numericals (if applicable), practice at least 20 problems of varying difficulty.
  • Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
  • Detailed Notes: ch10-work-and-energy.html
  • Practice Questions: 100+ questions with answers in 05-practice-questions/
  • Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
  • Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
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