Class 9 · Advanced ScienceNCERT 2026 Revised Syllabus
Chapter 5: Work and Energy
Science Advanced
Conservative and non-conservative forces, conversion between potential and kinetic energy, PE = mgh, and energy stored in a stretched wire.
Find complete NCERT solutions for Class 9 Advanced Science, Science Advanced Chapter 5. Every question has a clear, step-by-step answer.
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Reflect (Chapter Opening Questions)(1 questions)
1
Why does a thrown ball come back down? Why does a stretched rubber band regain its shape? Why does a sliding book eventually stop?
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Ans.
Solution:- Gravity, a conservative force, pulls the ball back down.
- Spring force restores the rubber band's shape.
- Friction, a non-conservative force, converts the book's kinetic energy into heat, stopping it.Answer: Gravity pulls the ball down, spring force restores the rubber band, and friction stops the sliding book.
Quick Check(5 questions)
1
Define a conservative force with one example.
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Ans.
Solution:- A conservative force is one whose work done is independent of the path taken.
- Work is zero over a closed path.
- Example: gravitational force.Answer: A conservative force does work independent of the path (zero over a closed path); e.g. gravity.
2
Why is gravitational force called a conservative force?
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Ans.
Solution:- Work done by gravity depends only on the initial and final positions, not the path taken.
- It equals zero over any closed path.Answer: Work by gravity depends only on positions and is zero over a closed path.
3
Why is friction called a non-conservative force?
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Ans.
Solution:- Work done against friction depends on the actual path length travelled.
- It converts kinetic energy into heat, which cannot be fully recovered.Answer: Friction is non-conservative because its work depends on the path and converts kinetic energy into unrecoverable heat.
4
What happens to energy when a non-conservative force acts on an object?
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Ans.
Solution:- Mechanical energy is not conserved.
- Some energy is dissipated as heat, sound, or deformation.
- This reduces the object's total kinetic energy over time.Answer: Mechanical energy is not conserved; some energy is dissipated as heat, sound, or deformation.
5
If there were no friction on Earth, how would motion be different? Explain.
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Ans.
Solution:- Objects once set in motion would continue moving forever at constant velocity.
- Ordinary actions like walking, stopping, or driving would become impossible without friction.Answer: Objects would keep moving at constant velocity forever, and walking, stopping or driving would become impossible.
Check Your Understanding(6 questions)
1
Explain the conversion of potential energy to kinetic energy when a ball is thrown upward.
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Ans.
Solution:- As the ball rises, kinetic energy converts into potential energy.
- At the highest point, all kinetic energy has become potential energy.
- The process reverses as the ball falls.Answer: KE converts into PE as the ball rises; the process reverses as it falls.
2
Why is gravitational potential energy considered to arise from a conservative force?
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Ans.
Solution:- Gravitational force is conservative.
- Its work depends only on the change in height.
- This allows potential energy to be uniquely defined at each position.Answer: Gravity is conservative, so PE is uniquely defined at each position.
3
Hard
Calculate the potential energy of a 5 kg object kept on top of a 30 m high building (PE = 0 at the base).
Board Level Answers
Ans.
Solution:
$PE = mgh = 5 \times 9.8 \times 30 = 1470 \text{ J}$Answer: Potential energy = 1470 J.4
Hard
What is the increment in its potential energy?
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Ans.
Solution:- Increase in PE from the base to the top
$= mgh = 5 \times 9.8 \times 30 = 1470 \text{ J}$.Answer: The increment in potential energy = 1470 J.5
Hard
A 10 kg weight is hung from a 5 m wire, causing it to stretch by 1 mm. Calculate the energy stored.
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Ans.
Solution:
$F = mg = 10 \times 9.8 = 98 \text{ N}$, extension $x = 0.001 \text{ m}$.Energy stored $= \frac{1}{2} F x = 0.5 \times 98 \times 0.001 = 0.049 \text{ J}$Answer: Energy stored = 0.049 J.6
Hard
Calculate the work done by an external force to lift a 2 m long rod from a horizontal to a vertical position.
Board Level Answers
Ans.
Solution:- The centre of mass rises by half the rod's length, i.e.
$1 \text{ m}$.
- Work done $= mg \times 1 = 9.8 \times m$ joules ($m$ = rod's mass).Answer: Work done = $9.8m$ joules, where m is the rod's mass.Activities — How to Do(4 questions)
1
Activity 5.2: hang masses on a spring, record extension each time, and plot Force (Y-axis, N) against Extension (X-axis, m/cm). What should the graph look like, and what does it show?
Board Level Answers
Ans.
Tabulate load vs stretch; plot Force (Y) vs Extension (X) with labelled axes and units. Points form a straight line through the origin (within the elastic limit): extension is proportional to force (Hooke’s law). The slope gives the spring’s stiffness; bending marks the elastic limit.
2
Reflect: if there were no friction, would a moving object ever stop?
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Ans.
No. Without friction (or any opposing force), net force is zero, so by Newton’s First Law the object keeps its constant velocity forever. Real objects stop only because friction and air drag continuously oppose motion.
3
Why do pendulums slowly stop after some time?
Board Level Answers
Ans.
Each swing loses a little energy to air resistance on the bob and friction at the pivot, converted to heat. With less total mechanical energy, the amplitude shrinks until the pendulum stops — energy is conserved overall, just no longer in the swing.
4
Why do machines require lubrication?
Board Level Answers
Ans.
Moving parts rub and waste energy as heat while wearing out. A lubricant film separates surfaces, lowering friction — so machines run cooler, parts last longer, fuel/electricity use drops, and seizure is prevented.