Work and Energy – Very Short Answer Type Questions
CBSE Class 9
Physics — Chapter 11: Work and Energy
50 Very Short Answer Type Questions & Answers — NCERT-aligned for CBSE Class 9 board exam preparation
CBSE Board Examinations — Quick guide
These Very Short Answer (VSA) questions are strictly per NCERT Class 9 Chapter 11 "Work and Energy". They are ideal for 1–2 mark questions in CBSE unit tests and board-style revision.
- Chapter focus: Definitions, formulae, units, examples, and simple conceptual reasoning.
- Question length: Very short answers (one-line to two-line answers).
- Use the details toggles below to expand each question and see the answer.
Part A — Basics of Work (Q1–Q12)
Q1. What is work in physics?
A:
Work is said to be done when a force causes displacement of an object in its line of action.
Q2. State the SI unit of work.
A:
SI unit of work is joule (J), where 1 J = 1 N·m.
Q3. Give the formula for work done by a constant force.
A:
W = F × s × cosθ, where F is force, s is displacement and θ is angle between them.
Q4. When is work said to be zero?
A:
Work is zero if there is no displacement or if force is perpendicular to displacement (cos90° = 0).
Q5. What is positive work?
A:
When the component of force along displacement is in the same direction as displacement, work is positive.
Q6. What is negative work?
A:
When the force component along displacement is opposite to displacement (e.g., friction opposing motion), work is negative.
Q7. Give one example of zero work in everyday life.
A:
Holding a heavy box stationary requires force but no displacement, so work done is zero.
Q8. Is work a scalar or vector quantity?
A:
Work is a scalar quantity (it has magnitude only).
Q9. What is the work done when a force of 10 N moves an object by 2 m in the direction of force?
A:
W = F × s = 10 × 2 = 20 J.
Q10. If displacement is perpendicular to the force, what is the work done?
A:
Zero; because cos90° = 0 so W = 0.
Q11. Define displacement in the context of work.
A:
Displacement is the straight-line distance in a specified direction through which the point of application of the force moves.
Q12. How does angle θ = 180° affect work done?
A:
If θ = 180°, cos180° = −1, so work is negative and equals −F·s.
Part B — Energy (Q13–Q26)
Q13. What is energy?
A:
Energy is the capacity to do work.
Q14. What are the SI units of energy?
A:
SI unit of energy is joule (J), same as work.
Q15. Name two broad forms of mechanical energy.
A:
Kinetic energy and potential energy.
Q16. State the formula for kinetic energy of a moving body.
A:
Kinetic energy, K = 1/2 · m · v², where m is mass and v is speed.
Q17. What is potential energy?
A:
Potential energy is energy stored in a body due to its position or configuration, e.g., gravitational potential energy.
Q18. Give the formula for gravitational potential energy near Earth's surface.
A:
U = m · g · h, where m is mass, g is acceleration due to gravity, h is height above reference.
Q19. What happens to kinetic energy when speed doubles?
A:
Kinetic energy becomes four times larger (since KE ∝ v²).
Q20. Can potential energy be negative?
A:
Yes—potential energy depends on chosen reference level; it can be negative relative to that reference.
Q21. Give one example of stored (elastic) potential energy.
A:
A compressed spring or a stretched rubber band stores elastic potential energy.
Q22. What is mechanical energy?
A:
Mechanical energy is the sum of kinetic and potential energies of a system.
Q23. How is work related to change in energy?
A:
Work done on a body results in change in its energy (work–energy principle).
Q24. What is the work–energy theorem (short)?
A:
The net work done on an object equals its change in kinetic energy.
Q25. Give an everyday example of conversion of potential energy to kinetic energy.
A:
A falling object: gravitational potential energy converts to kinetic energy as it falls.
Q26. Why is energy considered a scalar quantity?
A:
Energy has only magnitude and no direction, hence it is scalar.
Part C — Conservation & Examples (Q27–Q38)
Q27. State the law of conservation of energy (one line).
A:
Energy cannot be created or destroyed, only transformed from one form to another.
Q28. Give an example showing conservation of mechanical energy (neglecting friction).
A:
A pendulum: potential energy at highest point converts to kinetic at lowest point and back, total mechanical energy remains constant (neglecting air resistance).
Q29. Does friction conserve mechanical energy?
A:
No. Friction converts mechanical energy into thermal energy, so mechanical energy is not conserved.
Q30. What form of energy does chemical energy convert into in a candle flame?
A:
Chemical energy converts into thermal (heat) and light energy.
Q31. A block slides down frictionless incline—what happens to its potential energy?
A:
Its gravitational potential energy decreases and converts into kinetic energy.
Q32. Does energy transformation change the total energy of an isolated system?
A:
No, total energy of an isolated system remains constant during transformations.
Q33. What happens to the mechanical energy of a moving car when brakes are applied?
A:
Mechanical energy is dissipated as heat (thermal energy) due to friction in brakes.
Q34. Can electrical energy be converted into mechanical energy? Give an example.
A:
Yes—for example, an electric motor converts electrical energy into mechanical energy.
Q35. What is the source of energy for photosynthesis?
A:
Sunlight (solar energy) is converted to chemical energy in plants via photosynthesis.
Q36. What form of energy is associated with motion of particles?
A:
Thermal energy (internal energy) is associated with particle motion and vibration.
Q37. If a system loses energy, where does it go (brief)?
A:
It is transferred to surroundings or converted into other forms such as heat, sound, or radiation.
Q38. Name one practical device that converts potential energy into electrical energy.
A:
Hydroelectric dam—gravitational potential energy of water converts to mechanical then electrical energy.
Part D — Power & Calculations (Q39–Q46)
Q39. What is power?
A:
Power is the rate at which work is done or energy is transferred.
Q40. State the SI unit of power.
A:
SI unit of power is watt (W), where 1 W = 1 J/s.
Q41. Express power in terms of work and time.
A:
P = W / t, where W is work done and t is time taken.
Q42. A motor does 500 J of work in 10 s. What is its power?
A:
P = 500 / 10 = 50 W.
Q43. How is horsepower related to watt (short)?
A:
1 horsepower ≈ 746 W (approximate conversion).
Q44. What is the instantaneous power formula using force and velocity?
A:
Instantaneous power P = F · v (force dot velocity in same direction).
Q45. If twice the work is done in the same time, how does power change?
A:
Power doubles (since P = W/t).
Q46. A person lifts a 20 kg box by 2 m in 4 s. (g = 9.8 m/s²) Find power used (approx).
A:
Work = mgh = 20×9.8×2 = 392 J. Power = 392/4 ≈ 98 W.
Part E — Short Concept Checks & Examples (Q47–Q50)
Q47. Does lifting an object slowly or quickly change the work done against gravity?
A:
No. Work against gravity (mgh) is same regardless of time; only power changes.
Q48. Why does a moving vehicle require fuel even on a level road at constant speed?
A:
To overcome resistive forces such as friction and air resistance which convert mechanical energy into heat; fuel supplies energy to balance losses.
Q49. A person carries a tray of food horizontally at constant speed—what is the work done by the person on the tray?
A:
Zero, if the force is vertical and displacement is horizontal (force perpendicular to displacement) and height does not change.
Q50. Write one exam tip about remembering KE and PE formulas.
A:
Memorise KE = 1/2 mv² and PE = mgh; practise units (J) and simple substitutions to avoid careless errors.
Tip: Use this sheet during quick revision before tests.