Class 9th Science Work and Energy Important QAs

🔹 Very Short Answer (1 mark)

Q1. Define work.
Ans. Work is said to be done when a force is applied on an object and it is displaced in the direction of the force.

Q2. State the SI unit of work.
Ans. Joule (J).

Q3. Define 1 Joule of work.
Ans. Work done when a force of 1 N displaces an object by 1 m in the direction of force.

Q4. Define energy.
Ans. The capacity to do work.

Q5. State the SI unit of energy.
Ans. Joule (J).

Q6. Define kinetic energy.
Ans. Energy possessed by a body due to its motion.

Q7. Define potential energy.
Ans. Energy possessed by a body due to its position or configuration.

Q8. Give the formula for work done.
Ans. W = F × d × cosθ (θ = angle between force and displacement).

Q9. What is the work done when force is perpendicular to displacement?
Ans. Zero.

Q10. Define power.
Ans. Rate of doing work.

🔹 Short Answer (2–3 marks)

Q11. SI unit of power?
Ans. Watt (W).

Q12. 1 Watt = ?
Ans. 1 Joule per second (1 J/s).

Q13. Give two examples of kinetic energy.
Ans. Moving car, flowing river.

Q14. Give two examples of potential energy.
Ans. Stretched bow, water in elevated tank.

Q15. State work-energy theorem.
Ans. Work done on a body is equal to change in its kinetic energy.

Q16. Define mechanical energy.
Ans. Sum of kinetic and potential energy.

Q17. When is work said to be positive?
Ans. When force and displacement are in the same direction.

Q18. When is work said to be negative?
Ans. When force and displacement are in opposite directions.

Q19. Define gravitational potential energy.
Ans. Energy possessed by a body due to its position in a gravitational field.

Q20. State law of conservation of energy.
Ans. Energy can neither be created nor destroyed; it can only change from one form to another.

🔹 Long Answer (4–5 marks)

Q21. Write formula for kinetic energy and define variables.
Ans. KE = ½ mv², where m = mass, v = velocity.

Q22. Write formula for potential energy and define variables.
Ans. PE = mgh, where m = mass, g = acceleration due to gravity, h = height.

Q23. Differentiate between work and energy.

Q24. Differentiate between kinetic and potential energy.

Q25. Explain law of conservation of energy with example.
Ans. In pendulum motion, at highest point: PE max, KE min; at lowest point: KE max, PE min → total energy constant.

Q26. Define mechanical work with example.
Ans. Work done by a force when object is displaced. Eg: Pushing a cart.

Q27. Explain positive and negative work with examples.
Ans. Positive: Pulling a rope in same direction → work done on object.
Negative: Friction opposing motion → work done by object on friction is negative.

Q28. What is power? Give example.
Ans. Rate of doing work. Eg: Electric motor.

Q29. Difference between energy and power.

Q30. Explain work done in case of inclined plane.
Ans. Work = force × displacement along plane; less force required over longer distance compared to vertical lift.

🔹 Case Study / Application-Based (5–6 marks)

Q31. Case Study – Lifting Water
A boy lifts a bucket from well to terrace.
(i) Which type of energy increases?
(ii) Why?
Ans.
(i) Potential energy increases.
(ii) Because height above ground increases → PE = mgh.

Q32. Case Study – Moving Car
A car accelerates on road.
(i) What energy changes occur?
Ans. Chemical energy of fuel → kinetic energy of car.

Q33. Case Study – Pendulum
At highest point → KE = 0, PE = max; at lowest → KE = max, PE = min → total energy constant.

Q34. Why is work zero when holding an object stationary?
Ans. No displacement → W = F × d × cosθ → d = 0 → W = 0.

Q35. Why does friction do negative work?
Ans. Friction opposes motion → displacement opposite to force → negative work.

Q36. Why can energy change forms but total remain same?
Ans. Law of conservation of energy → e.g., electric → mechanical → heat.

Q37. How does a stretched rubber band store energy?
Ans. Elastic potential energy due to deformation.

Q38. Why is mechanical energy important for machines?
Ans. Machines convert energy to do work efficiently (KE → PE or vice versa).

Q39. Case Study – Roller Coaster
At top → PE max, KE low; at bottom → KE max, PE low → total energy constant.

Q40. Why does faster moving vehicle have more kinetic energy?
Ans. KE ∝ v² → small increase in speed → large increase in KE.

🔹 Extra Important (41–50)

Q41. Define work done by variable force.
Ans. Work = area under force-displacement graph.

Q42. What is gravitational work?
Ans. Work done by or against gravity → W = mgh.

Q43. Why does energy have same unit as work?
Ans. Both represent capacity to cause motion → Joule.

Q44. Difference between conservative and non-conservative forces.

Q45. Why is potential energy zero at reference point?
Ans. By convention, energy measured relative to chosen zero level.

Q46. Case Study – Electric Bulb
Electric energy → light + heat energy. Shows energy transformation.

Q47. Why is power important in daily life?
Ans. Determines how quickly work can be done → helps choose appliances efficiently.

Q48. How is work done when pulling a cart at an angle?
Ans. Only component of force along displacement does work → W = F cosθ × d.

Q49. Why is energy conserved in absence of friction?
Ans. No energy lost to heat → total mechanical energy constant.

Q50. Example of energy transformation in daily life.
Ans. Battery → electrical → light in torch.