Class 9 General Science Chapter 11 Work and Energy
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Work and Energy
Answers to Questions Within the Lesson(1)
Q1. A force of 7 N is applied on an object. Suppose the displacement of the object in the direction of the force is 8 m (Figure 11.3). Assume that the force acts on the object throughout the displacement. How much work is done?
Ans:
Given, force = 7 N and displacement = 8 m
Work done = Force × Displacement
= 7 × 8 = 56 J
Answers to Questions Within the Lesson(2)
Q2. When do we say that work is done?
Ans: Work is said to be done when a force applied on an object and causes displacement.
Q3. Write the expression for work done when a force is applied in the direction of displacement.
Ans: Work = Force × Displacement
Q4. Define 1 joule of work.
Ans: One joule is the amount of work done when a force of 1 N displaces an object by 1 m in the direction of the force.
Q4. A pair of bullocks applies a force of 1 N on a plough. The field is 15m long. Find the work done in ploughing the field.
Ans:
Force, F = 1 N
Displacement, s = 15 m
Work done = F × s = 1 × 15 = 15 J
Answers to Questions Within the Lesson(3)
Q1. What is meant by the kinetic energy of an object?
Ans: Kinetic energy is the energy possessed by an object due to its motion.
Q2. Write the expression for the kinetic energy of an object.
Ans: The kinetic energy of a moving object is given by:
Eₖ= 1/2mv²
Q3. The kinetic energy of a body moving with a velocity of 5 m/s is 25J. What will be its kinetic energy if the velocity is doubled? What will it beif the velocity is tripled?
Ans:
Since kinetic energy is proportional to the square of velocity,
When velocity is doubled:
New kinetic energy = 25 × (2²) = 25 × 4 = 100 J
When velocity is tripled:
New kinetic energy = 25 × (3²) = 25 × 9 = 225 J
Answers to Questions Within the Lesson(4)
Q1. What is meant by power?
Ans: Power is the rate at which work is done or energy is consumed.
Q2. Define 1 watt of power.
Ans: One watt is the power when 1 joule of work is done in 1 second.
Q3. A bulb consumes 100 J of electrical energy in 10 s. What is its power?
Ans:
Power = Energy / Time = 100 / 10 = 10 WQ
4. Define average power.
Ans: Average power is the total work done or total energy consumed divided by the total time taken.
Exercise: Questions and Answers
Q1. Observe the following activities. Based on your understanding of work, state whether work is done or not and justify your answer.
(a) A person is swimming in a pond.
(b) A donkey is carrying a load.
(c) A windmill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Crops are drying in the heat of the Sun.
(g) A sailboat is moving using wind energy.
Ans:
(a) Work is done because there is displacement while swimming.
(b) Work is done because the donkey uses energy and displacement occurs.
(c) Work is not done by the windmill itself if it does not undergo displacement.
(d) Work is not done because there is no displacement of the plant.
(e) Work is done because the engine uses energy and causes displacement of the train.
(f) Work is not done because only heat energy is used without displacement.
(g) Work is done because wind energy causes the boat to move, producing displacement.
Q2. An object projected at an angle to the ground moves along a curved path and returns to the ground. The initial and final points of its path lie on the same horizontal level. How much work is done by gravity on the object?
Ans: Since the initial and final positions are at the same height, the net work done by gravity is zero.
Q3. A battery lights a bulb. Describe the energy transformations involved in the process.
Ans: A battery stores chemical energy, which is converted into electrical energy when the circuit is completed. In the bulb, electrical energy is converted into light energy and heat energy.
Q4. A force acts on a body of mass 20 kg and changes its velocity from 5 m/s to 2 m/s. Find the work done by the force.
W = 1/2m(v²- u²)
Ans:
Given, m = 20 kg, u = 5 m/s, v = 2 m/s
W = (1/2) × 20 × (2² – 5²)
= 10 × (4 – 25)
= 10 × (-21)
= -210 J
Work done = -210 J
Q5. A body of mass 10 kg is at point A on a table. It is moved to point B. If the line joining A and B is horizontal, how much work is done by the gravitational force on the body? Explain your answer.
Ans: If the body is moved horizontally from A to B at the same level, the work done by the gravitational force is zero. This is because gravity acts vertically downward, while displacement is horizontal, so no work is done.
Q6. The potential energy of a freely falling body gradually decreases. Does it violate the law of conservation of energy? Why?
Ans: No, it does not violate the law of conservation of energy. As the body falls, its potential energy decreases, but an equal amount of kinetic energy increases. Thus, potential energy is converted into kinetic energy, and total energy remains constant.
Q7. What are the different energy transformations that occur while riding a bicycle?
Ans: While riding a bicycle, the chemical energy of our body is converted into kinetic energy. Some energy is also converted into heat due to the friction between the tyres and the road.
