In routine life, while performing many activities, we do work by using force, e.g. displace an object from one place and put it at another place, push a vehicle, walking, running, etc. We need energy to perform work. Animals and we get energy from food that we eat while machines get energy from the fuel. Power is also an important concept associated with work. We consider a person or engine etc. more powerful than the other if that person or engine could perform the work faster than the other. In this chapter, we shall study work, energy and power.

In this article, we have compiled RBSE Class 10 solutions for Chapter 11 Science which are prepared by a team of subject matter experts. With the help of these solutions, students can score well in their exam. Students can take help of these solutions to cross-check their answers and rectify their mistakes. These solutions provide students with the correct method of writing an answer. Keep these solutions in handy as it helps in clearing doubts of the difficult and tricky concepts.

### RBSE Class 10 Science Chapter 11: Objective Textbook Questions and Solutions

**Q1. Unit of work is?**

(A) Newton

(B) Joule

(C) Watt

(D) None of these

**Answer:** B

**Q2. If the angle between the force F and displacement s is θ then the work done will be given by**

(A) FS sinθ

(B) FS θ

(C) FS cos θ

(D) FS tan θ

**Answer:** C

**Q3. If an object of mass m is moving with v velocity then its kinetic energy will be given as**

(A) mv

(B) mgv

(C) mv²

(D) 1/2 mv²

**Answer: **D

**Q4. What will be the potential energy of an object of mass m at a height of h?**

(A) mgh

(B) mg/h

(C) mh/g

(D) ½ mgh²

**Answer:** A

**Q5. Unit of power is**

(A) Newton

(B) Watt

(C) Joule

(D) Newton meter

**Answer:** B

**Q6. Calculate the work done in taking object of 1 kg mass at 4m height? (g = 10m/s ^{2})**

(A) 1 Joule

(B) 4 Joule

(C) 20 Joule

(D) 40 Joule

**Answer:** D

**Q7. Total energy of an object falling freely towards earth will**

(A) keep on increasing

(B) keep on decreasing

(C) remains constant

(D) be zero

**Answer:** C

**Q8. If the velocity of an object is doubled then what will be its kinetic energy?**

(A) One fourth

(B) Half

(C) Double

(D) Four times

**Answer:** D

**Q9. What is the commercial unit of electric energy?**

(A) Joule

(B) Watt – second

(C) Kilowatt hour

(D) Kilowatt per hour

**Answer:** C

**Q10. If a spring is compressed under elastic limit to x distance then what will be the acquired potential energy (spring constant = k)**

(A) kx

(B) 1, kx²

(C) kx²

(D) None of these

**Answer:** B

### RBSE Class 10 Science Chapter 11: Very Short Answer Type Questions and Solutions

**Q11. Define work and write its unit.**

**Answer:** Work is said to be done when a body or object moves with the application of external force. We can define work as an activity involving a movement and force in the direction of the force. For example, a force of 30 newtons (N) pushing an object 3 meters in the same direction of the force will do 90 joules (J) of work.

If a force of 5 newtons is applied to an object and it moves 2 meters, the work will be 10 newton-meter. Newton meter is termed as the unit of Work.

**Q12. What is energy? Write the unit of energy?**

**Answer: **Energy is the ability to perform work. Energy can neither be created nor destroyed. It can only be transformed from one kind to another. The unit of Energy is the same as of Work i.e. Joules. Energy is found in many things and thus there are different types of energy. All forms of energy are either kinetic or potential. The energy in motion is known as Kinetic Energy whereas Potential Energy is the energy stored in an object and is measured by the amount of work done.

The SI unit of energy is joules (J), which is named in honour of James Prescott Joule.

**Q13. What do you understand by kinetic energy?**

**Answer:** The kinetic energy of an object is the energy that it possesses due to its motion. Kinetic energy definition is given as: The energy of an object because of its motion or the energy gained by an object from its state of rest to motion. The SI unit of kinetic energy is Joule which is equal to 1 kg.m^{2}.s^{-2}.

