- Dr.Sanjaykumar Pawar
Power Physics Notes PDF | NEET Work, Energy and Power Chapter
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| Power in Physics – Complete NEET Notes with Formulas, Units, Examples and Quick Revision |
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Complete Class 11 Physics Notes
NEET Physics Formula Handbook
Work, Energy and Power · Unit IV
Power — Complete Question Bank
A full CBSE Class 11 Physics practice set on Power: MCQs, assertion–reason, case studies, match-the-columns, numericals, and a quick answer key — click any answer to reveal it.
Section A Multiple Choice Questions
Tap "Show Answer" under any question to reveal the solution.
Level · Easy
Q1.Power is defined as:
Easy- (a) Total work done
- (b) Rate of doing work
- (c) Energy stored in a body
- (d) Force applied per unit area
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Q2.The SI unit of power is:
Easy- (a) Joule
- (b) Newton
- (c) Watt
- (d) Pascal
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Q3.1 horsepower is equal to:
Easy- (a) 500 W
- (b) 746 W
- (c) 1000 W
- (d) 980 W
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Q4.A machine does 1000 J of work in 10 s. Its power is:
Easy- (a) 10 W
- (b) 100 W
- (c) 1000 W
- (d) 10000 W
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Q5.The dimensional formula of power is:
Easy- (a) [ML²T⁻²]
- (b) [ML²T⁻³]
- (c) [MLT⁻²]
- (d) [M²L²T⁻²]
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Q6.Power is a:
Easy- (a) Vector quantity
- (b) Scalar quantity
- (c) Neither scalar nor vector
- (d) Tensor quantity
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Q7.A pump lifts 200 kg of water to a height of 5 m in 10 s (g = 10 m/s²). The power of the pump is:
Easy- (a) 100 W
- (b) 500 W
- (c) 1000 W
- (d) 2000 W
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Q8.If force and velocity are perpendicular to each other, the power is:
Easy- (a) Maximum
- (b) Minimum
- (c) Zero
- (d) Infinite
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Level · Medium
Q9.A car engine exerts a force of 500 N while moving at a constant speed of 20 m/s. The power of the engine in hp is approximately:
Medium- (a) 10.7 hp
- (b) 13.4 hp
- (c) 15.2 hp
- (d) 20.0 hp
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Q10.The position of a particle is given by x = 3t² + 2t (x in m, t in s). A force of 6 N acts on it. The power at t = 2 s is:
Medium- (a) 72 W
- (b) 84 W
- (c) 96 W
- (d) 108 W
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Q11.A body of mass 2 kg is moved by a force of 10 N at constant velocity of 5 m/s at 60° to the direction of force. The power is:
Medium- (a) 50 W
- (b) 25 W
- (c) 43.3 W
- (d) 0 W
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Q12.An engine of power 2 kW can do how much work in 1 minute?
Medium- (a) 120 J
- (b) 2000 J
- (c) 120000 J
- (d) 20000 J
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Q13.A force F acts on a body moving with velocity v. If the angle between F and v is 120°, the power is:
Medium- (a) Fv
- (b) Fv/2
- (c) Zero
- (d) −Fv/2
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Q14.The power of a pump that can lift 5000 kg of water per minute to a height of 20 m is: (g = 10 m/s²)
Medium- (a) 16.67 kW
- (b) 10 kW
- (c) 100 kW
- (d) 1.67 kW
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Q15.A man of mass 60 kg climbs up a staircase carrying a load of 20 kg. If the total height gained is 10 m in 20 s, the average power is: (g = 10 m/s²)
Medium- (a) 200 W
- (b) 300 W
- (c) 400 W
- (d) 800 W
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Level · Hard
Q16.A particle of mass m moves along a circular path of radius r with uniform speed v. The power delivered by the centripetal force is:
Hard- (a) mv²/r
- (b) mv³/r
- (c) Zero
- (d) mv²r
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Q17.The power delivered to a body moving in a straight line is given by P = 3t² + 2t (in watts). The work done in the first 2 seconds is:
Hard- (a) 10 J
- (b) 12 J
- (c) 14 J
- (d) 16 J
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Q18.A body of mass 1 kg is thrown vertically upward with initial velocity 20 m/s. The instantaneous power due to gravity at t = 1 s is: (g = 10 m/s²)
Hard- (a) 100 W
- (b) −100 W
- (c) 200 W
- (d) −200 W
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Q19.A pump motor is rated at 5 hp. How many kilograms of water can it raise in 1 minute through a height of 10 m? (g = 10 m/s², 1 hp = 746 W)
Hard- (a) 1492 kg
- (b) 2238 kg
- (c) 2984 kg
- (d) 3730 kg
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Q20.A vehicle of mass m accelerates uniformly from rest to velocity v in time t. The instantaneous power delivered by the engine at time t/2 is:
Hard- (a) mv²/2t
- (b) mv²/4t
- (c) mv²/t
- (d) 3mv²/4t
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Section B Very Short Answer Questions (1 Mark Each)
Q1.Define power.