Q8. If you try to push a large stone with full effort but fail to move it, does any energy transformation occur? Where does the energy you used go?
Ans: Yes, energy transformation occurs. Even though the stone does not move, the energy used is converted into heat energy in the muscles and surroundings, and some is stored as internal energy in the body.
Q9. In a house, a total of 250 ‘units’ of energy is consumed in one month. What is this energy in joules?
Ans:
1 unit of electrical energy = 1 kWh = 3.6 × 106 J
Therefore,
250 units = 250 × 3.6 × 106
= 9 × 108 J
Energy consumed = 9 × 108 J
Q10. A body of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the body is allowed to fall, find its kinetic energy when it has covered half the distance.
Ans:
Given, m = 40 kg, h = 5 m, g = 9.8 m/s²
Potential energy = 40 × 9.8 × 5 = 1960 J
When the body falls through half the distance, height fallen = 2.5 m Loss in potential energy = gain in kinetic energy
Kinetic energy = mgh = 40 × 9.8 × 2.5 = 980 J
Potential energy = 1960 J
Kinetic energy at half distance = 980 J
Q11. How much work is done by gravitational force on a satellite revolving around the Earth? Justify your answer.
Ans: The work done by gravitational force on a satellite revolving around the Earth is zero. This is because the gravitational force acts towards the centre of the Earth, while the motion of the satellite is perpendicular to this force. Hence, no work is done.
Q12. Is it possible for an object to have displacement without the application of force? Think and discuss.
Ans: No, displacement cannot occur without the application of force. For a body to change its position, a force must act on it. Without force, a body cannot start moving or change its state of motion.
Q13. A person stands for half an hour holding a bundle of hay on his head and feels tired. Has he done any work? Justify your answer.
Ans: No work is done in the physical sense. Work is done only when force causes displacement. In this case, although energy is spent by the person,
there is no displacement of the load, so no work is done.
Q14. An electric iron has a power of 1500 W. How much energy will it consume in 10 hours?
Ans:
Power, P = 1500 W
Time, t = 10 h = 10 × 3600 = 36000 s
Energy consumed = P × t
= 1500 × 36000
= 54,000,000 J
Energy consumed = 5.4 × 107 J
Q15. A pendulum bob is pulled to one side and then released to oscillate. Explain the energy transformations involved and use this to explain the law of conservation of energy. Why does the bob finally come to rest? What happens to its energy? Does it violate the law of conservation of energy?
Ans: When the pendulum is pulled to one side, it has maximum potential energy. When released, this potential energy converts into kinetic energy as it
moves downward. At the lowest point, kinetic energy is maximum and potential energy is minimum. As it rises again, kinetic energy converts back into potential energy.
This continuous conversion between potential and kinetic energy shows the law of conservation of energy, which states that energy can neither be created nor destroyed, only transformed from one form to another. The pendulum eventually comes to rest due to air resistance and friction at the pivot. Its mechanical energy is gradually converted into heat and sound energy. This does not violate the law of conservation of energy, as energy is only transformed into other forms.
Q16. A body of mass m is moving with constant velocity. How much work is required to bring it to rest?
Ans: The work required to bring the body to rest is equal to its initial kinetic energy.
Work done = – (1/2)mv²
(The negative sign indicates that the work is done against the motion.)
Q17. A car of mass 1500 kg is moving with a speed of 60 km/h. Find the work required to stop it.
Ans:
Speed, v = 60 km/h = 60 ×1000/3600 = 16.67) m/s
Work required to stop the car = loss of kinetic energy
W = -1/2mv²
W = 1/2×1500 (16.67)^2
W≈ -208375 J ≈ -2.08 × 105J
Work required = 2.08 × 105 J (magnitude)
Q18. In each of the following cases, a force F is applied to a body of mass m. The direction of displacement is shown from west to east by along arrow. Observe the figures carefully and determine whether the work done by the force is negative, positive, or zero.
Ans:
(i) In the first case, the work done is zero.
(ii) In the second case, the work done is positive.
(iii) In the third case, the work done is negative.
Q19. Soni says that even when several forces act on a body, its acceleration can be zero. Do you agree with her? Why?
Ans: Yes, this statement is correct. If multiple forces acting on a body are balanced (i.e., their resultant force is zero), then the acceleration of the body will be zero. In this case, there is no change in the state of motion.
Q20. Each of four devices has a power of 500 W. If they operate for 10 hours, find the total energy consumed in units.
Ans:
Power of one device = 500 W = 0.5 kW
Total power of 4 devices = 4 × 0.5 = 2 kW
Time = 10 hours
Energy consumed = Power × Time = 2 × 10 = 20 kWh
Energy consumed = 20 units
Q21. Objects in free fall eventually reach the ground and come to rest. What happens to their kinetic energy?