**Q14. Write the law of conservation of energy?**

**Answer:** The law of conservation of energy states that energy can neither be created nor be destroyed. Although, it may be transformed from one form to another. If you take all forms of energy into account, the total energy of an isolated system always remains constant. All the forms of energy follow the law of conservation of energy. In brief, the law of conservation of energy states that in a closed system, i.e., a system that is isolated from its surroundings, the total energy of the system is conserved.

**Q15. What are the common forms in which dissipation of energy takes place?**

**Answer:** When energy transformation takes place a part of energy is dissipated as heat, sound, light, etc.

**Q16. Can you make a hundred percent efficient system?**

**Answer:** No, it is not possible because there will be some loss of energy in the system. Hence, for every engine, there is a limit for its efficiency. The efficiency is maximum for a reversible engine such as Carnot heat engine.

**Q17. What do you mean by electrical energy?**

**Answer:** Electrical energy is the energy derived from electric potential energy or kinetic energy of the charged particles. In general, it is referred to as the energy that has been converted from electric potential energy. We can define electrical energy as the energy generated by the movement of electrons from one point to another. The movement of charged particles along/through a medium (say wire) constitute current or electricity.

**Q18. Write the name of any three types of electricity plants?**

**Answer:** The three types of electricity plants are:

- Nuclear power plants
- Hydroelectric power plants
- Solar power plants

**Q19. What is power? Write unit of the power.**

**Answer:** We can define power as the rate of doing work, it is the work done in unit time. The SI unit of power is Watt (W) which is joules per second (J/s). Sometimes the power of motor vehicles and other machines are given in terms of Horsepower (hp), which is approximately equal to 745.7 watts.

The unit for standard metric work is the Joule and the standard metric unit for time is the second, so the standard metric unit for power is a Joule / second, defined as a Watt and abbreviated W.

**Q20. Which light will be useful in reducing consumption of electric energy at home?**

**Answer: **In order to lower the consumption of electricity at homes we should use CFL and LED lights.

**Q21. Which important point should we remember while purchasing new electrical home appliances?**

**Answer:** While purchasing new electrical appliances we should check the star rating. More star ratings means more energy efficiency. Energy efficient appliances use around 30% less electricity.

**Q22. An object displaces 10m when 20 N force is applied on it. Calculate the work done. (200 Joule).**

**Answer:** Given, Force = 20N

Displacement = 10m

Work = Force x Displacement

= 20N x 10m = 200Nm.

**Q23. It takes one minute to uplift a 30kg object to the height of 2m. Calculate the power used. (10W).**

**Answer: **Given,

Mass = 30kg

Height = 2m

Time = 1min = 60sec and g = 10m/s^{2}

We can calculate energy spent by using the formula

mgh/t = 30kg x 10ms^{-2} x 2m/60s = 10W

**Q24. A 60W bulb is used 8 hour per day for 30 days then what will be the energy consumption in electric unit. (14.4 units)**

**Answer: **Given,

Power of bulb = 60 W, Time per day = 8 hour, No. of days = 30

Energy consumed: 60W x 8h x 30d = 14400 W = 14.4 kWh = 14.4 unit

### RBSE Class 10 Science Chapter 11: Short Answer Type Questions and Solutions

**Q25. What do you mean by work? If the direction of displacement is different from the direction of the force then how the work is calculated? Explain with example.**

**Answer: **We can define work as: The product of the component of the force in the direction of the displacement and the magnitude of this displacement.

If the direction of force is different than the direction of displacement of the object, then the component of force along the direction of the displacement is used to calculate work.

Let force F is acting at point A in such a way that during the displacement of object till point B the direction of force is making an angle θ with respect to the direction of displacement. The component of force along with the direction of displacement.

