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Free PDF — Class 9 Work and Energy Chapter Guide for Science 2025-26
Master Work and Energy: CBSE Class 9 Science Chapter 11 Guide 2025-26
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FAQs on Master Work and Energy: CBSE Class 9 Science Chapter 11 Guide 2025-26
1. What type of numerical problems are considered most important from Chapter 11, Work and Energy, for the Class 9 exams (2025-26)?
For the Class 9 exams, it is important to focus on numericals based on the following formulas:
Work Done (W = F × s): Calculating work when force and displacement are given.
Kinetic Energy (E_k = ½ mv²): Finding the kinetic energy of a moving object or calculating the velocity if energy is known.
Potential Energy (E_p = mgh): Calculating the potential energy of an object raised to a certain height.
Power (P = W/t or P = E/t): Calculating the power of a person or machine based on work done or energy consumed in a given time.
Practising problems that involve the conversion between kinetic and potential energy is also a high-yield area.
2. How can one state and prove the Law of Conservation of Energy for a freely falling body? This is a frequently asked 5-mark question.
The Law of Conservation of Energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another. The total energy of an isolated system remains constant. For a freely falling body, the sum of its potential and kinetic energy remains constant at all points during its fall. You can prove this by showing that the Total Mechanical Energy (E_p + E_k) is the same at the highest point, any point in between, and just before it hits the ground.
3. What are the essential conditions for work to be done, and when is the work done on an object considered zero?
For work to be done, two conditions must be met:
A force must be applied to the object.
- The object must have a displacement in the direction of the applied force.
Work done is considered zero if: 1) The displacement is zero (e.g., pushing a wall), or 2) The angle between the force and displacement is 90° (e.g., a satellite orbiting the Earth, where gravitational force is perpendicular to its motion).
4. What are the key differences between Kinetic Energy and Potential Energy, a common 3-mark question?
The main differences are:
Basis: Kinetic energy is the energy an object possesses due to its motion. Potential energy is the energy stored in an object due to its position or configuration.
Formula: Kinetic energy is calculated as E_k = ½ mv², depending on mass and velocity. Gravitational potential energy is calculated as E_p = mgh, depending on mass and height.
Example: A flying bird possesses kinetic energy. A stretched rubber band possesses potential energy.
5. Why does a coolie carrying a heavy load on his head and walking on a horizontal road do no work against gravity?
This is a key conceptual question. Work done is calculated by the formula W = Fs cosθ, where θ is the angle between the force and displacement. In this case, the coolie applies an upward force to support the load, which is against the force of gravity (acting vertically downwards). However, his displacement is horizontal. The angle between the force (vertical) and displacement (horizontal) is 90 degrees. Since the cosine of 90° is zero, the work done against gravity is zero.
6. How are work and power different? Explain why a more powerful engine can do the same amount of work faster.
Work is the measure of energy transfer when a force moves an object, while power is the rate at which work is done or energy is transferred. The key difference is time. A more powerful engine has a higher power rating, meaning it can convert energy or do work at a faster rate. Therefore, it can complete the same amount of work (e.g., lifting a car) in less time compared to a less powerful engine.
7. What does the commercial unit of energy, 'kilowatt-hour (kWh)', represent, and why is it an important term for exams?
A kilowatt-hour (kWh) is the amount of energy consumed when an electrical appliance with a power rating of 1 kilowatt operates for 1 hour. It is important for exams because you are often asked to convert it into Joules. The conversion is: 1 kWh = 3.6 × 10⁶ Joules. This unit is used for electricity bills because the Joule is too small for measuring large-scale energy consumption, making kWh a more practical commercial unit.
8. Can an object possess energy without momentum? And can it possess momentum without energy? Explain this important concept.
An object can possess energy without having momentum. For example, a rock at the top of a hill has gravitational potential energy but has zero velocity, and therefore, zero momentum. However, an object with mass cannot possess momentum without having kinetic energy. Momentum (p = mv) requires the object to have a non-zero velocity. If velocity is not zero, then its kinetic energy (E_k = ½ mv²) must also be greater than zero.
9. How would you derive the formula for the kinetic energy of an object? This derivation is an important question for exams.
The derivation is based on the work done to accelerate an object. Consider an object of mass 'm' at rest (u=0). A force 'F' is applied, causing it to move a distance 's' and reach a final velocity 'v'.
The work done is W = F × s.
According to Newton's second law, F = ma.
From the third equation of motion, v² - u² = 2as. Since u=0, we get v² = 2as, which means s = v² / 2a.
Substitute 'F' and 's' into the work equation: W = (ma) × (v² / 2a).
By simplifying, we get W = ½ mv². This work done is stored as the kinetic energy of the object. Thus, E_k = ½ mv².