Ans: In this case, the kinetic energy is transformed into other forms of energy such as heat, sound, and deformation energy when the object hits the ground.
Additional Questions & Answers
Q1. What are the two conditions required for work to be done?
Ans: (i) A force must act on the object.
(ii) The object must undergo displacement.
Q2. Is work a scalar or vector quantity? What is its unit?
Ans: Work is a scalar quantity. Its unit is joule (J).
Q3. What is potential energy?
Ans: Potential energy is the energy possessed by an object due to its position or configuration.
Q4. On what does gravitational potential energy depend?
Ans: Gravitational potential energy depends on the initial and final vertical positions (height) of the object.
Q5. What is the expression for the work done against gravity in raising a body of mass m to a height h?
W = mgh
Ans: The work done against gravity is given by W = mgh.
Q6. State the law of conservation of energy.
Ans: Energy can neither be created nor destroyed; it can only be transformed from one form to another.
Multiple Choice Questions (MCQ)
1. Work is said to be done when
(a) force is applied only
(b) displacement occurs only
(c) force causes displacement
(d) object is at rest
Ans: (c)
2. The SI unit of work is
(a) watt
(b) joule
(c) newton
(d) pascal
Ans: (b)
3. Work done is given by
(a) force × time
(b) force × displacement
(c) mass × velocity
(d) force / displacement
Ans: (b)
4. If force and displacement are in the same direction, work done is
(a) zero
(b) negative
(c) positive
(d) constant
Ans: (c)
5. If force acts perpendicular to displacement, work done is
(a) maximum
(b) zero
(c) negative
(d) infinite
Ans: (b)
6. Work done against gravity is stored as
(a) kinetic energy
(b) heat energy
(c) potential energy
(d) sound energy
Ans: (c)
7. The energy possessed by a body due to motion is called
(a) potential energy
(b) heat energy
(c) kinetic energy
(d) chemical energy
Ans: (c)
8. The kinetic energy of a body depends on
(a) mass only
(b) velocity only
(c) both mass and velocity
(d) temperature
Ans: (c)
9. A stretched spring possesses
(a) kinetic energy
(b) potential energy
(c) heat energy
(d) light energy
Ans: (b)
10. A moving car possesses
(a) potential energy
(b) kinetic energy
(c) sound energy
(d) chemical energy
Ans: (b)
11. The energy possessed by a body due to its position is called
(a) kinetic energy
(b) mechanical energy
(c) potential energy
(d) heat energy
Ans: (c)
12. Power is defined as
(a) work done
(b) energy stored
(c) rate of doing work
(d) force applied
Ans: (c)
13. The SI unit of power is
(a) joule
(b) watt
(c) newton
(d) pascal
Ans: (b)
14. One watt is equal to
(a) 1 J/s
(b) 1 N
(c) 1 m/s
(d) 1 kg
Ans: (a)
15. If no displacement occurs, work done is
(a) maximum
(b) minimum
(c) zero
(d) negative
Ans: (c)
16. Work done by friction is
(a) positive
(b) negative
(c) zero
(d) constant
Ans: (b)
17. Mechanical energy is the sum of
(a) heat and sound energy
(b) potential and kinetic energy
(c) light and heat energy
(d) chemical and electrical energy
Ans: (b)
18. The law of conservation of energy states that
(a) energy can be created
(b) energy can be destroyed
(c) energy is always constant
(d) energy can neither be created nor destroyed
Ans: (d)
19. When a body falls freely, its potential energy
(a) increases
(b) decreases
(c) remains same
(d) becomes zero
Ans: (b)
20. When a body falls freely, its kinetic energy
(a) decreases
(b) increases
(c) remains same
(d) becomes zero
Ans: (b)
21. The commercial unit of energy is
(a) joule
(b) watt
(c) kilowatt-hour
(d) newton
Ans: (c)
22. 1 kWh is equal to
(a) 3.6 × 106 J
(b) 1 J
(c) 1000 J
(d) 360 J
Ans: (a)
23. If velocity is doubled, kinetic energy becomes
(a) double
(b) half
(c) four times
(d) same
Ans: (c)
24. If mass is doubled, kinetic energy becomes
(a) double
(b) half
(c) four times
(d) same
Ans: (a)
25. The work done by gravity on a freely falling object is
(a) negative
(b) zero
(c) positive
(d) infinite
Ans: (c)
26. The work done in lifting a body against gravity is
(a) positive
(b) negative
(c) zero
(d) constant
Ans: (a)
27. If time increases, power
(a) increases
(b) decreases
(c) remains same
(d) becomes zero
Ans: (b)
28. Energy is measured in
(a) joule
(b) watt
(c) newton
(d) pascal
Ans: (a)
