Important Questions Class 11 Physics Chapter 5: Work, Energy and Power

Work measures energy transferred by a force during displacement, while power measures the rate of that transfer. Mechanical energy remains conserved when only conservative forces do work on a system.

Work, energy and power convert force-based mechanics into a method based on scalar quantities and conservation laws. Important Questions Class 11 Physics Chapter 5 help students practise scalar product, work done, kinetic energy, work-energy theorem, variable force, potential energy, mechanical energy conservation, spring energy, power and collisions. The CBSE 2026 chapter also explains conservative and non-conservative forces, elastic collisions, inelastic collisions, and one-dimensional collision formulas.

Key Takeaways

• Work Done: Work by a constant force is W = F · d = Fd cos θ.
• Kinetic Energy: A body of mass m and speed v has K = 1/2 mv².
• Work-Energy Theorem: Net work done on a body equals its change in kinetic energy.
• Power: Instantaneous power is P = F · v.

Important Questions Class 11 Physics Chapter 5 Structure 2026

Important Questions Class 11 Physics Chapter 5 with Answers

Work, Energy and Power questions often need a clear choice between force analysis and energy analysis.
Students should identify the force doing work before applying any formula.
These work energy and power class 11 important questions follow the NCERT 2026 sequence.

1. What does Important Questions Class 11 Physics Chapter 5 mainly test?

Important Questions Class 11 Physics Chapter 5 mainly test work, kinetic energy, potential energy, conservation of energy, power and collisions. The chapter also uses scalar product to calculate work.

1. Work Skill: Use W = Fd cos θ.
2. Energy Skill: Use K = 1/2 mv² and U = mgh.
3. Conservation Skill: Apply K + V = constant.
4. Collision Skill: Use momentum and kinetic energy rules.
5. Final Result: The chapter tests energy transfer and conservation.

2. Why is work in physics different from everyday work?

Work in physics needs both force and displacement. A large force with zero displacement does no physical work.

1. Physics Meaning: Work = force component × displacement.
2. Example: Pushing a rigid wall gives zero displacement.
3. Formula Used: W = Fd cos θ.
4. Final Result: No displacement means zero work.

3. What are the SI units of work, energy and power?

The SI unit of work and energy is joule, and the SI unit of power is watt. One watt equals one joule per second.

1. Work Unit: Joule.
2. Energy Unit: Joule.
3. Power Unit: Watt.
4. Unit Relation: 1 W = 1 J/s.
5. Final Result: Work and energy share the same unit.

Scalar Product Class 11 Physics Questions

Scalar product helps calculate work because work depends on the component of force along displacement.
It converts two vectors into one scalar value.
These scalar product class 11 physics questions cover dot product, projection and angle.

4. What is scalar product of two vectors?

Scalar product of two vectors is A · B = AB cos θ. It gives a scalar quantity.

1. Vector 1: A.
2. Vector 2: B.
3. Angle: θ between A and B.
4. Formula Used: A · B = AB cos θ.
5. Final Result: Scalar product has no direction.

5. When is scalar product zero?

Scalar product is zero when two non-zero vectors are perpendicular. The angle between them is 90°.

1. Formula Used: A · B = AB cos θ.
2. At θ = 90°: cos 90° = 0.
3. Result: A · B = 0.
4. Final Result: Perpendicular vectors have zero dot product.

6. Find the scalar product of A = 3i + 4j − 5k and B = 5i + 4j + 3k.

The scalar product is 16. Multiply corresponding components and add them.

1. Given Data:
   A = 3i + 4j − 5k
   B = 5i + 4j + 3k
2. Formula Used: A · B = AxBx + AyBy + AzBz.
3. Calculation:
   A · B = 3 × 5 + 4 × 4 + (−5) × 3
   A · B = 15 + 16 − 15
   A · B = 16
4. Final Result: A · B = 16.

7. Why is scalar product useful in work calculation?

Scalar product is useful because work uses force component along displacement. The dot product automatically includes that component.

1. Work Formula: W = F · d.
2. Component Used: F cos θ.
3. Displacement: d.
4. Final Result: Work is the scalar product of force and displacement.

