NCERT Solutions for Class 9 Science Chapter 10 - Work and Energy
Have you ever thought of how you feel exhausted after climbing up a flight of stairs or how a moving bicycle slows down as you use the brakes? This is discussed through these examples of everyday life, in NCERT Solutions for Class 9 Science Chapter 10 – Work and Energy in the simplest way, where new and very important concepts such as force, energy and power, besides work and energy, are introduced. The NCERT Class IX Science Chapter 10 Solutions are devised in such a way that they facilitate both easy learning and exam-focused learning. This chapter is also crucial in CBSE board examinations, school exams, as well as national competitive exams such as NTSE and Olympiads, since there is always one or two questions in school tests on this topic.
This Story also Contains
- NCERT Solutions for Class 9 Science Chapter 10: Work and Energy
- Work and Energy Class 9 NCERT: Solved Exercise Questions
- Class 9 Science NCERT Chapter 10: Higher Order Thinking Skills (HOTS) Questions
- Class 9 Science Chapter 10 Work and Energy: Topics
- Approach to Solve Questions of Work and Energy Class 9
- Benefits of NCERT Class 9 Science Work and Energy Solutions
- How Can NCERT Solutions for Class 9 Science Chapter 10 Help in Exam Preparation?
- NCERT Solutions for Class 9 Science Chapter-wise
All 21 questions in the textbook in this chapter are answered in a step-by-step manner and accompanied by 5 Higher Order Thinking Skills (HOTS) questions. With easy explanations, solved examples, and useful diagrams, the NCERT solutions discuss some of the most important concepts of work, energy, power, and the law of conservation of energy. Important formulas have also been provided as a complete listing to enable you to revise within a short duration, besides being able to solve numerical questions effortlessly. These NCERT Solutions of Science Class 9 Work and Energy are made by subject experts and are in line with the latest CBSE curriculum, and have been represented in a very simple and easy manner. Students will also be able to learn more quickly, construct powerful ideas, and have a higher sense of confidence when taking tests using these class 9 science chapter 10 Work and Energy question answers.
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NCERT Solutions for Class 9 Science Chapter 10: Work and Energy
The NCERT Solutions for Class 9 Science Chapter 10: Work and Energy provide clear, step-by-step answers to all textbook exercises, making complex topics easy to understand. Students can also download the free PDF for quick revision and exam preparation as per the latest CBSE guidelines.
Work and Energy Class 9 NCERT: Solved Exercise Questions
The NCERT Solutions of Class 9 Science Chapter 10: Work and Energy; Exercise Questions comprise well-formatted responses to all the problems in the chapter textbook covering important concepts such as work, energy, power and the law of conservation of energy. These class 9 science chapter 10 Work and Energy question answers guide students to reinforce ideas and enhance precision in tests.
• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A windmill 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:
(i) Work done by Suma is negative as the force and displacement are in opposite directions.
(ii) Work done is zero as the gravity on the load is acting vertically downward, whereas its displacement is in a horizontal direction.
(iii) Work done is positive as both force and displacement are in an upward direction.
(iv) Work done is zero as there is no displacement involved.
(v) Work done is positive as the force is acting in the direction of the motion.
(vi) Work done is zero as there is no displacement of the grains.
(vii) Work done by wind force is positive as it supports the motion of the boat.
Answer:
We can see that there is no net displacement in the vertical direction. It has only horizontal displacement. Thus, the work done by vertical forces on the stone will be zero.
The force of gravity is acting vertically downward; thus, the work done by it is zero.
Q3. A battery lights a bulb. Describe the energy changes involved in the process.
Answer:
The chemical energy stored in a battery is transformed into the heat energy which glows the filament of the bulb. Further, the heat energy is converted into light energy.
Answer:
By the equations of motion, we can write :
$v^2\ =\ u^2\ +\ 2as$
$s\ =\ \frac{v^2\ -\ u^2}{2a}$
$=\ \frac{2^2\ -\ 5^2}{2a}\ =\ \frac{-21}{2a}\ m$
The work done is :
$W\ =\ F.s$
$=\ 20a\times \frac{-21}{2a}$
$=\ -210\ J$
Thus work done is - 210 J.
Answer:
The displacement of the object is horizontal on the table. We know that the gravitational force is acting in a downward direction. There is no displacement vertically.
Thus, the work done by the gravitational force is zero.
