Work and Energy – Short Answer Type Questions
CBSE Class 9
Physics — Chapter 11: Work and Energy
50 Short Answer Type Questions & Answers — NCERT-aligned for CBSE Class 9 board exam preparation
CBSE Board Examinations — How to use this sheet
- Questions follow NCERT Chapter 11 topics: definition of work, energy types, work–energy theorem, power, conservation and examples.
- Short answers are suitable for 2–4 mark questions; practice writing equations and units where required.
Part A — Work: Definitions, formulae and examples (Q1–Q10)
Q1. Define work done by a constant force.
Work done by a constant force is the product of the component of the force along the displacement and the magnitude of the displacement: W = F·s·cosθ. It represents energy transferred to or from an object.
Q2. State the SI unit of work and show its base units.
The SI unit of work is the joule (J). One joule equals one newton metre: 1 J = 1 N·m = 1 kg·m²·s⁻².
Q3. When is the work done by a force zero? Give two situations.
Work is zero if there is no displacement or if the force is perpendicular to the displacement. Examples: (i) holding a book stationary, (ii) carrying a tray horizontally with no vertical displacement while the lifting force is vertical.
Q4. Explain positive, negative and zero work with examples.
Positive work occurs when force has a component along displacement (pushing a trolley). Negative work when force opposes displacement (friction slowing a sliding box). Zero work when force is perpendicular to displacement (centripetal force on uniform circular motion).
Q5. A force of 15 N acts on an object and moves it 3 m in the force direction. Calculate the work done.
W = F·s = 15 N × 3 m = 45 J. Since force and displacement are in same direction, cosθ = 1.
Q6. Define displacement in the context of work.
Displacement is the straight-line vector distance through which the point of application of the force moves. Only the displacement of the point where force is applied matters for calculating work.
Q7. Why is work considered a scalar quantity even though force and displacement are vectors?
Work equals the scalar (dot) product of force and displacement, producing a single numerical value (magnitude with sign) and no direction; hence it is scalar.
Q8. How does the angle between force and displacement affect work?
Work depends on cosθ. If θ = 0°, cosθ = 1 (maximum positive work); θ = 90°, cosθ = 0 (zero work); θ = 180°, cosθ = −1 (maximum negative work).
Q9. Can normal force do work? Explain with example.
Normal force can do work only if its point of application moves in its direction. Example: normal force from a moving ramp on a block sliding down while ramp itself moves—otherwise, for a stationary horizontal floor supporting a moving object horizontally, normal does no work.
Q10. Distinguish between work and energy in one sentence.
Work is the process by which energy is transferred from one body to another, while energy is the capacity of a system to do work.
Part B — Energy: Types, formulas and properties (Q11–Q20)
Q11. Define kinetic energy and write its expression.
Kinetic energy (KE) is the energy possessed by a body due to its motion. For a body of mass m moving at speed v, KE = ½·m·v².
Q12. Define gravitational potential energy near Earth's surface and provide the formula.
Gravitational potential energy (PE) of an object of mass m at height h above a reference is U = m·g·h, where g is acceleration due to gravity (≈9.8 m/s²).
Q13. A body of mass 2 kg moves at 3 m/s. Compute its kinetic energy.
KE = ½·m·v² = 0.5 × 2 × 3² = 9 J.
Q14. Explain elastic potential energy with formula for a spring.
Elastic potential energy stored in a spring compressed/stretched by x is U = ½·k·x², where k is spring constant. It represents energy due to elastic deformation.
Q15. How does kinetic energy change if speed doubles? Show mathematically.
KE ∝ v². If v → 2v, KE_new = ½ m (2v)² = 4 × ½ m v² = 4 KE_old; kinetic energy becomes four times.
Q16. Can potential energy be negative? Give a brief reason.
Yes. Potential energy depends on chosen reference level. If the reference level is set above the object, mgh may be negative relative to that reference.
Q17. What is internal (thermal) energy and when does it increase?
Internal energy is the total microscopic kinetic and potential energy of particles in a body. It increases when temperature rises or when work is done against internal forces (e.g., friction).
Q18. Define mechanical energy of a system.
Mechanical energy is the sum of kinetic and potential energies of a system: E_mech = KE + PE. It describes macroscopic energy available for mechanical work.
Q19. Give two examples of energy conversion commonly seen in daily life.
Examples: (i) Electric heater converts electrical energy into thermal energy. (ii) Hydroelectric plant converts gravitational potential energy of water into electrical energy via turbines and generators.
Q20. Why is energy considered a scalar quantity?
Energy has magnitude but no direction and is described by numerical values (e.g., joules); therefore it is scalar.
Part C — Work–Energy Theorem, calculations & examples (Q21–Q30)
Q21. State the work–energy theorem in one sentence.
The net work done on an object by all forces equals the change in its kinetic energy: W_net = ΔKE.
Q22. A 5 kg object speeds up from 2 m/s to 6 m/s. Find the net work done on it.
ΔKE = ½·m·(v_f² − v_i²) = 0.5×5×(36−4) = 2.5×32 = 80 J. So net work done is 80 J.
Q23. How does doing positive work on a body affect its kinetic energy?
Positive work increases the body's kinetic energy; the work done appears as an increase in KE according to the work–energy theorem.
Q24. A force of 20 N acts at 60° to displacement of 4 m. Compute the work done.
W = F·s·cosθ = 20×4×cos60° = 80×0.5 = 40 J.