= F.cosθ

Thus the work done

W = (Component of force along the direction of displacement) x displacement

=F cosθ x s

=FS cosθ

**Q26. When force F is applied on an object moving with velocity u then its velocity increases to v. If the distance travelled by the object during this change of velocity is S then calculate the increase in the kinetic energy.**

**Answer: **By using third equation of motion

v^{2} = u^{2} + 2as

a = v^{2 }– u^{2}/2s

As per Newton’s second law of motion

F = ma

F = m (v^{2 }– u^{2}/2s)

F.s = 1/2m (v^{2 }– u^{2})

Since, W = F.s

Hence, W = 1/2m (v^{2 }– u^{2})

Or W = 1/2mv^{2} – 1/2mu^{2}

This shows that work done is equal to change in kinetic energy of object.

**Q27. What is potential energy? An ideal spring having spring constant k is compressed by x distance, derive the formula of acquired potential energy of the spring.**

**Answer: **As we know, an object can store energy as a result of its position. In the case of a bow and an arrow, when the bow is drawn, it stores some amount of energy, which is responsible for the kinetic energy it gains, when released. We can define potential energy as: The energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.

For the Second Part of Answer, please visit here to know in detail.

**Q28. An object is moving with a uniform velocity v. If the mass of the object is m, then find how much work will have to be done to bring this object to rest?**

**Answer: **For detailed Answer, please visit here**.**

**Q29. What do you mean by conservation of mechanical energy?**

**Answer: **Mechanical energy is the sum of kinetic energy and potential energy in an object that is used to do a particular work. In other words, it describes the energy of an object because of its motion or position, or both.

According to the principle of conservation of mechanical energy, the total mechanical energy of a system is conserved i.e., the energy can neither be created nor be destroyed; it can only be internally converted from one form to another, if the forces doing work on the system are conservative in nature.

**Q30. An object falls freely from a height and its potential energy is continuously decreasing. Explain how mechanical energy is conserved in this process?**

**Answer:** For detailed answer visit here.

**Q31. How does energy dissipation take place?**

**Answer:** When one type of energy is transformed to another type then a part of the energy is dissipated as heat, sound, light, etc. From dissipation of energy we mean that during the process of transformation or transmission, a part of the energy is converted in such form that is either not required or not usable by us. To understand energy dissipation, the best example is electricity that we use at home.

**Q32. From the production of electrical energy to its consumption at home how does energy dissipation takes place?**

**Answer:** First of all, steam is produced from the heat energy to rotate the turbine. Mechanical energy of turbine, as kinetic energy, is used to operate the generator. In this process also a part of energy is lost. Turbine helps in electricity production in the generator. Electric energy produced by the generator is converted into kinetic energy of electric charges. With the help of conductors this electrical energy is transmitted to our houses. During the process a part of electrical energy is lost during transmission, distribution and storage.

**Q33. How are work, energy and power related to each other?**

**Answer:** Work – Work is said to be done when a body or object moves with the application of external force. We can define work as an activity involving a movement and force in the direction of the force. For example, a force of 30 newtons (N) pushing an object 3 meters in the same direction of the force will do 90 joules (J) of work.

Energy – Energy is the ability to perform work. Energy can neither be created nor destroyed. It can only be transformed from one kind to another. The unit of Energy is same as of Work i.e. Joules. Energy is found in many things and thus, there are different types of energy.

All forms of energy are either kinetic or potential. The energy in motion is known as Kinetic Energy whereas Potential Energy is the energy stored in an object and is measured by the amount of work done.

Power – Power is a physical concept that has several different meanings, depending on the context and the information that is available. We can define power as the rate of doing work. It is the amount of energy consumed per unit time.

**Q34. What do you mean by electrical energy? How is electrical energy obtained from a coal power plant?**

**Answer: **Electrical energy is the energy derived from electric potential energy or kinetic energy of the charged particles. In general, it is referred to as the energy that has been converted from electric potential energy. We can define electrical energy as the energy generated by the movement of electrons from one point to another. The movement of charged particles along/through a medium (say wire) constitutes current or electricity.

