When is work done said to be zero?

Work is zero when: (1) Force is perpendicular to displacement (θ=90°, cos90°=0), e.g., gravity does no work when carrying a bag horizontally; (2) No displacement occurs, e.g., pushing against a wall; (3) No force is applied. Formula: W = F×d×cosθ — if θ=90° or d=0, W=0.

What is the difference between kinetic and potential energy?

Kinetic energy: energy due to motion; KE = ½mv². Potential energy: energy due to position; PE = mgh (gravitational). A stone at height has max PE, zero KE. When falling, PE converts to KE. At ground, KE = max, PE = 0. Total mechanical energy (KE + PE) remains constant (conservation of energy).

What is a kilowatt-hour and how is it used?

A kilowatt-hour (kWh) is the commercial unit of electrical energy. 1 kWh = energy used by a 1 kW (1000 W) device in 1 hour = 3.6 × 10⁶ J = 3600 kJ. Electricity bills measure consumption in units (kWh). A 100W bulb running for 10 hours consumes 1 kWh = 1 unit.

Work, Power and Energy

Work and Energy • Class 9 Science • NCERT • CBSE

Work is done when a force causes displacement in its direction: W = F × d × cosθ. Kinetic energy = ½mv². Potential energy = mgh. The law of conservation of energy states that energy can neither be created nor destroyed, only converted from one form to another.

Key Formulas

W = F × d × cosθ (Work formula)
KE = ½mv² (Kinetic Energy)
PE = mgh (Gravitational Potential Energy)
P = W/t (Power)
1 kWh = 3.6 × 10⁶ J
Work-Energy theorem: W = ΔKE = ½mv² − ½mu²

Frequently Asked Questions

When is work done said to be zero?: Work is zero when: (1) Force is perpendicular to displacement (θ=90°, cos90°=0), e.g., gravity does no work when carrying a bag horizontally; (2) No displacement occurs, e.g., pushing against a wall; (3) No force is applied. Formula: W = F×d×cosθ — if θ=90° or d=0, W=0.
What is the difference between kinetic and potential energy?: Kinetic energy: energy due to motion; KE = ½mv². Potential energy: energy due to position; PE = mgh (gravitational). A stone at height has max PE, zero KE. When falling, PE converts to KE. At ground, KE = max, PE = 0. Total mechanical energy (KE + PE) remains constant (conservation of energy).
What is a kilowatt-hour and how is it used?: A kilowatt-hour (kWh) is the commercial unit of electrical energy. 1 kWh = energy used by a 1 kW (1000 W) device in 1 hour = 3.6 × 10⁶ J = 3600 kJ. Electricity bills measure consumption in units (kWh). A 100W bulb running for 10 hours consumes 1 kWh = 1 unit.

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