Work Done Class 11 Physics Questions

Work done depends on force, displacement and the angle between them.
It can be positive, negative or zero depending on the direction of force.
These work done class 11 physics questions cover common NCERT cases.

8. What is work done by a constant force?

Work done by a constant force is W = Fd cos θ. Here θ is the angle between force and displacement.

1. Force: F.
2. Displacement: d.
3. Angle: θ.
4. Formula Used: W = Fd cos θ.
5. Final Result: Work is force component along displacement times displacement.

9. When is work done positive?

Work done is positive when force has a component along displacement. The angle lies between 0° and 90°.

1. Angle Range: 0° ≤ θ < 90°.
2. Cosine Value: cos θ is positive.
3. Example: Force pulling a box forward.
4. Final Result: Positive work increases kinetic energy.

10. When is work done negative?

Work done is negative when force acts opposite to displacement. The angle lies between 90° and 180°.

1. Angle Range: 90° < θ ≤ 180°.
2. Cosine Value: cos θ is negative.
3. Example: Friction on a sliding block.
4. Final Result: Negative work reduces kinetic energy.

11. When is work done zero?

Work done is zero when displacement is zero or force is perpendicular to displacement. It is also zero when force is zero.

1. Case 1: No displacement.
2. Case 2: Force perpendicular to displacement.
3. Case 3: No force.
4. Final Result: Zero work occurs when Fd cos θ = 0.

12. A cyclist stops in 10 m due to a 200 N frictional force. Find work done by the road.

The work done by the road is −2000 J. Friction acts opposite to displacement.

1. Given Data:
   F = 200 N
   d = 10 m
   θ = 180°
2. Formula Used: W = Fd cos θ.
3. Calculation:
   W = 200 × 10 × cos 180°
   W = −2000 J
4. Final Result: Work done = −2000 J.

13. Why does gravity do no work on a body moving horizontally?

Gravity does no work because it acts vertically while displacement is horizontal. The angle between them is 90°.

1. Force: mg downward.
2. Displacement: Horizontal.
3. Angle: 90°.
4. Calculation: W = Fd cos 90° = 0.
5. Final Result: Gravity does zero work in horizontal motion.

Work Energy Theorem Class 11 Questions

The work-energy theorem connects net work with kinetic energy change.
It is a scalar form of Newton’s second law over a displacement.
These work energy theorem class 11 questions cover constant and variable force cases.

14. State the work-energy theorem.

The work-energy theorem states that net work done on a body equals change in its kinetic energy. It is written as Wnet = Kf − Ki.

1. Initial Kinetic Energy: Ki.
2. Final Kinetic Energy: Kf.
3. Net Work: Wnet.
4. Formula Used: Wnet = ΔK.
5. Final Result: Net work equals change in kinetic energy.

15. How does work-energy theorem follow from constant acceleration?

For constant acceleration, v² − u² = 2as. Multiplying by m/2 gives the work-energy theorem.

1. Kinematic Relation: v² − u² = 2as.
2. Multiply by m/2: 1/2mv² − 1/2mu² = mas.
3. Using F = ma: mas = Fs.
4. Final Result: Kf − Ki = W.

16. A 2 kg body goes from rest to 5 m/s. Find net work done.

The net work done is 25 J. Use Wnet = ΔK.

1. Given Data:
   m = 2 kg
   u = 0
   v = 5 m/s
2. Formula Used: Wnet = 1/2mv² − 1/2mu².
3. Calculation:
   Wnet = 1/2 × 2 × 25 − 0
   Wnet = 25 J
4. Final Result: Net work = 25 J.

17. A raindrop reaches ground with kinetic energy 1.25 J, while gravity does 10 J work. Find work by air resistance.

The work done by air resistance is −8.75 J. Use work-energy theorem.

1. Given Data:
   ΔK = 1.25 J
   Wg = 10 J
2. Formula Used: ΔK = Wg + Wr.
3. Calculation:
   Wr = ΔK − Wg
   Wr = 1.25 − 10
   Wr = −8.75 J
4. Final Result: Air resistance does −8.75 J work.