In coal power plants, heat energy or mechanical energy is produced by different processes. A part of energy is lost during this process. Steam is produced from the heat energy to rotate the turbine. Mechanical energy of turbine, as kinetic energy, is used to operate the generator. In this process also a part of energy is lost. Turbine helps in electricity production in the generator. Electric energy produced by the generator is converted into kinetic energy of electric charges.

**Q35. How is electrical energy produced from hydroelectric plants?**

**Answer: **Hydroelectricity is produced by utilizing the gravitational force of falling water. To this end, the hydropower plant requires a dam. This dam is placed on a source of water, preferably a river. The dam is a massive wall that blocks the flow of the river, therefore, a lot of water collects behind the dam. Near the bottom of the dam, there is an intake from which the water is let into the dam. This intake leads to a drop through the penstock inside the dam. The device used to obtain energy from the falling water here is a turbine. Thermal power plants also use turbines but there are some major differences between the steam turbines and hydro turbines. The turbine is connected by a shaft to the generator. When the water rotates the turbine, electrical energy is generated.

**Q36. How can we reduce the dissipation of electrical energy?**

**Answer: We can reduce the **dissipation of electric energy by following the mentioned below points:

- We should switch off electrical appliances when not in use.
- Use LED or CFL in place of incandescent bulb.
- Use electrical appliances with star rating.

**Q37. What improvements can be done to make air conditioning more efficient in homes?**

**Answer: We should take following steps to make air-conditioning more effective:**

- Walls and roofs of houses should have insulators in them.
- Outer walls should be painted in white or light colour.
- Hollow bricks should be used while making the walls.

**Q38. What is electric power? How is the consumption of electric power calculated? Explain by giving an example.**

**Answer: **Electric power is the rate at which work is done or energy is transformed in an electrical circuit. Simply put, it is a measure of how much energy is used in a span of time.

The formula for electric power is given by P = V× I

where,

P is the power

V is the potential difference in the circuit

I is the electric current

Example: A 9V battery is connected to a resistor having the resistance of 10 Ω. What is the current and power across the resistor?

Solution:

I = V/R = 9/100 = 0.9 A

P = VI = 9 × 0.9 = 8.1 J/s or 8.1 W

**Q39. When we switch on a bulb to get light then what energy transformations take place?**

**Answer:** Electric energy is converted into light energy and heat energy, when we switch on a bulb to get light. This happens because of heating effect of electric current. The filament becomes red hot due to heating effect and starts emitting light.

### RBSE Class 10 Science Chapter 11: Essay Type Questions and Solutions

**Q40. What is energy? Prove that the work performed by an object is equal to the difference between kinetic energy of the objects in two different states.**

**Answer: **We can define energy as the strength to do any kind of physical activity. Thus, they say, “Energy is the ability to do work”.

By using third equation of motion:

v^{2} = u^{2} + 2as

a = v^{2} – u^{2}/2s

According to the Newton’s Second Law of Motion

F = ma

Or F = m (v^{2} – u^{2}/2s)

Or F.s = 1/2m (v^{2} – u^{2})

Since, W = F.s

Hence, W = 1/2m (v^{2} – u^{2})

Or W = 1/2mv^{2} – 1/2mu^{2}

The equation shows that work done is equal to change in kinetic energy of object.

**Q41. What is electric energy? How is electrical energy produced in the following plants? Explain.**

(a) Hydro electric plant

(b) Wind energy plant

(c) Solar energy plant

**Answer:** Electrical energy is the energy derived from electric potential energy or kinetic energy of the charged particles. In general, it is referred to as the energy that has been converted from electric potential energy. We can define electrical energy as the energy generated by the movement of electrons from one point to another. The movement of charged particles along/through a medium (say wire) constitute current or electricity.

a) Hydroelectric power: In hydroelectric plants potential energy of water is increased by increasing the height of water level mostly by forming dams. This energy is converted to kinetic energy by flow of water and is fed to turbine. The turbine rotates and the associated generator produces electrical energy.

b) Wind energy plant: In a wind mill the turbine is rotated by the kinetic energy of wind and generator associated with turbine produces electricity. This renewable energy resource is most environment friendly as compared to other power plants.

c) Solar energy plant – Energy from Sun is concentrated with the help of lens and mirrors and then it is converted into heat. The heat is used to get steam. This steam rotates the turbine and the associated generator produces electricity.

