Work and Energy Class 9 Question Answer Advanced Science Chapter 5

Chapter-wise Class 9 Advanced Science Solutions and NCERT Class 9 Advanced Science Chapter 5 Work and Energy Question Answer are useful for focused study.

Work and Energy Class 9 Questions and Answers

Work and Energy Question Answer Class 9

Quick Check

Question 1.
Define a conservative force with one example.
Answer:
A conservative force is one where the work done moving an object between two points depends only on the start and end positions, not the path taken. The total work over a closed loop is zero.

Example: Gravitational force.

Question 2.
Why is gravitational force called a conservative force?
Answer:
Gravitational force is considered conservative because the work done in moving an object depends solely on its vertical displacement (initial and final height) and is completely independent of the path taken.

Whether you lift an object vertically or slide it up a long, winding ramp to the same height, the work done against gravity remains W = mgh. Furthermore, in a closed loop (returning an object to its starting point), the total work done by gravity is zero, meaning the energy is fully recovered.

Question 3.
Why is friction called a non-conservative force?
Answer:
Friction is non-conservative because work done depends on the path length, not just displacement. Energy is dissipated as heat rather than stored and the total work over a closed loop is never zero.

Question 4.
What happens to energy when a non-conservative force acts on an object?
Answer:
Mechanical energy is dissipated and transformed into non-recoverable forms like heat, sound, or light. Unlike conservative forces, this energy cannot be stored as potential energy and is lost from the object’s total mechanical energy pool.

Question 5.
If there were no friction on Earth, how would motion be different? Explain.
Answer:
Without friction, motion would be uncontrollable. Objects in motion would never stop unless they hit an obstacle, as there would be no force to dissipate their kinetic energy.

Walking, driving, or even gripping objects would be impossible, as surfaces would offer zero traction to initiate or change direction.

Check Your Understanding

Question 1.
Explain the conversion of potential energy to kinetic energy when a ball is thrown upward.
Answer:
When a ball is projected vertically upward, it initially passes as maximum kinetic energy due to its velocity.
As the ball rises, it moves against the gravitational force acting downward.
Because of this,

• the velocity of the ball gradually decreases.
• its kinetic energy decreases.
• at the something its gravitational potential energy increases.

At the highest point

• velocity becomes zero.
• kinetic energy becomes zero.
• potential energy becomes maximum.
• Therefore, at the higher point, all the energy has been converted into potential energy.

During downward motion

• The stored potential energy is converted into kinetic energy.
• Velocity increases continuously.

This process demonstrates the law of conservation of mechanical energy which states that total mechanical energy (KE + PE) remains constant in the absence of non-conservative forces.

Question 2.
Why is the gravitational force conservative?
Answer:
A force is said to be conservative if the work done by it depends only on initial and final positions, not on the path followed

• Work done is path independent.
• Work done in closed loop is zero.
• Energy can be fully recovered.
  Hence, gravitational force is a perfect conservative force.

Question 3.
Calculate the potential energy of a 5 kg object kept on the top of a 30 m high building. (Considering potential energy to be zero at the base of the building.)
Answer:
Potential energy, PE = mgh
= 5 × 9.8 × 30
= 1470 J

Question 4.
What is the increment in its potential energy?
Answer:
The change in potential energy is called the increment of potential energy
∆PE = PE_final – PE_initial

Question 5.
A 10 kg weight is hung from a 5 m wire, causing it to stretch by 1 mm. Calculate the energy stored.
Answer:
Given, mass =10 kg
Force, F = mg
= 10 × 9.8 = 98 N
Extention, x = 1 mm
= 1 × 10^-2 m
U = \(\frac{1}{2}\) × Fx²
= \(\frac{1}{2}\) × 98 × (1 × 10^-2)²
= 49 × 10^-2 J

Question 6.
Calculate the work done by an external force to lift a 2 m long rod from a horizontal to a vertical position.
Answer:
Given, l = 2 m
Height of the centre of gravity,
h = \(\frac{l}{2}\)
= \(\frac{2}{2}\) = 1 m
W = mgh
= m × 9.8 × 1
W = 9.8 m J

Work and Energy Class 9 MCQ

Question 1.
What is the primary purpose of using lubricants like oil or grease in machinery?
(A) To increase the weight of the machine.
(B) To reduce friction and prevent wear and tear of parts.
(C) To make the machine parts move slower.
(D) To convert heat energy back into mechanical energy.
Answer:
(B) To reduce friction and prevent wear and tear of parts.