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Q2.Write the SI unit of power.
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Q3.What is 1 horsepower in watts?
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Q4.Is power a scalar or vector quantity? Why?
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Q5.Write the dimensional formula of power.
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Q6.A force acts perpendicular to the velocity of a body. What is the power?
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Q7.What is the relation between power, force, and velocity?
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Q8.A machine does no work. Can it have power?
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Q9.What is the commercial unit of power?
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Q10.Write the expression for instantaneous power.
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Section C Short Answer Questions (2 Marks Each)
Q1.Distinguish between average power and instantaneous power.
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| Average Power | Instantaneous Power |
|---|---|
| Defined as total work divided by total time | Defined as power at a particular instant |
| Formula: Pavg = W/t | Formula: Pinst = F·v |
| Used when total work and time are given | Used when force or velocity varies with time |
Q2.Derive the relation P = Fv.
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Q3.A pump delivers 1000 liters of water per minute to a tank at a height of 20 m. Find the power of the pump. (Density of water = 1000 kg/m³, g = 10 m/s²)
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Q4.Show that the power delivered by the centripetal force in uniform circular motion is zero.
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Q5.The power of an engine is 5 kW. How much work can it do in 10 minutes?
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Q6.A car of mass 1000 kg moves up an incline of 1 in 20 at a constant speed of 10 m/s. Find the power of the engine. (g = 10 m/s², neglect friction)
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Q7.Why is the power of a body moving with constant velocity on a frictionless horizontal surface zero?
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Q8.A 2 kW motor pump is used to pump water from a well 10 m deep. How much water can be pumped per minute? (g = 10 m/s²)
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Section D Long Answer Questions (3–5 Marks Each)
Q1.(a) Define power and derive its SI unit. (2 marks)
(b) A pump can throw 8000 kg of water per minute to a height of 15 m. Calculate the power of the pump. (g = 9.8 m/s²) (3 marks)
Show Answer
(b) Given: m = 8000 kg (per minute), h = 15 m, t = 60 s, g = 9.8 m/s². Work done per minute = mgh = 8000 × 9.8 × 15 = 1,176,000 J. Power P = W/t = 1,176,000/60 = 19,600 W = 19.6 kW
Q2.(a) Derive the expression for instantaneous power. (2 marks)
(b) The position of a body of mass 2 kg is given by x = 2t³ + 3t² + 5, where x is in meters and t in seconds. A constant force of 12 N acts on the body in the direction of motion. Calculate the power delivered at t = 2 s. (3 marks)
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(b) x = 2t³ + 3t² + 5 → v = dx/dt = 6t² + 6t. At t = 2 s: v = 6(4) + 6(2) = 24 + 12 = 36 m/s. Power P = F × v = 12 × 36 = 432 W
Q3.(a) Prove that power can also be expressed as the scalar product of force and velocity. (2 marks)
(b) An engine of power 10 hp is used to pump water from a well 8 m deep. How many kilograms of water can be pumped in 1 hour? (1 hp = 746 W, g = 9.8 m/s²) (3 marks)
Show Answer
(b) P = 10 hp = 10 × 746 = 7460 W. t = 1 hour = 3600 s. Total work W = P × t = 7460 × 3600 = 26,856,000 J. m = W/(gh) = 26,856,000/(9.8 × 8) = 26,856,000/78.4 ≈ 342,551 kg
Q4.(a) What is the difference between kilowatt and kilowatt-hour? (2 marks)
(b) A family uses a 2 kW electric heater for 4 hours daily. Calculate the energy consumed in 30 days in kWh and joules. (3 marks)
Show Answer
| Kilowatt (kW) | Kilowatt-hour (kWh) |
|---|---|
| Unit of power | Unit of energy |
| 1 kW = 1000 W | 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J |
| Measures rate of energy consumption | Measures total energy consumed |
(b) Power P = 2 kW, daily usage t = 4 h. Daily energy = 2 × 4 = 8 kWh. 30-day energy = 8 × 30 = 240 kWh. In joules: 240 kWh = 240 × 3.6 × 10⁶ = 8.64 × 10⁸ J
Section E Assertion and Reason Questions
Choose: (a) Both true, Reason correctly explains Assertion | (b) Both true, Reason does NOT explain Assertion | (c) Assertion true, Reason false | (d) Assertion false, Reason true
Q1.Assertion: Power is a scalar quantity.
