Welcome to the complete solutions for Class 9 Science Work and Energy exercises. These answers are prepared for both CBSE and State Board students to help you understand concepts clearly and score full marks in exams. The questions are provided exactly as in the textbook, with step-by-step explanations and examples.
1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.
- Suma is swimming in a pond.
- A donkey is carrying a load on its back.
- A wind-mill is lifting water from a well.
- A green plant is carrying out photosynthesis.
- An engine is pulling a train.
- Food grains are getting dried in the sun.
- A sailboat is moving due to wind energy.
Answer:
| Activity | Work Done? | Explanation |
|---|---|---|
| Suma is swimming in a pond | Yes | Work is done on water molecules because she exerts force on water, causing displacement backward to move forward. |
| A donkey is carrying a load on its back | No | The force applied is vertical, displacement is horizontal. Since force and displacement are perpendicular, work done = 0. |
| A wind-mill is lifting water from a well | Yes | Water is displaced upwards against gravity. Work is done as force is applied in the direction of displacement. |
| A green plant is carrying out photosynthesis | No | No mechanical work is done. Energy from sunlight is converted into chemical energy stored in glucose. |
| An engine is pulling a train | Yes | Engine applies force along the direction of motion, moving the train forward, so work is done. |
| Food grains are getting dried in the sun | No | Heat energy is absorbed, but no mechanical work is done as there is no displacement due to applied force. |
| A sailboat is moving due to wind energy | Yes | The wind exerts force on the sails in the direction of motion, causing displacement; hence work is done. |
2. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
Answer: The vertical displacement of the object is zero because it starts and ends at the same height. Work done by gravity = 0 J. Gravity does positive work while the object is falling and negative work while it is rising, which cancels out.
3. A battery lights a bulb. Describe the energy changes involved in the process.
Answer: Chemical energy in the battery → Electrical energy in the wires → Light energy + Heat energy in the bulb. Total energy is conserved.
4. Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.
Answer:
Work done, W = ½ m (vf² - vi²) = ½ × 20 × (2² - 5²) = 10 × (-21) = -210 J
Negative sign indicates work is done against motion.
5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Answer: Displacement is horizontal, force (weight) is vertical → θ = 90° → cos90° = 0 → Work done = 0 J.
6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Answer: No. As potential energy decreases, kinetic energy increases. Total energy remains constant. Energy is conserved.
7. What are the various energy transformations that occur when you are riding a bicycle?
Answer: Chemical energy (from food) → Mechanical energy (pedaling) → Kinetic energy (motion of bicycle) → Heat energy (friction in brakes and tires).
8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Answer: Yes, energy is used in muscles and converted into internal energy (heat). No mechanical work is done on the rock because there is no displacement.
9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer: 1 unit = 1 kWh = 3.6 × 10⁶ J → 250 × 3.6 × 10⁶ = 9 × 10⁸ J
10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Answer: PE = mgh = 40 × 10 × 5 = 2000 J
At half height (h = 2.5 m): PE = 40 × 10 × 2.5 = 1000 J → KE = 2000 - 1000 = 1000 J
11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Answer: Satellite moves in circular orbit; displacement is perpendicular to gravity → Work = 0 J.
12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.
Answer: Yes. An object can move with constant velocity even when no net force acts on it (Newton’s First Law).
13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
Answer: No mechanical work is done on the hay because there is no displacement. Fatigue is due to internal energy spent by muscles.
14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer: Energy = Power × Time = 1500 × 10 × 3600 = 5.4 × 10⁷ J
15. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
Answer:
- Highest point: PE maximum, KE = 0
- Lowest point: KE maximum, PE = 0
- During oscillation: PE ↔ KE transformations
- Bob eventually stops due to friction and air resistance → energy converts to heat and sound
- Total energy remains conserved → Conservation of energy is not violated
16. An object of mass m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?
Answer: Work required = Kinetic Energy = ½ mv²
17. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer: v = 60 km/h = 16.67 m/s → Work = ½ × 1500 × 16.67² ≈ 208,333 J
18. In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
Answer:
- Force along displacement → Positive work
- Force opposite displacement → Negative work
- Force perpendicular → Zero work
19. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
Answer: Yes. If forces are balanced (net force = 0), acceleration = 0. Newton’s First Law explains this.
20. Find the energy in joules consumed in 10 hours by four devices of power 500 W each.
Answer: Total power = 4 × 500 = 2000 W → Energy = 2000 × 10 × 3600 = 7.2 × 10⁷ J
21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Answer: Kinetic energy is transformed into heat, sound, and deformation energy of the object and ground. Total energy remains conserved.
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