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Work, Power & Energy

General Science · Physics

Useful for: Railway RRB, CTET, OTET, OSSTET, OSSC RHT — Regular High School Teacher, CSIR NET, CTSA / STSS, Junior Teacher

and 12 more exams

OAVS, OSEPA Upper Primary — Science, OSSC Science, OSSC Teacher Readiness, OSSSC Govt TGT Science — CBZ, OSSSC Govt TGT Science — PCM, Sainik Schools, Special OTET, SSB Lecturer (Degree Colleges), SSB School Teachers, SSB TGT CBZ, SSB TGT PCM

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Overview

Work is done when a force moves a body; energy is the capacity to do work; power is the rate of doing work.

Key points

Work W = F × s × cos θ; unit joule.
Kinetic energy = ½mv²; potential energy = mgh.
Power = work ÷ time; unit watt; 1 horsepower = 746 W.
Energy is conserved; it only changes form.
No work is done if the displacement is perpendicular to the force.

Explanation

Doubling speed makes kinetic energy four times.
Efficiency = useful output ÷ input × 100%.
A falling body converts potential energy into kinetic energy.
1 kWh is the commercial unit of energy.

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

A 10 N force moves a box 5 m in its direction: work = 50 J.
A 2 kg mass at 10 m height (g = 10): PE = 200 J.
A 2 kg body at 3 m/s: KE = ½ × 2 × 9 = 9 J.

Traps and advanced points

Trap: Thinking work is done when holding a load still.
Trap: KE doubles when speed doubles (it quadruples).
Trap: Mixing power and energy units.

Quick revision

W = Fs cos θ.
KE = ½mv², PE = mgh.
1 hp = 746 W.

Practice MCQs

Which lens is used to correct simple myopia?
1. Convex lens
2. Bifocal lens always
3. Concave lens
4. Plane glass only
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Answer: (c) Concave lens. Concave lens is used to correct simple myopia.
Which option correctly answers this General Science check: chemical formula of water?
1. beta cells of pancreas
2. rate of reaction
3. H₂O
4. sodium
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Answer: (c) H₂O. One molecule has two H atoms and one O atom.
Which option correctly answers this General Science check: lens used to correct hypermetropia?
1. convex lens
2. ½mv²
3. 9.8 m/s²
4. mitochondria
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Answer: (a) convex lens. A converging lens helps focus near-object rays on the retina.
Which option correctly answers this General Science check: symbol Na represents?
1. rate of reaction
2. vitamin K
3. sodium
4. convex lens
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Answer: (c) sodium. Na comes from Latin natrium.
AIDS is caused by which virus?
1. Influenza virus
2. HIV
3. HBV
4. Dengue bacterium
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Answer: (b) HIV. AIDS is caused by HIV.
ALP Basic Science & Engineering: Complete the statement: “Work is measured in ____.”
1. small lengths/diameters with finer precision
2. engineering drawing
3. Personal Protective Equipment
4. joule
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Answer: (d) joule. 1 joule = 1 newton-metre.
ALP Basic Science & Engineering: Complete the statement: “Power is measured in ____.”
1. small lengths/diameters with finer precision
2. greenhouse gases
3. direction
4. watt
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Answer: (d) watt. Power is rate of doing work.
ALP Basic Science & Engineering: Which statement opening is correctly completed by “joule”?
1. first-angle and third-angle projection are methods used in
2. a fuse is primarily used to
3. work is measured in
4. electrical power in a DC circuit can be written as
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Answer: (c) work is measured in. 1 joule = 1 newton-metre.
ALP Basic Science & Engineering: Which statement opening is correctly completed by “watt”?
1. SI base unit of mass
2. power is measured in
3. an isometric drawing is mainly used to
4. a strong password is improved by
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Answer: (b) power is measured in. Power is rate of doing work.
Which part of the human brain controls balance and posture?
1. Cerebellum
2. Cerebrum
3. Medulla oblongata
4. Hypothalamus
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Answer: (a) Cerebellum. The cerebellum coordinates voluntary movements, balance and posture.
Which gas is mainly responsible for ozone layer depletion?
1. Nitrogen
2. Carbon dioxide
3. Methane
4. Chlorofluorocarbons (CFCs)
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Answer: (d) Chlorofluorocarbons (CFCs). Chlorine released from CFCs breaks down ozone in the stratosphere; the Montreal Protocol phases them out.
Which is a renewable source of energy?
1. coal
2. petrol
3. solar energy
4. natural gas
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Answer: (c) solar energy. Sunlight does not run out; fossil fuels do.
The main source of energy for the Earth is the —
1. Moon
2. Sun
3. wind
4. coal
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Answer: (b) Sun. Almost all energy on Earth comes from the Sun.
A stretched rubber band has — energy.
1. kinetic
2. potential
3. sound
4. nuclear
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Answer: (b) potential. Energy stored due to change of shape is potential energy.