Question 10
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Given,
Mass (m) = 40 kg
Acceleration due to gravity = 10 ms-2
Height (h) = 5 m
Potential energy = ?
We know,
Potential energy = m g h
Substituting we get,
P.E. = 40 × 10 × 5 = 2000 J
Height when halfway down : = = 2.5 m
Potential energy when halfway = ?
Substituting we get,
P.E. = 40 × 10 × 2.5 = 1000 J
According to the law of conservation of energy:
Total potential energy = potential energy halfway down + kinetic energy halfway down
2000 = 1000 + K.E. halfway down
K.E. at halfway down = 2000 - 1000 = 1000 J
Hence, Potential energy = 2000 J and kinetic energy at halfway down = 1000 joules.
Chapter 10: Work and Energy — Quick Reference
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
- 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.
- 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