Reason: Power is the ratio of two scalar quantities, work and time.
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Q2.Assertion: When a body moves in a circular path with uniform speed, the power delivered by the centripetal force is zero.
Reason: The centripetal force is always perpendicular to the velocity of the body.
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Q3.Assertion: A body moving with constant velocity on a frictionless horizontal surface has zero power.
Reason: No force is required to maintain constant velocity on a frictionless surface.
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Q4.Assertion: 1 horsepower is equal to 1000 watts.
Reason: Horsepower is the commercial unit of power.
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Q5.Assertion: The instantaneous power of a body can be negative.
Reason: Power is always positive because it is the rate of doing work.
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Q6.Assertion: Average power is always equal to instantaneous power.
Reason: Average power is calculated over a time interval while instantaneous power is at a specific instant.
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Q7.Assertion: The power delivered by gravity to a body thrown vertically upward is negative.
Reason: The gravitational force acts opposite to the direction of motion during upward journey.
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Q8.Assertion: A pump requires more power to lift the same amount of water to a greater height.
Reason: Power is directly proportional to the height through which water is lifted.
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Section F Fill in the Blanks
Q1.Power is defined as the ______ of doing work.
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Q2.The SI unit of power is ______, named after the scientist ______.
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Q3.1 horsepower = ______ watts.
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Q4.The dimensional formula of power is ______.
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Q5.When force and velocity are perpendicular to each other, the power is ______.
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Q6.The commercial unit of power is ______.
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Q7.For a body moving with constant velocity on a frictionless surface, the power is ______.
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Q8.Instantaneous power is given by the scalar product of ______ and ______.
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Q9.1 kilowatt = ______ watts.
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Q10.The power of a pump lifting mass m through height h in time t is given by P = ______.
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Section G Case Study Based Questions
Case Study 1 · Hydroelectric Power Plant
A hydroelectric power plant uses water falling from a height to generate electricity. Water from a reservoir at a height of 100 m flows down through penstocks to turn turbines. The plant has a capacity to process 5000 kg of water per second. (Take g = 10 m/s²)
Q1.What is the power generated by the falling water?
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Q2.If the efficiency of the turbine-generator system is 80%, what is the actual electrical power output?
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Q3.How much energy is produced in 1 hour at this actual power output?
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Case Study 2 · Electric Vehicle
An electric car of mass 1500 kg accelerates from rest to a speed of 30 m/s in 10 seconds. The motor delivers constant power during this time.
Q1.What is the acceleration of the car?
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Q2.What is the average power delivered by the motor during acceleration?
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Q3.If the car maintains a constant speed of 30 m/s on a level road with a frictional force of 500 N, what power is required to overcome friction?
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Case Study 3 · Human Power Output
A person of mass 70 kg climbs a staircase of 50 steps, each 20 cm high, in 20 seconds. (g = 9.8 m/s²)
Q1.What is the total height climbed?
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Q2.What is the work done against gravity?
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Q3.What is the average power output of the person?