Kinetic Energy Class 11 Physics Questions

Kinetic energy is the energy possessed by a body due to motion.
It is always non-negative because it depends on the square of speed.
These kinetic energy class 11 physics questions focus on formula and speed changes.

18. What is kinetic energy?

Kinetic energy is energy possessed by a body due to motion. Its formula is K = 1/2mv².

1. Mass: m.
2. Speed: v.
3. Formula Used: K = 1/2mv².
4. SI Unit: Joule.
5. Final Result: Kinetic energy depends on mass and speed squared.

19. Why is kinetic energy a scalar quantity?

Kinetic energy is scalar because it depends on speed squared, not velocity direction. It has magnitude only.

1. Formula: K = 1/2mv².
2. Speed: Scalar.
3. Mass: Scalar.
4. Final Result: Kinetic energy has no direction.

20. If speed doubles, what happens to kinetic energy?

Kinetic energy becomes four times when speed doubles. It is proportional to v².

1. Initial: K = 1/2mv².
2. New Speed: 2v.
3. New Energy: K' = 1/2m(2v)² = 4K.
4. Final Result: Doubling speed quadruples kinetic energy.

21. A 50 g bullet has speed 200 m/s. Find its kinetic energy.

The kinetic energy is 1000 J. Convert mass to kilogram first.

1. Given Data:
   m = 50 g = 0.05 kg
   v = 200 m/s
2. Formula Used: K = 1/2mv².
3. Calculation:
   K = 1/2 × 0.05 × 200²
   K = 1000 J
4. Final Result: K = 1000 J.

22. If the same bullet emerges with 10% of its initial kinetic energy, find its speed.

The emergent speed is 63.2 m/s. Use K = 1/2mv².

1. Initial Kinetic Energy: 1000 J.
2. Final Kinetic Energy: 100 J.
3. Formula Used: v = √(2K/m).
4. Calculation:
   v = √(2 × 100/0.05)
   v = √4000
   v = 63.2 m/s
5. Final Result: Emergent speed = 63.2 m/s.

Variable Force Class 11 Questions

A variable force changes with position, so simple W = Fd may not apply.
The work done equals the area under the force-displacement graph.
These variable force class 11 questions cover graphs and definite integrals.

23. How is work done by a variable force calculated?

Work done by a variable force is calculated by integrating force over displacement. It equals area under the F-x graph.

1. Small Work: dW = F(x) dx.
2. Total Work: W = ∫F(x) dx.
3. Graph Meaning: Area under force-displacement curve.
4. Final Result: Variable force work equals ∫F dx.

24. What does area under a force-displacement graph represent?

Area under a force-displacement graph represents work done. Area above the axis is positive, and below the axis is negative.

1. Graph Axis: Force on y-axis and displacement on x-axis.
2. Positive Area: Positive work.
3. Negative Area: Negative work.
4. Final Result: Area under F-x graph gives work.

25. A force decreases linearly from 100 N to 50 N over 10 m. Find work done.

The work done is 750 J. The graph forms a trapezium.

1. Given Data:
   F1 = 100 N
   F2 = 50 N
   d = 10 m
2. Formula Used: Work = 1/2(F1 + F2)d.
3. Calculation:
   W = 1/2(100 + 50) × 10
   W = 750 J
4. Final Result: Work done = 750 J.

26. A woman pushes with 100 N for 10 m, then force decreases from 100 N to 50 N over the next 10 m. Find total work.

The total work done is 1750 J. Add rectangular and trapezium areas.

1. First Part:
   W1 = 100 × 10
   W1 = 1000 J
2. Second Part:
   W2 = 1/2(100 + 50) × 10
   W2 = 750 J
3. Total:
   W = 1000 + 750
   W = 1750 J
4. Final Result: Total work = 1750 J.

Potential Energy Class 11 Physics Questions

Potential energy is stored energy due to position or configuration.
It can be defined for conservative forces such as gravity and spring force.
These potential energy class 11 physics questions cover gravitational potential energy and conservative force.