Answer:
No, there is no violation of the law of conservation of energy. At the maximum height, the energy is in the form of potential energy. When the object reaches the ground, its potential energy decreases, whereas its kinetic energy is increasing (as the velocity of the object is increasing). Thus, there is no loss of total energy (energy transformation may take place ).
Q7. What are the various energy transformations that occur when you are riding a bicycle?
Answer:
The muscular energy of a person is transformed into the form of mechanical energy, which helps to rotate the wheel of a bicycle.
Answer:
Since the rock doesn't move, the net displacement is zero. As a result, the work done by the force is zero.
The energy that we apply to the rock gets transformed into the form of heat.
Answer:
We know that 1 unit of energy is given by: 1 unit = 1 kWh.
Also, $1\ KWh\ =\ 3.6\times 10^6\ J$
Thus, 250 units in joules is given by :
$E\ =\ 250\times 3.6\times 10^6\ =\ 9\times 10^8\ J$
Answer:
The potential energy of an object of mass m at a height h is given as = mgh
The potential energy of the given object is :
$P.E.\ =\ mgh\ =\ 40\times 10\times 5\ =\ 2000\ J$
The potential energy is being converted into the K.E..
Thus, at halfway kinetic energy of the object is :
$K.E.\ =\ \frac{P.E.}{2}\ =\ \frac{2000}{2}\ =\ 1000\ J$
Answer:
The work done by the gravitational force is zero. This is because the satellite is moving in a circular orbit. Thus, the direction of displacement of the satellite is perpendicular to the force of gravity. Hence work done is zero.
Answer:
This depends on the initial state of the object. If the object is in motion initially, then it will continue to be in this state as no external force is acting. But if the object is at rest initially, then the object can't move without an external force.
Answer:
Since there was no displacement thus the work done by man is zero. This work should not be considered relatable to the term 'work' we use in daily life.
The weight against gravity led the man to get tired.
Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer:
The relation between energy and power is given by :
Energy = Power $\times$ Time
Thus, the energy used in 10 hours is :
$E\ =\ 1500\times 10\ =\ 15000\ Wh\ =\ 15\ KWh$
Hence, the energy used by the heater is 15 kWh.
Answer :
In the above figure, point B is the mean position about which the bob rotates.
When the bob is released from point C, it attains some velocity while moving down (up to B) and decelerates and stops at point A.
Thus, points A and C are the maximum height points. And the velocity of the bob at point B will be maximum.
The total energy at points A and C is only the potential energy, as their velocity at these points is zero. And at point B, as the height of the bob is zero thus the total energy is just the kinetic energy.
Thus, in this manner, the conservation of energy takes place (by transforming into some other form).
It eventually comes to rest due to the air resistance. It decelerates the motion of the bob. (as it is a frictional force.)
There is no violation of the energy conservation law, as some amount of energy is converted in the form of heat.
Answer:
At this moment, the energy of the object is :
$K.E.\ =\ \frac{1}{2}mv^2$
Thus, in bringing the object to rest, the work needed is: $=\ K.E.$
$W\ =\ \frac{1}{2}mv^2\ J$
Q17. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer:
Firstly, convert the velocity into SI units.
$v\ =\ 60\times \frac{5}{18}\ =\ \frac{50}{3}\ m/s$
Thus, the work done to stop the car is equal to the kinetic energy of the car.
$W\ =\ \frac{1}{2}mv^2\ =\ \frac{1}{2}\times 1500\times \left ( \frac{50}{3} \right )^2$
$=\ 208333.33\ J\ or\ 208.33\ KJ$
- Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
Answer:
(i) In the first case, the force acting on the object is perpendicular to the displacement of the body. Thus, the work done by the force is zero.
(ii) In this, the force is in the direction of the displacement, so the work done is positive.
(iii) In this case, the direction of force and displacement are opposite to each other. Thus, the work done by the force is negative.
Answer:
Yes. If all the external forces acting on the body balance each other, then the net force acting on the object is zero. Thus, the acceleration produced is zero even if forces are acting on the body. The necessary condition for the zero acceleration is that the net force acting on the body should be zero.
Q20. Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Answer:
The energy consumed by 1 device is given by :
$E\ =\ 500\times 10\ =\ 5000\ Wh$
Thus the energy consumed by 4 devices is $=\ 4\times 5000\ =\ 20\ KWh$
Q21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Answer:
When an object is falling from a height, there is an increase in the kinetic energy of the object. Thus, just before hitting the ground, the kinetic energy of the object is very high. But after hitting the ground, its velocity comes to zero, and thus the kinetic energy becomes zero. The kinetic energy is transformed into other forms of energy, such as heat, sound or some deformation on the ground.
