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Work, Energy, and Simple Machines – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 7. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Work is done when a force moves a body. Energy is the capacity to do work, it changes form but is conserved, and simple machines make work easier.

Work and energy

Work W = F × s (in the direction of force); unit joule. No displacement → no work.
Kinetic energy KE = ½mv²; potential energy PE = mgh.
Law of conservation of energy: energy can neither be created nor destroyed; a falling body turns PE into KE.

Power

Power P = W/t; unit watt (W). 1 horsepower = 746 W.
1 kWh = 3.6 × 10⁶ J.

Simple machines

Lever: effort × effort arm = load × load arm. Classes by position of fulcrum, load and effort.
Pulley, inclined plane, wheel and axle, wedge, screw.
Mechanical advantage MA = load/effort; efficiency = output work / input work × 100 (always < 100% due to friction).

Key terms

Joule: work done when 1 N moves a body 1 m
Mechanical advantage: ratio of load to effort
Fulcrum: the fixed point about which a lever turns

Formulas

W = F s cos θ
KE = ½mv², PE = mgh
P = W/t
Lever: E × effort arm = L × load arm

Important questions with answers

Is work done when a person holds a heavy bag without moving?2 marks

No. Work = force × displacement in the direction of force; the displacement is zero, so no work is done on the bag (though the person gets tired).

A 10 kg box is lifted to a shelf 2 m high. Find the work done and the potential energy gained (g = 10 m/s²). If it falls, what is its kinetic energy just before hitting the ground?3 marks

Work = mgh = 10 × 10 × 2 = 200 J, equal to the PE gained. By conservation of energy, KE just before hitting the ground = 200 J (ignoring air resistance).

Explain mechanical advantage and efficiency of a machine. A lever lifts a 600 N load with an effort of 150 N. Find its MA. Name the three classes of levers with one example each.5 marks

Mechanical advantage = load/effort; efficiency = (work output/work input) × 100%, always less than 100% due to friction. MA = 600/150 = 4. Class I: fulcrum between effort and load (seesaw, scissors). Class II: load between fulcrum and effort (wheelbarrow, nutcracker). Class III: effort between fulcrum and load (tongs, human forearm).

Quick revision

Energy is conserved.
Power unit watt = J/s.
Efficiency of a real machine is less than 100%.

Exam tips

A porter carrying a load on a level road does no work against gravity (force ⟂ displacement).
Doubling speed makes KE four times.

Practice MCQs

SI unit of work is
1. newton
2. watt
3. joule
4. pascal
Show answer
Answer: (c) joule. Joule.
Energy possessed by a body due to its motion is
1. kinetic
2. potential
3. chemical
4. nuclear
Show answer
Answer: (a) kinetic. Kinetic.
A pair of scissors is a
1. lever
2. pulley
3. inclined plane
4. screw
Show answer
Answer: (a) lever. First-class lever.
KE of a 2 kg ball moving at 3 m/s is
1. 6 J
2. 18 J
3. 9 J
4. 3 J
Show answer
Answer: (c) 9 J. ½ × 2 × 9 = 9 J.
PE of a 5 kg box 4 m high (g = 10 m/s²) is
1. 20 J
2. 50 J
3. 200 J
4. 400 J
Show answer
Answer: (c) 200 J. 5 × 10 × 4.
A machine lifts a 600 N load with 200 N effort. MA is
1. 0.33
2. 3
3. 400
4. 800
Show answer
Answer: (b) 3. 600/200.
If the speed of a body doubles, its kinetic energy becomes
1. double
2. four times
3. half
4. same
Show answer
Answer: (b) four times. KE ∝ v².
A 60 kg boy climbs 10 m of stairs in 20 s (g = 10 m/s²). His power is
1. 3000 W
2. 600 W
3. 120 W
4. 300 W
Show answer
Answer: (d) 300 W. 6000 J / 20 s.

All chapters of Class 9 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.