27. What is potential energy?

Potential energy is stored energy due to position or configuration. It can convert into kinetic energy when constraints are removed.

1. Example 1: Raised stone has gravitational potential energy.
2. Example 2: Compressed spring has elastic potential energy.
3. Condition: It applies to conservative forces.
4. Final Result: Potential energy is stored mechanical energy.

28. What is gravitational potential energy near Earth’s surface?

Gravitational potential energy near Earth’s surface is V = mgh. It is measured relative to a chosen zero level.

1. Mass: m.
2. Acceleration Due to Gravity: g.
3. Height: h.
4. Formula Used: V = mgh.
5. Final Result: Gravitational potential energy equals mgh.

29. What is a conservative force?

A conservative force is a force whose work depends only on initial and final positions. Its work over a closed path is zero.

1. Path Dependence: No dependence on path.
2. Closed Path Work: Zero.
3. Example: Gravitational force.
4. Final Result: Conservative force has an associated potential energy.

30. Why is friction non-conservative?

Friction is non-conservative because its work depends on the path length. Its work over a closed path is not zero.

1. Friction Direction: Opposes motion.
2. Work: Usually negative.
3. Closed Path: Work remains negative.
4. Final Result: Friction has no single potential energy function.

31. A 2 kg body is lifted to height 5 m. Find its potential energy using g = 10 m/s².

The potential energy is 100 J. Use V = mgh.

1. Given Data:
   m = 2 kg
   g = 10 m/s²
   h = 5 m
2. Formula Used: V = mgh.
3. Calculation:
   V = 2 × 10 × 5
   V = 100 J
4. Final Result: Potential energy = 100 J.

Conservation of Mechanical Energy Class 11 Questions

Mechanical energy is the sum of kinetic and potential energies.
It stays constant when only conservative forces do work.
These conservation of mechanical energy class 11 questions cover falling bodies and pendulum motion.

32. State the principle of conservation of mechanical energy.

The principle states that total mechanical energy remains constant when only conservative forces do work. It is written as K + V = constant.

1. Kinetic Energy: K.
2. Potential Energy: V.
3. Total Mechanical Energy: E = K + V.
4. Condition: Only conservative forces do work.
5. Final Result: Mechanical energy remains conserved.

33. What happens to energy during free fall?

During free fall, gravitational potential energy decreases and kinetic energy increases. Their sum remains constant without air resistance.

1. Top Point: Energy is mostly potential.
2. Middle Point: Energy is partly kinetic and partly potential.
3. Ground Level: Energy becomes kinetic.
4. Final Result: Potential energy converts into kinetic energy.

34. A ball is dropped from height H. Find speed just before reaching ground.

The speed is √(2gH). Use conservation of mechanical energy.

1. Initial Energy: Ei = mgH.
2. Final Energy: Ef = 1/2mv².
3. Equate: mgH = 1/2mv².
4. Final Result: v = √(2gH).

35. Find speed at height h when a ball falls from height H.

The speed is √[2g(H − h)]. The loss in potential energy becomes kinetic energy.

1. Initial Energy: mgH.
2. Energy at Height h: mgh + 1/2mv².
3. Equate: mgH = mgh + 1/2mv².
4. Final Result: v = √[2g(H − h)].

36. Why is tension in a pendulum not included in work done?

Tension does no work because it is perpendicular to displacement along the circular path. Only gravity changes mechanical energy.

1. Tension Direction: Along string.
2. Displacement Direction: Tangential.
3. Angle: 90°.
4. Final Result: Tension does zero work in pendulum motion.

Spring Potential Energy Class 11 Questions

Spring force is a variable conservative force.
Its potential energy depends on extension or compression squared.
These spring potential energy class 11 questions cover Hooke’s law, stored energy and compression.

37. What is Hooke’s law for a spring?

Hooke’s law states that spring force is proportional to displacement from equilibrium. Its formula is Fs = −kx.

1. Spring Constant: k.
2. Displacement: x.
3. Negative Sign: Force acts opposite to displacement.
4. Final Result: Spring force is Fs = −kx.