Q25. Explain how friction affects the mechanical energy of a moving object.
Friction does negative work on moving objects, converting mechanical energy into thermal energy of the surfaces, thus reducing mechanical energy of the system.
Q26. A spring (k = 200 N/m) is compressed by 0.1 m. Find stored elastic potential energy.
U = ½·k·x² = 0.5 × 200 × (0.1)² = 100 × 0.01 = 1 J.
Q27. Why is the work done by gravity independent of path?
Gravity is a conservative force; work done by gravity depends only on initial and final heights (change in potential energy) and not on the path between them.
Q28. Show briefly how lifting an object at constant speed involves work and energy change.
Lifting object at constant speed requires upward force equal to weight; work done = mgh, which increases gravitational potential energy by same amount; kinetic energy remains constant.
Q29. A 10 N force moves an object 3 m at 120° to the force direction. What is work done?
cos120° = −1/2, so W = 10×3×(−1/2) = −15 J. Negative work indicates force opposes displacement.
Q30. How can you experimentally show that work changes energy of an object?
Example experiment: pull a mass with known force through known displacement and measure change in speed; calculate work W = F·s and ΔKE = ½ m(v_f²−v_i²) to verify W = ΔKE.
Part D — Conservation of Energy & practical examples (Q31–Q40)
Q31. State the law of conservation of energy in a sentence.
Energy cannot be created or destroyed in an isolated system; it can only be transformed from one form to another, total energy remaining constant.
Q32. Describe energy changes of a pendulum (neglecting friction).
At highest points the pendulum has maximum potential and minimum kinetic energy; at the lowest point it has maximum kinetic and minimum potential energy. Total mechanical energy remains constant.
Q33. Why does mechanical energy decrease in real pendulums over time?
Due to non-conservative forces—air resistance and friction at pivot—mechanical energy is converted into thermal energy, causing amplitude to decrease.
Q34. Explain how hydroelectric power converts energy from one form to another.
Gravitational potential energy of stored water converts into kinetic energy as it falls, turning turbines (mechanical energy) which drive generators to produce electrical energy.
Q35. A cyclist brakes and stops. Where does the kinetic energy go?
Kinetic energy is converted into thermal energy in the brakes and the road due to friction, and some energy is dissipated as sound.
Q36. Give an example where chemical potential energy becomes kinetic energy.
Burning fuel in a car engine converts chemical energy into thermal energy, producing expanding gases that do work on pistons, yielding mechanical (kinetic) energy of the car.
Q37. What is a conservative force? Name two examples.
A conservative force is one for which work done is path-independent and potential energy can be defined. Examples: gravitational force and electrostatic force.
Q38. What is non-conservative force? Give an example and its effect.
Non-conservative forces (like friction) depend on the path; they dissipate mechanical energy into thermal energy and mechanical energy is not conserved when they act.
Q39. Explain briefly why a satellite in circular orbit has constant mechanical energy.
For a satellite in circular orbit neglecting drag, gravitational potential and kinetic energies remain constant in time; no net work is done by gravity over a full orbit, so mechanical energy is constant.
Q40. How is energy efficiency of a device defined?
Efficiency = (useful energy output / total energy input) × 100%. It measures how well a device converts input energy into the desired form without losses.
Part E — Power, applications and concept checks (Q41–Q50)
Q41. Define power and state its SI unit.
Power is the rate at which work is done or energy is transferred: P = W/t. Its SI unit is watt (W), where 1 W = 1 J/s.
Q42. A machine does 3600 J of work in 2 minutes. What is its power?
Time = 2 min = 120 s. Power = 3600 J / 120 s = 30 W.
Q43. Relate power, force and velocity for instantaneous power.
Instantaneous power delivered by a constant force is P = F·v when force and velocity are in same direction (dot product form). This gives power in watts if F in newtons and v in m/s.
Q44. Explain why two people doing the same work in different times have different power outputs.
Power = W/t. If the work W is same but time t differs, the person who does the work faster (smaller t) has a larger power output.
Q45. A 50 kg lift is raised 5 m in 4 s. (g = 9.8 m/s²) Find approximate power required (neglect friction).
Work = mgh = 50×9.8×5 = 2450 J. Power = 2450 / 4 ≈ 612.5 W ≈ 613 W.
Q46. Why does carrying an object at constant height require no work against gravity?
Because the vertical displacement is zero, work done against gravity (mgh) is zero; however energy may be expended by muscles due to internal biological processes, not counted as mechanical work on the object.
Q47. Explain briefly why constant speed on a level road still requires fuel in vehicles.
To overcome resistive forces—rolling friction and air resistance—that do negative work on the vehicle. Fuel provides energy to compensate these losses and maintain constant speed.
Q48. A man pulls a crate with force 100 N at 30° above horizontal for 5 m. Find work done by the man.
Horizontal component = 100 cos30° = 100 × (√3/2) ≈ 86.6 N. Work = (horizontal component) × 5 m ≈ 86.6×5 ≈ 433 J. (If displacement purely horizontal.)
Q49. Give one exam tip for solving numerical problems in work and energy.
Always sketch the situation, identify forces and directions, write known quantities with units, use W = F·s·cosθ or energy methods (ΔKE, mgh, ½kx²) and check unit consistency at the end.
Q50. Summarise in two lines how work, energy and power are related.
Work is a transfer of energy; energy is the capacity to do work; power measures how fast that energy transfer (work) occurs. Mathematically: P = dW/dt and W changes a system's energy.