**Q42. Total energy of an ideal simple pendulum is conserved. Prove this statement by calculating its energy in different stages of its oscillations.**

**Answer: **When the bob of a simple pendulum is displaced to one side of its equilibrium position then its centre of gravity is lifted up. The work done in the process is stored as potential energy of the pendulum. When bob is released from this position then it returns towards equilibrium. Its potential energy decreases during this movement from B to A. At A its potential energy is lowest and because of speed of bob it has highest kinetic energy at point A.

Because of this kinetic energy the pendulum moves towards the other side of the equilibrium. During this movement its kinetic energy decreases and again its potential energy increases. At point C the speed of pendulum is zero. At this point the kinetic energy of the pendulum is zero and its potential energy is highest. The pendulum returns to its equilibrium position because of this acquired potential energy.

Let m is the mass of the bob and l is the length of string to which it is suspended. The potential energy for displacement x of the bob.

E_{p} = ½ mg/l .x^{2}

If k = mg/l ( ∵ m,g and l are constant)

Then potential energy E_{p} = ½ kx^{2}

Similarly, when a spring, whose spring constant k, is displaced by distance x from the equilibrium with the elastic limits then the potential energy of the spring will be 1/2kx^{2}.

E_{p} = 1/2kx^{2}

**Q43. Explain various energy dissipations during transformations of energy. What can be done to reduce the dissipation of energy?**

**Answer: **When one type of energy is transformed to another type then a part of the energy is dissipated in the form of heat, light and sound energy. For example; When a ceiling fan is switched on, electrical energy is converted into kinetic energy. During this change, some of the energy is converted into heat energy which is evident from ceiling fan becoming hot.

Following steps can be taken to reduce dissipation of energy:

- Machine should be maintained properly and timely repaired.
- In machines, lubricants should be used.
- More efficient machines should be designed; eg: a car with better mileage.
- Electrical appliances with five star rating should be used.

**Q44. Prove that the mechanical energy of a freely falling object remains constant at all points of its path under gravitational field.**

**Answer: **When we uplift any object from the surface of the earth then we have to do work against the gravitational acceleration. This work done on the body increases the energy of the object and part of the potential energy of that object.

Maximum force required to uplift the object from the surface of the earth is equal to the weight object. If an object of mass m is lifted to a height of h from the surface of the earth then minimum force required is equal to its weight mg.

Potential energy of the object at height h = work done against the gravitational force.

= Force × Displacement

=mg h

Therefore, potential energy of an object of mass m at a height h from the surface of the earth will be mgh. Potential energy depends upon the height of the object from the earth and does not depend upon the height of the object from the earth and doesn’t depend upon the path to reach that height.

### RBSE Class 10 Science Chapter 11: Numerical Questions and Solutions

**Q45. An electron is moving with a velocity of 1.2 x 10 ^{6} m/s. If the mass of the electron is 9.1 x 10^{-31} kg then find its kinetic energy. (6.55 x 10^{-19}J).**

**Answer:** Given,

Velocity = 1.2 x 10^{6} m/s

Mass = 9.1 x 10^{-31} kg

We can calculate the kinetic energy by using the formula

KE = 1/2 mv^{2}

= 1/2 x 9.1 x 10^{-31} x (1.2 x 10^{6})^{2}

= 1/2 x 9.1 x 10^{-31} x 1.44 x 10^{12}

= 6.55 x 10^{-19}J

**Q46. A machine takes an object of 40 kg mass to a height of 10 m. Calculate the work done. (g = 9.8m/s ^{2}). (3.92 kJ).**