38. What is spring potential energy?

Spring potential energy is V = 1/2kx². It applies to both extension and compression.

1. Spring Constant: k.
2. Displacement: x.
3. Formula Used: V = 1/2kx².
4. Final Result: Spring energy depends on x².

39. Why is spring force conservative?

Spring force is conservative because work done depends only on initial and final positions. Work over a complete cycle is zero.

1. Force Law: Fs = −kx.
2. Potential Energy: V = 1/2kx².
3. Closed Path: Net work is zero.
4. Final Result: Spring force has potential energy.

40. A spring has k = 200 N/m and is compressed by 0.1 m. Find stored energy.

The stored energy is 1 J. Use V = 1/2kx².

1. Given Data:
   k = 200 N/m
   x = 0.1 m
2. Formula Used: V = 1/2kx².
3. Calculation:
   V = 1/2 × 200 × 0.1²
   V = 1 J
4. Final Result: Spring energy = 1 J.

41. A 1000 kg car moving at 18 km/h compresses a spring of k = 5.25 × 10³ N/m. Find maximum compression.

The maximum compression is 2.18 m approximately. Use kinetic energy equals spring potential energy.

1. Given Data:
   m = 1000 kg
   v = 18 km/h = 5 m/s
   k = 5.25 × 10³ N/m
2. Energy Equation:
   1/2mv² = 1/2kx²
3. Calculation:
   x = v√(m/k)
   x = 5√(1000/5250)
   x ≈ 2.18 m
4. Final Result: Maximum compression ≈ 2.18 m.

Power Class 11 Physics Questions

Power measures how fast work is done or energy is transferred.
It is a scalar quantity and can be average or instantaneous.
These power class 11 physics questions cover watt, horsepower, kWh and motor power.

42. What is power in physics?

Power is the rate of doing work or transferring energy. Its formula is P = W/t.

1. Work Done: W.
2. Time Taken: t.
3. Formula Used: P = W/t.
4. SI Unit: Watt.
5. Final Result: Power measures rate of energy transfer.

43. What is instantaneous power?

Instantaneous power is the power at a given instant. It is given by P = F · v.

1. Force: F.
2. Velocity: v.
3. Formula Used: P = F · v.
4. Final Result: Instantaneous power equals force dot velocity.

44. What is one horsepower?

One horsepower equals 746 W. It is still used for engines and vehicles.

1. Unit: Horsepower.
2. Conversion: 1 hp = 746 W.
3. Use: Automobiles and motors.
4. Final Result: 1 hp = 746 W.

45. What is one kilowatt-hour?

One kilowatt-hour is an energy unit equal to 3.6 × 10⁶ J. It appears in electricity bills.

1. Power: 1 kW = 1000 W.
2. Time: 1 hour = 3600 s.
3. Energy: 1000 × 3600 J.
4. Final Result: 1 kWh = 3.6 × 10⁶ J.

46. An elevator of total mass 1800 kg moves upward at 2 m/s against friction 4000 N. Find motor power.

The motor power is 44000 W or about 59 hp. Use P = Fv.

1. Given Data:
   m = 1800 kg
   g = 10 m/s²
   friction = 4000 N
   v = 2 m/s
2. Total Downward Force:
   F = mg + friction
   F = 1800 × 10 + 4000
   F = 22000 N
3. Power:
   P = Fv
   P = 22000 × 2
   P = 44000 W
4. Horsepower:
   P = 44000/746 ≈ 59 hp
5. Final Result: Power ≈ 44000 W or 59 hp.

Collision Class 11 Physics Questions

Collisions use conservation of momentum for the system of colliding bodies.
Kinetic energy may or may not remain conserved after collision.
These collision class 11 physics questions cover elastic and inelastic cases.

47. What is a collision in physics?

A collision is an interaction where two bodies exert large forces on each other for a short time. Momentum remains conserved for the system.

1. Interaction Time: Very small.
2. Forces: Large internal impulsive forces.
3. Conservation: Total momentum remains conserved.
4. Final Result: Collisions are analysed using momentum and energy.

