Work, Energy and Power — Previous Year Questions (Class 11 Physics)
Work, Energy and Power are fundamental concepts linking force and motion. Master energy conservation and power calculations essential for board exams.
TL;DR: Work, Energy and Power are fundamental concepts linking force and motion. Master energy conservation and power calculations essential for board exams.
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Work, Energy and Power are fundamental concepts linking force and motion. Master energy conservation and power calculations essential for board exams.
Work, Energy and Power — Previous Year Questions with Solutions
Q (2023, 2 marks): A force of 10 N acts on a body at an angle of 30° to the horizontal. If the body is displaced 5 m horizontally, calculate the work done by the force.
Answer: Given: F = 10 N, θ = 30°, s = 5 m
Work done W = F · s · cos(θ)
W = 10 × 5 × cos(30°)
W = 50 × (√3/2)
W = 25√3 J ≈ 43.3 J
Final Answer: W = 25√3 J or 43.3 J
Q (2022, 3 marks): A 2 kg object is lifted vertically from the ground to a height of 10 m. Calculate (i) work done against gravity, (ii) gravitational potential energy gained. (g = 10 m/s²)
Answer: (i) Work done against gravity:
W = mgh = 2 × 10 × 10 = 200 J
(ii) Gravitational potential energy gained:
PE = mgh = 2 × 10 × 10 = 200 J
Note: Work done against gravity equals the potential energy gained.
Final Answer: Work = 200 J, PE = 200 J
Q (2023, 3 marks): An object of mass 5 kg is moving with a velocity of 10 m/s. Calculate its kinetic energy. If the velocity is doubled, how does the kinetic energy change?
Answer: Initial KE: KE₁ = (1/2)mv² = (1/2) × 5 × 10² = 250 J
When velocity is doubled (v = 20 m/s):
KE₂ = (1/2) × 5 × 20² = (1/2) × 5 × 400 = 1000 J
Ratio: KE₂/KE₁ = 1000/250 = 4
Conclusion: Kinetic energy increases by a factor of 4 (becomes 4 times).
Final Answer: Initial KE = 250 J, Final KE = 1000 J, increases 4 times
Q (2021, 2 marks): A machine does 500 J of work in 10 seconds. Calculate the power delivered by the machine.
Answer: Given: Work done W = 500 J, Time t = 10 s
Power P = W/t = 500/10 = 50 W
Final Answer: P = 50 W
Q (2022, 3 marks): A body of mass 4 kg falls freely from a height of 20 m. Using energy conservation, find its velocity just before hitting the ground. (g = 10 m/s²)
Answer: Using conservation of energy:
Initial energy = Final energy
PE₀ + KE₀ = PEf + KEf
mgh + 0 = 0 + (1/2)mv²
gh = (1/2)v²
v² = 2gh = 2 × 10 × 20 = 400
v = √400 = 20 m/s
Final Answer: v = 20 m/s
Q (2023, 5 marks): A pump lifts 100 kg of water per minute to a height of 5 m. Calculate the power required. (g = 10 m/s²)
Answer: Mass lifted per minute = 100 kg
Height h = 5 m
g = 10 m/s²
Time t = 1 minute = 60 s
Work done W = mgh = 100 × 10 × 5 = 5000 J
Power P = W/t = 5000/60 = 83.33 W ≈ 83.3 W
Alternatively, using instantaneous power:
P = (dm/dt) × g × h = (100/60) × 10 × 5 = 83.33 W
Final Answer: P ≈ 83.3 W or 250/3 W
Frequently Asked Questions
What is the difference between work and energy?
Work is the process of transferring energy by applying force over a distance. Energy is the capacity to do work. Work is done when force causes displacement; energy is the state property of a system.
Why is power important in practical applications?
Power indicates the rate at which work is done. Two machines may do the same work, but the one delivering higher power does it faster, which is crucial for efficiency in industrial and household applications.
More Class 11 Physics PYQs
- Current Electricity
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Electromagnetic Induction
- Alternating Current
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