Question 2.
Why is a real pendulum different from an ideal one in a vacuum?
(A) Real pendulums are not affected by gravity.
(B) Real pendulum experiences air resistance and friction at the point of suspension.
(C) Real pendulum only use conservative force.
(D) There is no difference between the two.
Answer:
(B) Real pendulum experiences air resistance and friction at the point of suspension.

Question 3.
If a spring with a spring constant k = 100 N/m is stretched by 0.2m, how much elastic potential energy is stored in it?
(A) 2 J
(B) 4 J
(C) 10 J
(D) 20 J
Answer:
(A) 2 J
Applying U = \(\frac{1}{2}\) kx²
= \(\frac{1}{2}\) × 100 × (0.2)²
= 50 × 0.04 = 2 J

Question 4.
If the extension of a spring is doubled while remaining within its elastic limit, what happens to the stored elastic potential energy?
(A) The energy is halved.
(B) The energy remains the same.
(C) The energy is doubled.
(D) The energy becomes four times the original value.
Answer:
(D) The energy becomes four times the original value.
Since U = \(\frac{1}{2}\) kx², doubling x (making it 2x) results in (2x²) which is 4 times the original energy.

Assertion-Reason Questions

Directions (Q.Nos. 1 – 3): In each of the following questions, a statement of Assertion is given by the corresponding statement of Reason. Of the statements, mark the correct answer as
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
(C) Assertion is true but Reason is false.
(D) Assertion is false but Reason is true.

Question 1.
Assertion (A): The work done by gravitational force in moving a body from one point to another is independent of the path followed.
Reason (R): Gravitational force is a conservative force and for such forces, work done depends only on the initial and final positions.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Question 2.
Assertion (A): If friction were to suddenly disappear on Earth, a moving car would be unable to stop by applying brakes.
Reason (R): Braking relies on the conversion of kinetic energy into thermal energy through friction between the tires, brake pads, and the road.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Question 3.
Assertion (A): Potential energy can only be defined for conservative forces like gravity and spring forces.
Reason (R): Non-conservative forces dissipate energy as heat, making it impossible to “store” that energy for later recovery by the system.
Answer:
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Work and Energy Class 9 Extra Questions and Answers

Case/Source Based Questions

Question 1.
A ball is thrown vertically upward. When it rises, the gravitational force does negative work on it, decreasing its kinetic energy. As the ball descends, the gravitational force does positive work on it, increasing its kinetic energy. The ball falls back to the point of projection with same velocity and kinetic energy with which it was thrown up. The net work done by the gravitational force on the ball during the round trip is zero because work done by gravity on displacing a body from one point to another points depends only on the ends positions of the body.

i. Find the work done by the gravitational force on the ball, when it moves upward direction.
ii. When the ball moves vertically upward, then work done by gravitational force is negative. Why?
iii. Find the net work done by the conservative force during the round trip of a body.
Or
Name the energy of the ball which remains the same during round trip of the ball.
Answer:
i. Work done by the gravity when the ball moves upward will be,
W = – mgh.

ii. When the ball moves vertically upward, then angle between the direction of gravitational force and displacement remains 180°.
∴ Work done, W = Fs cos 180° = – Fs
Therefore, work done by gravitational force on the ball during upward movement is negative.

iii. Work done by the conservative force depends only on initial and final position of the object. During round trip of a body, initial and final position of the body coincides, hence work done by the conservative force is zero.
Or
Mechanical energy (sum of kinetic and potential energy).

Very Short Answer Type Questions

Question 1.
What does the area under a force-extension graph represent?
Answer:
The area under the linear slope represents the work done on the spring, which is stored as elastic potential energy.

Question 2.
What is the SI unit of the spring constant?
Answer:
The SI unit is newton per metre (N/m), representing the force required to stretch or compress a spring by one metre.