48. What is an elastic collision?

An elastic collision is a collision in which total momentum and total kinetic energy remain conserved. Ideal billiard ball collisions approximate it.

1. Momentum: Conserved.
2. Kinetic Energy: Conserved after collision.
3. Deformation: Energy is recovered.
4. Final Result: Elastic collision conserves kinetic energy.

49. What is an inelastic collision?

An inelastic collision is a collision in which kinetic energy is not conserved. Some kinetic energy changes into heat, sound or deformation.

1. Momentum: Conserved.
2. Kinetic Energy: Not conserved.
3. Energy Change: Other forms appear.
4. Final Result: Inelastic collision loses kinetic energy.

50. What is a completely inelastic collision?

A completely inelastic collision occurs when the bodies move together after collision. Momentum remains conserved.

1. After Collision: Bodies stick together.
2. Common Velocity: Same final velocity.
3. Kinetic Energy: Maximum loss among inelastic collisions.
4. Final Result: Bodies move together in completely inelastic collision.

Elastic Collision Class 11 Questions

One-dimensional elastic collision gives direct formulas for final velocities.
Special cases help solve problems quickly without lengthy algebra.
These elastic collision class 11 questions follow NCERT collision results.

51. What are final velocities in one-dimensional elastic collision when second body is initially at rest?

The final velocities are found from momentum and kinetic energy conservation. For masses m1 and m2, use the standard elastic collision formulas.

1. Initial Condition: m1 moves with speed v1i.
2. Target Body: m2 is at rest.
3. Formula 1: v1f = [(m1 − m2)/(m1 + m2)]v1i.
4. Formula 2: v2f = [2m1/(m1 + m2)]v1i.
5. Final Result: Elastic collision conserves both momentum and kinetic energy.

52. What happens in an elastic collision of equal masses?

The first mass stops, and the second mass moves with the first mass’s initial speed. This happens in a head-on elastic collision.

1. Condition: m1 = m2.
2. Initial Motion: m1 moves, m2 rests.
3. Final Result: v1f = 0 and v2f = v1i.
4. Final Result: Equal masses exchange velocities.

53. What happens when a light body hits a very heavy body elastically?

The light body reverses its velocity, and the heavy body remains nearly at rest. This is the m2 >> m1 case.

1. Mass Condition: m2 is much larger than m1.
2. Final Speed of Heavy Body: Nearly zero.
3. Final Speed of Light Body: Nearly −v1i.
4. Final Result: Light body rebounds with nearly same speed.

54. Why is momentum conserved in all collisions?

Momentum is conserved because internal forces are equal and opposite during collision. Their impulses cancel for the system.

1. Newton’s Third Law: F12 = −F21.
2. Impulse: Force × time.
3. System Result: Total impulse is zero.
4. Final Result: Total linear momentum remains conserved.

55. Is kinetic energy conserved during contact in an elastic collision?

No, kinetic energy need not stay constant during contact. It is conserved only before and after the collision.

1. During Contact: Bodies deform.
2. Temporary Storage: Kinetic energy can become elastic potential energy.
3. After Collision: Kinetic energy returns.
4. Final Result: Elastic collision conserves final total kinetic energy.

NCERT Class 11 Physics Chapter 5 Questions

NCERT questions often combine signs, units, work-energy theorem, power and collisions.
Students should state the force doing work before calculating numerical values.
These NCERT Class 11 Physics Chapter 5 questions follow the 2026 exercise pattern.

56. What is the sign of work done by a man lifting a bucket from a well?

The work done by the man is positive. The applied force and displacement are both upward.

1. Force Direction: Upward.
2. Displacement Direction: Upward.
3. Angle: 0°.
4. Final Result: Work done by the man is positive.

57. What is the sign of work done by gravity while lifting a bucket?

The work done by gravity is negative. Gravity acts downward while displacement is upward.

1. Force Direction: Downward.
2. Displacement Direction: Upward.
3. Angle: 180°.
4. Final Result: Work done by gravity is negative.

