Important Question Work and Energy Chapter 10 Science Class 9 CBSE Board

Important Questions For All Chapters – Science Class 9

Short Questions

1. What are the two essential conditions for work to be done?
Answer: (i) A force should act on an object.
(ii) The object must be displaced in the direction of the force.

2. Write the formula for work done.
Answer: Work done (W) = Force (F) × Displacement (s)

3. What is the SI unit of work?
Answer: The SI unit of work is joule (J).

4. When is work said to be positive?
Answer: Work is positive when the force and displacement are in the same direction.

5. What is 1 joule of work?
Answer: 1 joule of work is done when a force of 1 newton displaces an object by 1 metre in the direction of force.

6. Define energy.
Answer: Energy is the capacity of an object to do work.

7. Write the expression for kinetic energy.
Answer: Kinetic Energy (Ek) = ½ mv²

8. Write the expression for potential energy.
Answer: Potential Energy (Ep) = mgh

9. State the law of conservation of energy.
Answer: Energy can neither be created nor destroyed; it can only change from one form to another. The total energy remains constant.

10. Define power.
Answer: Power is the rate of doing work or rate of transfer of energy.

Long Questions

1. Explain the scientific conception of work with suitable examples.
Answer: In science, work is said to be done when a force acts on an object and causes displacement in the direction of force. For example, pushing a pebble so that it moves is work. However, if we apply force on a rock and it does not move, no work is done scientifically because there is no displacement.

2. Derive the formula for work done by a constant force.
Answer: Let a constant force F act on an object and displace it by a distance s in the direction of the force. Then,
Work done (W) = Force × Displacement
W = F × s.
If F = 1 N and s = 1 m, then W = 1 J (1 joule).

3. What is the difference between positive and negative work? Give examples.
Answer:

• Positive work: When force and displacement are in the same direction. Example: A person lifting a load upward.
• Negative work: When force acts opposite to the displacement. Example: Frictional force on a moving object.

4. Define energy and explain its unit.
Answer: Energy is the capacity to do work. The unit of energy is the same as that of work, i.e., joule (J). 1 joule is the energy required to do 1 joule of work. A larger unit is kilojoule (1 kJ = 1000 J).

5. What is kinetic energy? Derive an expression for it.
Answer: Kinetic energy is the energy possessed by a body due to its motion.
If an object of mass m moves with velocity v, then
Work done W = ½ mv² – ½ mu².
If initial velocity u = 0,
Kinetic Energy Ek = ½ mv².

6. Define potential energy and explain with examples.
Answer:
Potential energy is the energy possessed by an object due to its position or configuration.
Examples:

• A stretched bow has potential energy.
• Water stored at a height has potential energy.

7. Derive an expression for gravitational potential energy.
Answer:
Work done in raising an object of mass m through a height h against gravity is:
W = Force × Displacement = mg × h = mgh.
Therefore, Potential Energy (Ep) = mgh.

8. Explain the interconversion of energy with examples.
Answer:
Energy can change from one form to another.
Examples:

• In a hydroelectric plant, potential energy of water → kinetic energy → electrical energy.
• In photosynthesis, solar energy → chemical energy.
• In an electric bulb, electrical energy → light and heat energy.

9. State and explain the law of conservation of energy with an example.
Answer:
The law states that energy can neither be created nor destroyed, only transformed.
Example: During free fall of an object, potential energy decreases while kinetic energy increases, but total energy (PE + KE) remains constant.

10. Explain the transformation of energy in a simple pendulum.
Answer: At the extreme position, the pendulum has maximum potential energy and zero kinetic energy. As it moves down, potential energy converts into kinetic energy. At the mean position, kinetic energy is maximum. This transformation continues, showing total energy remains constant.

11. Define power and derive its formula.
Answer: Power is the rate of doing work.
If work done = W and time taken = t,
Then, Power (P) = W / t.
The SI unit is watt (W). 1 W = 1 J/s.

12. Differentiate between work, energy, and power.
Answer: 

13. Explain the relation between kinetic and potential energy during free fall.
Answer:When an object falls freely from a height, its potential energy converts into kinetic energy. At the start, PE = mgh, KE = 0. As it falls, PE decreases, KE increases, but total energy remains constant (mgh = ½mv²).

14. A porter lifts 15 kg luggage to a height of 1.5 m. Calculate the work done.
Answer: Given: m = 15 kg, h = 1.5 m, g = 10 m/s²
Work done W = mgh = 15 × 10 × 1.5 = 225 J.

15. Two girls of weight 400 N each climb a height of 8 m. A takes 20 s, B takes 50 s. Find power of each.
Answer: Work done = mgh = 400 × 8 = 3200 J
Power of A = 3200 / 20 = 160 W
Power of B = 3200 / 50 = 64 W.