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Section H Statement Based Questions
Passage 1
"Power is the rate at which work is done. It is a scalar quantity. The SI unit of power is watt. When a force acts on a body in the direction of its motion, the power is given by P = Fv. If the force makes an angle θ with the velocity, then P = Fv cosθ."
Q1.Why is power called a scalar quantity?
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Q2.A force of 20 N acts on a body moving with a velocity of 4 m/s. The force makes an angle of 60° with the direction of motion. Calculate the power.
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Passage 2
"A pump is used to lift water from a well. The power of the pump depends on the mass of water lifted, the height to which it is lifted, and the time taken. The efficiency of the pump is defined as the ratio of useful power output to the total power input."
Q1.Write the expression for the power of a pump lifting water.
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Q2.A pump of power 2 kW and efficiency 75% is used to lift water through 10 m. How much water can it lift in 1 minute? (g = 10 m/s²)
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Section I Match the Columns
| Column A | Column B |
|---|---|
| (a) Unit of power | (p) [M¹L²T⁻³] |
| (b) Dimensional formula of power | (q) Joule |
| (c) Unit of work | (r) Watt |
| (d) 1 hp | (s) 746 W |
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| Column A (Physical Quantity) | Column B (Expression) |
|---|---|
| (a) Average power | (p) F·v |
| (b) Instantaneous power | (q) W/t |
| (c) Power in lifting | (r) mgh/t |
| (d) Power when F ⊥ v | (s) Zero |
Show Answer
| Column A (Situation) | Column B (Power) |
|---|---|
| (a) Body moving with constant velocity on frictionless surface | (p) Positive |
| (b) Body thrown upward against gravity | (q) Negative |
| (c) Body falling freely under gravity | (r) Zero |
| (d) Body in uniform circular motion | (s) Variable |
Show Answer
| Column A | Column B |
|---|---|
| (a) James Watt | (p) Unit of energy |
| (b) Joule | (q) Commercial unit of power |
| (c) Horsepower | (r) SI unit of power |
| (d) Kilowatt-hour | (s) Improved steam engine |
Show Answer
| Column A (Value) | Column B (Equivalent) |
|---|---|
| (a) 1 kW | (p) 3.6 × 10⁶ J |
| (b) 1 kWh | (q) 1000 W |
| (c) 1 hp | (r) 746 W |
| (d) 1 W | (s) 1 J/s |
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Section J True or False
Q1.Power and work have the same dimensions.
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Q2.1 kilowatt-hour is a unit of power.
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Q3.The power of a body can be negative.
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Q4.A body in uniform circular motion has zero power due to centripetal force.
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Q5.The SI unit of power is named after James Prescott Joule.
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Section K Numerical Problems (With Detailed Solutions)
Q1.A pump lifts 1200 kg of water per minute to a height of 25 m. Calculate the power of the pump in kW. (g = 10 m/s²)
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Q2.A car of mass 1000 kg moves on a level road at a constant speed of 20 m/s against a resistance of 400 N. Find the power of the engine in horsepower.
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Q3.The power of an engine is 5 kW. How much time will it take to lift a load of 500 kg to a height of 40 m? (g = 10 m/s²)
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Q4.A force F = (3i + 4j) N acts on a body moving with velocity v = (4i − 3j) m/s. Calculate the power.
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Q5.A motor pump is rated at 3 hp. Calculate the maximum mass of water it can lift in 2 minutes through a height of 15 m. (1 hp = 746 W, g = 9.8 m/s²)
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Answer Key (Quick Reference — MCQs & Assertion-Reason)
| Section | Q. No. | Answer |
|---|---|---|
| MCQ (Easy) | 1 | (b) |
| 2 | (c) | |
| 3 | (b) | |
| 4 | (b) | |
| 5 | (b) | |
| 6 | (b) | |
| 7 | (c) | |
| 8 | (c) | |
| MCQ (Medium) | 9 | (b) |
| 10 | (b) | |
| 11 | (b) | |
| 12 | (c) | |
| 13 | (d) | |
| 14 | (a) | |
| 15 | (c) | |
| MCQ (Hard) | 16 | (c) |
| 17 | (d) | |
| 18 | (b) | |
| 19 | (b) | |
| 20 | (a) | |
| Assertion-Reason | 1 | (a) |
| 2 | (a) | |
| 3 | (a) | |
| 4 | (c) | |
| 5 | (c) | |
| 6 | (d) | |
| 7 | (a) | |
| 8 | (a) |
Exam Tips for CBSE Class 11
- Formula Sheet: Memorize P = W/t, P = Fv, P = Fv cosθ, and P = mgh/t
- Unit Conversions: Always convert to SI units first (especially hp → W, min → s)
- Sign Convention: Power is positive when force aids motion, negative when it opposes
- Scalar Nature: Remember power is scalar — never add vectorially
- Time Management: Most Power questions take 1–2 minutes max
NEET Physics Notes
Chapter: POWER
Power is the rate at which work is done or energy is transferred.