58. A force F = −3i + 2j + 3k N moves a body 4 m along z-axis. Find work done.

The work done is 12 J. Only the z-component contributes.

1. Given Data:
   F = −3i + 2j + 3k N
   displacement = 4k m
2. Formula Used: W = F · d.
3. Calculation:
   W = (−3i + 2j + 3k) · 4k
   W = 12 J
4. Final Result: Work done = 12 J.

59. A pump fills 30 m³ water tank at height 40 m in 15 min with 30% efficiency. Find electric power consumed.

The electric power consumed is 43.6 kW approximately. Use output energy divided by time and efficiency.

1. Given Data:
   Volume = 30 m³
   mass = 30000 kg
   h = 40 m
   t = 15 min = 900 s
   efficiency = 0.30
2. Useful Power:
   Pout = mgh/t
   Pout = 30000 × 9.8 × 40/900
   Pout = 13067 W
3. Input Power:
   Pin = Pout/0.30
   Pin ≈ 43556 W
4. Final Result: Electric power ≈ 43.6 kW.

60. A 0.5 kg body has velocity v = 5x^(3/2). Find work from x = 0 to x = 2 m.

The work done is 100 J. Use work-energy theorem.

1. Given Data:
   m = 0.5 kg
   v = 5x^(3/2)
   x = 2 m
2. Final Speed:
   v = 5 × 2^(3/2)
   v² = 25 × 2³ = 200
3. Work Done:
   W = ΔK = 1/2mv²
   W = 1/2 × 0.5 × 200
   W = 50 J
4. Final Result: Work done = 50 J.

61. A pendulum bob starts from horizontal position. Length is 1.5 m and 5% energy dissipates. Find speed at lowest point.

The speed is 5.29 m/s approximately. Only 95% of initial potential energy becomes kinetic energy.

1. Given Data:
   L = 1.5 m
   usable energy fraction = 0.95
2. Energy Equation:
   1/2mv² = 0.95mgL
3. Calculation:
   v = √(2 × 0.95 × 9.8 × 1.5)
   v = 5.29 m/s
4. Final Result: Speed ≈ 5.29 m/s.

Class 11 Physics Chapter 5 Questions and Answers

Mixed questions in this chapter need careful distinction between force, work and energy.
Direction matters for work, but kinetic energy depends only on speed.
These class 11 physics chapter 5 questions and answers cover key formula decisions.

62. Can work done be negative?

Yes, work done can be negative when force opposes displacement. Friction usually does negative work on a moving body.

1. Angle: θ = 180°.
2. Cosine: cos 180° = −1.
3. Formula: W = Fd cos θ.
4. Final Result: Opposing force does negative work.

63. Can kinetic energy be negative?

No, kinetic energy cannot be negative. It depends on mass and speed squared.

1. Formula: K = 1/2mv².
2. Mass: Positive.
3. Speed Squared: Non-negative.
4. Final Result: Kinetic energy is never negative.

64. Can potential energy be negative?

Yes, potential energy can be negative depending on the chosen zero level. Only change in potential energy has direct physical meaning.

1. Zero Level: Chosen by convenience.
2. Example: Gravitational energy in universal gravitation is negative for bound systems.
3. Important Quantity: ΔV.
4. Final Result: Potential energy depends on reference level.

65. Why does work-energy theorem not give complete motion details?

Work-energy theorem gives energy change, not time information. It is an integral form of Newton’s second law.

1. Gives: Relation between work and kinetic energy.
2. Does Not Give: Exact time dependence directly.
3. Need For Time: Newton’s second law.
4. Final Result: Work-energy theorem gives scalar energy information.

66. Why is total energy conserved even in inelastic collision?

Total energy is conserved because lost kinetic energy changes into heat, sound or deformation. Only kinetic energy is not conserved.

1. Momentum: Conserved for isolated system.
2. Kinetic Energy: Decreases.
3. Other Energy Forms: Increase.
4. Final Result: Total energy remains conserved.

CBSE Class 11 Physics Chapter-Wise Important Questions