Work, Energy and Power: Class 11 Physics Q&A
18 free answers on Work, Energy and Power (Physics) for Class 11 students, across CBSE, ICSE and State boards. Includes questions such as “Why the work done by centripetal force is zero?”, “Which SI unit is N·m?”, “What is work on physics?”
Question & Answer
Questions
- PhysicsClass 11All
What is meant by positive work, negative work and zero work? Give one example of each work?
Positive work: If the force and displacement are in the same direction. Example: When a body of mass m falls vertically downwards through a height h under gravity. Work done by gravity is positive. Zero work: Force and displacement are at right angle to each other. Example: When a person holding a suitcase on his head moves on a horizontal road. Work done by gravity is zero. <
- PhysicsClass 11All
Why the work done by centripetal force is zero?
Because centripetal force is perpendicular to the displacement of the body.
- PhysicsClass 11All
Which SI unit is N·m?
N·m (newton-meter) is the SI unit of torque or work/energy, not momentum. For torque, it represents rotational force. For work or energy, newton-meters are equivalent to joules (J). This is different from momentum, which is measured in kg·m/s.
- PhysicsClass 11All
What is work on physics?
In physics, work is the energy transfer that occurs when a force causes displacement. It's defined mathematically as W = F × d × cos(θ) and measured in joules. Work can be: Positive: Force aids motion (lifting, pushing forward) Negative: Force opposes motion (friction, air resistance) Zero: No displacement or force perpendicular to motion Wo
- PhysicsClass 11All
How to calculate work and power
? Work calculation: W = F × d × cos(θ) Power calculation: P = W/t (work per unit time) Or P = F × v (force times velocity) Example: A 500N force moves an object 10m in 5 seconds: Work: W = 500 × 10 = 5,000 J Power: P = 5,000/5 = 1,000 W = 1 kW
- PhysicsClass 11All
What is W equal to?
W can represent different quantities depending on context: Work: W = F × d × cos(θ) (joules) Watt: Unit of power (1 W = 1 J/s) Weight: W = mg (in some contexts) Always check the context to determine whether W represents work, watts, or weight.
- PhysicsClass 11All
Is power P or W?
Power is denoted by P, measured in watts (W): Symbol: P (italicized in equations) Unit: W (watt, upright in text) Formula: P = W/t (work W divided by time) Don't confuse: P (symbol for power) with W (unit watt) and W (symbol for work).
- PhysicsClass 11All
How to calculate total work?
To calculate total work when multiple forces act: Method 1: Find net force (ΣF), then W_total = F_net × d × cos(θ) Method 2: Calculate work by each force separately, then sum: W_total = W₁ + W₂ + W₃ + ... Method 3: Use Work-Energy Theorem: W_total = ΔKE Example: If gravity does -50J and applied force does +80J, total work = 30J.
- PhysicsClass 11All
What are the three ways to calculate work?
Three primary methods to calculate work: Direct formula: W = F × d × cos(θ) - for constant force Work-Energy Theorem: W = ΔKE = ½m(v₂² - v₁²) Integration: W = ∫F·ds - for variable force Each method suits different scenarios: constant vs. variable force, known velocities vs. known forces.
- PhysicsClass 11All
What is the formula for work?
Work is calculated using the formula W = F × d × cos(θ), where W represents work, F is the applied force, d is the displacement, and θ is the angle between the force and displacement vectors. When force and displacement are in the same direction, this simplifies to W = F × d. Work measures the energy transferred when a force moves an object through a distance.
- PhysicsClass 11All
How to calculate power when mass, velocity and time are given?
Power is the rate of doing work. With mass and velocity, first find kinetic energy, then divide by time: P = KE/t = (½mv²)/t where m is mass, v is velocity, and t is time. Example: A 100 kg object reaches 20 m/s in 10 seconds. Power = (½ × 100 × 20²) / 10 = 20,000 / 10 = 2,000 watts.
- PhysicsClass 11All
. How is power calculated in combined mechanical and electrical systems?
Electromechanical systems (motors, generators) involve energy conversion between electrical and mechanical forms: For Electric Motors: Electrical Input Power: P_in = V × I × cos(φ) cos(φ) = power factor (for AC systems) Mechanical Output Power: P_out = τ × ω Efficiency: η = (P_out / P_in) × 100% <stron
- PhysicsClass 11All
How do I calculate mechanical power in dynamic systems?
Dynamic systems involve changing motion, requiring consideration of acceleration and forces: For rotational systems:P = τ × ω τ = torque (N·m) ω = angular velocity (rad/s) For linear motion with changing velocity:P = F × v F = force applied (N) v = instantaneous velocity (m/s) <stro
- PhysicsClass 11All
Should I use mechanical or electrical power formulas?
The choice depends on the type of system you're analyzing: Use Mechanical Power Formulas when: Analyzing engines, motors, or turbines (output) Calculating work done by forces Studying motion, velocity, and acceleration Working with gravitational or kinetic energy Common formulas: P = F × v, P = τ × ω (torque × angular velocity) Use Electrical Pow
- PhysicsClass 11All
What's the difference between calculating power with different given variables?
The approach changes based on what information you have: Given Variables Formula Best For Mass, height, time P = (m × g × h) / t Lifting objects vertically <t
- PhysicsClass 11All
When can't I use P = I²R instead of P = V²/R?
You can technically use either formula in any DC circuit, but practical considerations matter: Avoid P = I²R when: Current is difficult to measure accurately The circuit has multiple parallel branches You're working with high-impedance circuits Current fluctuates significantly Avoid P = V²/R when: Voltage varies unpredictably You're working with series circ
- PhysicsClass 11All
When should I use P = V²/R versus P = I²R?
Both formulas are correct but optimal in different situations: Use P = V²/R when: You know voltage and resistance Voltage is constant (like a fixed power supply) You're analyzing parallel circuits Resistance changes and you need to see power impact Use P = I²R when: You know current and resistance Current is constant (like series circuits)<
- PhysicsClass 11All
Is 1 joule eual to 1 erg?
No, 1 joule is not eual to 1 erg. Instead, 1 joule euals 10⁷ ergs (10 million ergs). The joule is the SI unit of energy, while the erg is the CGS unit. One erg is defined as 1 dyne·cm or 1 g·cm²/s², which euals 10⁻⁷ joules. The erg is a much smaller unit of energy than the joule.