Question 3.
When does Hooke’s law fail to apply?
Answer:
Hooke’s law fails when the deforming force exceeds the elastic limit, causing the material to undergo permanent or plastic deformation.

Short Answer Type Questions

Question 1.
Describe the relation between work done and elastic potential energy.
Answer:
When you stretch or compress a spring, you perform work against the internal restoring force. This energy is not lost but is stored within the molecular structure of the material as elastic potential energy. When the deforming force is removed, this stored energy is converted back into kinetic energy.

Question 2.
Why is the gravitational force classified as a conservative force?
Answer:
Gravity is a conservative force because the work it performs on an object depends solely on the initial and final vertical positions, not the path taken. In a closed loop, the net work done is zero. This allows the energy to be stored as recoverable potential energy.

Question 3.
Calculate the spring constant if 20 N causes a 0.05 m extension.
Answer:
Using Hooke’s law F = kx
After rearranging Hooke’s law,
k = F/x
Substituting the values,
k = \(\frac{20}{0.05}\) = 400 N/m
This constant represents the spring’s stiffness, indicating it requires 400 N of force to stretch it by one full metre.

Question 4.
What would happen to planetary motion if friction disappeared entirely?
Answer:
On a macro scale, orbits would remain stable since space is a vacuum. However, on Earth, motion would become uncontrollable. Without friction, objects would never stop sliding unless they collided, and humans could not walk or drive because there would be no traction to initiate or change direction.

Question 5.
How much energy is stored in a spring (k = 200 N/m) stretched by 0.1 m?
Answer:
The elastic potential energy is calculated using
U = \(\frac{1}{2}\) kx²
Substituting the values,
U = \(\frac{1}{2}\) × 200 × (0.1)²
U = 100 × 0.01 = 1 J
This 1 J of energy is stored within the spring’s structure and is ready to do work upon release.

Long Answer Type Questions

Question 1.
Define conservative and non-conservative forces. Explain how they differ in terms of path dependency and energy recovery, providing one detailed example for each.
Answer:
A conservative force is one where the work done in moving an object between two points is independent of the path taken (e.g. gravity). The work depends only on the initial and final positions. If an object moves in a closed loop, the net work done is zero, and the mechanical energy remains within the system as potential energy.

Conversely, a non-conservative force (e.g. friction) is path dependent. The longer the path, the more work is done against the force. In a closed loop, the work done is not zero because the force always opposes motion. Instead of being stored, the energy is dissipated into the environment as heat or sound.

Derivation of Work in a Conservative Field:
Consider a mass m moved from height h₁ to h₂ along a curved path.
At any point, the force is F = mg.
The work done is,
W = \(\int_{h_1}^{h_2}\) F . dh
= \(\int_{h_1}^{h_2}\) mg dh
W = mg (h₂ – h₁)
This shows W only depends on the change in height, not the horizontal distance or curve.

Question 2.
Derive the expression for the energy stored in a stretched spring. Explain why we must account for the variable nature of the force during this derivation.
Answer:
When stretching a spring, the force is not constant, it increases linearly from 0 to kx. Therefore, we cannot simply multiply the final force by distance. We must use integration or the average force.

Derivation:
The work done (dW) to move a spring by a tiny distance dx is
dW = F_ext . dx.
F_ext = kx
W = \(\int_0^x\) kx dx
W = k \(\left[\frac{x^2}{2}\right]_0^x\)
W = \(\frac{1}{2}\) kx²
This work is stored as elastic potential energy (U).
U = \(\frac{1}{2}\) kx²

Question 3.
Mathematically define the relationship between a conservative force and potential energy. Use this to explain the work done by gravity on a falling object.
Answer:
For a conservative force, we define the change in potential energy (∆U) as the negative of the work done (We) by that force
F = \(\frac{dU}{dx}\)
This means the force always points in the direction of decreasing potential energy.

Application to Gravity:
As an object falls, gravity does positive work (force and displacement are in the same direction).
W = ∫ F . dy = ∫ mg dy

According to the relation ∆U = – W, the potential decreases (becomes more negative or smaller) as the object falls. This “lost” potential energy is converted into kinetic energy.

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