Simple Meaning
Work tells us how much work is done, while Power tells us how fast the work is done.
Example
- Student A climbs 4 floors in 20 seconds.
- Student B climbs 4 floors in 40 seconds.
Both students do the same work, but Student A has more power because he completes the work in less time.
Average Power
Where
- P = Average Power (Watt)
- W = Work Done (Joule)
- t = Time Taken (Second)
Instantaneous Power
The power at a particular instant of time is called Instantaneous Power.
Power in Terms of Force
We know
Therefore
Since
Hence
General Formula
Special Cases
Case 1: Force and Velocity in Same Direction
P = Fv
Power is Maximum.
Case 2: Force Opposite to Velocity
P = -Fv
Negative power means energy is removed from the body.
Example: Braking a moving car.
Case 3: Force Perpendicular to Velocity
P = 0
Example: Uniform Circular Motion
The centripetal force changes only the direction of velocity, not its speed.
Nature of Power
Reason: It is obtained from the dot product of force and velocity.
Dimensions of Power
SI Unit
Named after James Watt, who improved the steam engine.
Definition of One Watt
A power of one watt means one joule of work is done every second.
Other Units
Horsepower is commonly used for engines, cars and motorcycles.
Electrical Energy
Electrical appliances are rated in watts.
- 60 W Bulb
- 100 W Bulb
- 1000 W Heater
Higher wattage means the appliance consumes energy faster.
Kilowatt-hour (kWh)
Electricity bills are measured in kilowatt-hour (kWh).
Conversion
= 3.6 × 10⁶ J
Example
A 100 W bulb runs for 10 hours.
= 1 kWh
Important Fact
Remember:
kWh is a unit of Energy, NOT Power.
Difference Between Power and Energy
| Power | Energy |
|---|---|
| Rate of doing work | Capacity to do work |
| P = W / t | W = Pt |
| Unit = Watt (W) | Unit = Joule (J) |
| Scalar Quantity | Scalar Quantity |
| Measures speed of work | Measures amount of work |
Graph Concept
NEET Formula Sheet
| Formula | Expression |
|---|---|
| Average Power | P = W / t |
| Instantaneous Power | P = dW / dt |
| Power by Force | P = F · v |
| General Formula | P = Fv cosθ |
| Parallel Force | P = Fv |
| Perpendicular Force | P = 0 |
| Opposite Force | P = -Fv |
| 1 Watt | 1 J/s |
| Horsepower | 1 hp = 746 W |
| 1 kWh | 3.6 × 10⁶ J |
NEET Quick Revision
- Power = Rate of doing work.
- Power = Speed of energy transfer.
- Power is a scalar quantity.
- P = Fv cosθ.
- Maximum power when θ = 0°.
- Zero power when θ = 90°.
- Negative power when θ = 180°.
- 1 Watt = 1 Joule/second.
- 1 Horsepower = 746 W.
- 1 Unit of electricity = 1 kWh = 3.6 × 10⁶ J.
- kWh is a unit of Energy, not Power.
Frequently Asked NEET Questions
Q1. What is Power?
Power is the rate at which work is done or energy is transferred.
Q2. Why is Power a Scalar Quantity?
Because Power is obtained from the dot product of Force and Velocity.
Q3. What is SI Unit of Power?
Watt (W).
Q4. Is kWh a unit of Power?
No. It is a unit of Energy.
Q5. What is the power when Force is perpendicular to Velocity?
P = 0
