Work, Energy, and Simple Machines — Class 9 MCQs with Answers

Work, Energy, and Simple Machines — important questions & MCQs with answers (Class 9 Science)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   SI unit of work is:

   ANewtonBPascalCWattDJoule
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   Answer: Joule

   Explanation: Work = force × displacement, so combining newtons and metres gives one SI unit: the joule (1 J = 1 N·m) — not the newton, pascal, or watt, which measure other quantities.

2. Q2 of 6Easy

   Kinetic energy of an object of mass m and speed v is:

   AmvB(1/2)mv²CmghDm/v²
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   Answer: (1/2)mv²

   Explanation: Kinetic energy is defined as KE = ½mv² — half the mass times speed squared, not mv or mgh, which describe momentum-like or potential energy instead.

3. Q3 of 6Medium

   If 600 J of work is done in 2 minutes, power =

   A300 WB10 WC60 WD5 W
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   Answer: 5 W

   Explanation: Power = W/t, and 2 minutes = 120 s, so P = 600 J / 120 s = 5 W — skipping the minutes-to-seconds conversion would wrongly give 300 W.

4. Q4 of 6Medium

   Assertion (A): A boy who pushes hard against a rigid wall for ten minutes does a large amount of work on the wall. Reason (R): If an object is not displaced, the work done on it is zero, even though a force is applied to it.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: the wall does not move, so the displacement is zero and the boy does no work on it, even though he feels tired. R is true: work done is zero when there is no displacement, whatever the force.

5. Q5 of 6Hard

   When a body falls freely, its:

   ATotal mechanical energy decreasesBBoth KE and PE increaseCKE alone increases without limitDTotal mechanical energy is conserved
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   Answer: Total mechanical energy is conserved

   Explanation: In free fall with no air resistance, PE steadily converts to KE, but their sum — total mechanical energy — stays constant throughout the fall.

6. Q6 of 6Hard

   Assertion (A): A weightlifter who lifts a 50 kg mass through 2 m in 4 s needs a power of 250 W (take g = 10 m s⁻²). Reason (R): One horsepower is equal to 746 W.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: Both statements are true. Work = mgh = 50 × 10 × 2 = 1000 J and power = 1000 J ÷ 4 s = 250 W; the value of one horsepower is a separate fact, so R does not explain A.

Work, Energy, and Simple Machines notes: key concepts (Class 9 Science)

Work is done only when a force produces displacement, and energy is the capacity to do work. This chapter defines work precisely, develops kinetic and potential energy, states the law of conservation of energy, and introduces power and the commercial unit of energy.

Work in science

Work is done only when a force acts on an object AND the object is displaced along the direction of the force.

Zero work

No work is done when there is no displacement, or when the force is perpendicular to the displacement, as with a coolie carrying a load horizontally.

Positive and negative work

Work is positive when force and displacement point the same way and negative when they oppose, as friction does.

Energy

The capacity to do work, measured in the same unit as work, the joule.

Kinetic energy

The energy an object possesses because of its motion, proportional to the square of its speed.

Potential energy

The energy an object possesses because of its position or configuration, such as a raised object or a stretched spring.

Gravitational potential energy

The work done in raising an object against gravity, equal to mgh and dependent only on the height gained, not the path taken.

Law of conservation of energy

Energy can neither be created nor destroyed, only transformed from one form to another; the total remains constant.

Energy transformation in a falling body

As an object falls its potential energy converts steadily into kinetic energy, while their sum stays constant.

Forms of energy

Mechanical, heat, light, sound, chemical, electrical and nuclear energy, each convertible into the others.

Power

The rate of doing work or transferring energy, measured in watts; one watt is one joule per second.

Average power

Total work done divided by the total time taken, used when the rate of working varies.

Commercial unit of energy

The kilowatt-hour, the unit in which electrical energy is billed, equal to 3.6 × 10⁶ joules.