Answer: Given,

Mass = 40kg

h = 10m and

g = 9.8m/s^{2}

So, we can calculate work done by using the formula

W = F.s = mgh

W = 40kg x 9.8m/s^{2} x 10

m = 3920J = 3.92kJ

**Q47. An object of mass 6 kg falls from a height of 5 m. Calculate the change in potential energy. (g = 10 m/s ^{2}) (300J)**

**Answer:** Given,

Mass = 6 kg, h = 5 m

PE = mgh

= 6 kg x 10 ms^{-2} x 5 ms

= 300 J

**Q48. Spring constant of a spring is 4 x 10 ^{3} N/m. How much work will have to be done to compress it by 0.04 m? (3.2J)**

**Answer: **We can calculate work done by using the formula,

W = 1/2 kx^{2}

= 1/4 x 4 x 10^{3}N/m (0.04m)^{2}

= 3.2N/m

= 3.2J

**Q49. 0.04J work is required to be done on a spring to stretch it by 0.02m When a spring is pulled up to 0.02 m. Calculate the spring constant. ( 2 x 10 ^{3} N/m)**

**Answer: **We can calculate spring constant by

W = 1/2 kx^{2}

K = 2W/x^{2}

= 2 x 0.4/(0.02)^{2}

= 0.8/0.0004

= 8 x 10^{-1}x 10^{4}/4

= 2 x 10^{3} J/m^{2}

= 2 x 10^{3 }Nm/m^{2}

= 2 x 10^{3} N/m

**Q50. Calculate the power used by an engine taking 200 kg mass at 50 m height in 10 secs. (g = 10m/s ^{2}) (10kW)**

**Answer:** Work done can be calculated as:

W = mgh

200 x 10 x 50 = 100000J

Power = P = W/t

= 100000J/10s

=10kW.

**Q51. In a house 5 electrical appliances are used for 10 hours per day. Out of these, two appliances are of 200 W and three appliances are of 400 W. Calculate the energy consumed in one day in terms of electrical units. (16 units).**

**Answer: **Total power of 5 electrical appliances

= 2 x 200 W + 3 x 400 W

= 200 W + 1200 W = 1600 W

= 1.6 kW

Units per day = 1.6 x 10 h = 16 units.

**Q52. An object of 40 kg mass is moving with velocity 2 m/s. Now, force is applied on the object such that its velocity is 5 m/s. Calculate the work done by the force. (420J).**

**Answer: **We can calculate work done by using the formula

W = 1/2 m(v^{2} – u^{2})

= 1/2 x 40(5^{2} – 2^{2})

= 20(5 + 2) (5 – 2)

=20 x 7 x 3

= 420J

**Q53. If an object of mass 50 kg is lifted to 3 m height then calculate its potential energy. Now let it freely fall and find its kinetic energy when it is just at middle of its path. (g = 10m/s ^{2}). (1.5kJ, 750kJ)**

**Answer: **We can calculate potential energy as

PE = mgh

= 50 x 10 x 3

= 1500J

From the given question, we know that KE is zero at that height hence mechanical energy of object at 3m height = 1500J3

At half th height i.e. 1.5m, potential energy can be calculated as follows:

PE = mgh

= 50 x 10 x 1.5

= 750kJ

At this stage also the mechanical energy would be 1500J

Hence, KE = Mechanical energy – Potential energy = 1500J – 750J = 750kJ

**Q54. A block of 8 kg is moving with 4m/son a frictionless surface. The block compresses a spring and comes to rest. If spring constant is 2 x 10 ^{4}N/m then find the compression in the spring. (0.08)**

Answer: Kinetic energy of the block can be calculated as follows:

1/2 mv^{2}

= 1/2 x 8 x 4^{2} = 64J

The KE will be equal to work done on compressing the spring. Hence, compression can be calculated as follows:

W = 1/2 kx^{2}

64 = 1/2 x 2 x 10^{4} x x^{2}

x^{2} = 64/10^{4}

x = 8/10^{2}

= 0.08m