Class 9 Science NCERT Chapter 10: Higher Order Thinking Skills (HOTS) Questions
The NCERT Chapter 10: Work and Energy HOTS Questions of Class 9 Science is structured to help develop critical thinking and the ability to solve problems. These are higher-order questions that are beyond the basic ones, so that the students can be in a position to practice the concept of work, power and conservation of energy in real-life scenarios.
Q.1 A car of mass 1000 kg accelerates from 0 to 20 m/s in 10 seconds. What power is developed by the car's engine?
Answer:
Power developed by the engine of the car P,
$
\begin{aligned}
& P=\text { force } \times \text { velocity } \\
= & (1 / 2) \times \text { mass } \times \text { velocity } ^2 / \text { time } \\
= & \frac{1}{2} \times 1000 \times 20^2=4000 \mathrm{~W}
\end{aligned}
$
Q.2 A car with a constant speed is moving on a straight road. What is the nature of the work done by the engine?
Answer:
Since the car is moving at a constant speed, there is no change in its kinetic energy. Therefore, the net work done on the car by all the forces acting on it is zero, including the work done by the engine.
Q.3 A uniform force of 20 N is applied to a box, displacing it by 5 m. What is the nature of the work done by the force?
Answer:
Work done is given by the equation W = Fd for constant force, where W is the work done, F is the force applied, and d is the distance traveled in the direction of the force. As the force and displacement are in the same direction, the work done is positive.
Q.4 An engine of a car of mass m = 1000 Kg changes its velocity from 5 m/s to 25 m/s in 5 minutes. The power (in KW) of the engine is
Answer:
Power expressed as the rate of change of kinetic Energy -
Power $=$ Work done $/$ time $=$ change in K.E $/$ time
$
\begin{aligned}
& P=\frac{\frac{1}{2} m\left[(25)^2-(5)^2\right]}{5 \times 60} \\
& =\frac{\frac{1}{2} \times 1000 \times 600}{5 \times 60}=1000 \mathrm{~W}
\end{aligned}
$
$
P=1 K W
$
Q.5 A truck of mass 30,000 kg moves up an inclined plane of slope 1 in 100 at a speed of 30 kmph. The power of the truck is (given g = 10 $\left.m s^{-2}\right)$
Answer:
$
\begin{aligned}
& \vec{P}=\vec{f} \cdot \vec{V} \\
& P=f v \sin \theta \\
& P=m g \sin \theta V \\
& =30000 \times 10 \times \frac{1}{100} \times \frac{30 \times 5}{18}
\end{aligned}
$
$P=25 \mathrm{~kW}$
Class 9 Science Chapter 10 Work and Energy: Topics
The Class 9 science Chapter 10: Work and Energy familiarises the students with the basics of transferring and using energy as work takes place. The concepts of work, different kinds of energy, power, and the law of conservation of energy are explained in this chapter, so it is critical in conceptual understanding as well as in preparation for exams.
10.1 Work
10.1.2 Scientific Conception Of Work
10.1.3 Work Done By A Constant Force
10.2 Energy
10.2.1 Forms Of Energy
10.2.4 Potential Energy Of An Object At A Height
10.2.5 Are Various Energy Forms Interconvertible?
10.2.6 Law Of Conservation Of Energy
10.3 Rate Of Doing Work
Approach to Solve Questions of Work and Energy Class 9
The problem-solving strategy of Work and Energy enables the students to know how to apply the concepts, not memorise the formulae. Students are able to relate real-life situations, such as those when lifting a body or when a moving car, etc., are compared with physics concepts by studying step-by-step instructions. This style instils confidence in problem-solving and enhances accuracy in theory and numerical problems in the exams.
- Read the question thoroughly to determine the given values and the unknown that is to be determined.
- Determine whether the question involves work done, energy (kinetic or potential) or power.
- Apply the formula of Work = Force x Displacement x cosθ when necessary.
- Use the formula of kinetic energy, KE = 1/2 mv2 and potential energy, P.E. = mgh, as required.
- Determine whether the law of conservation of energy can be used to simplify the problem.
- In numerical problems, all units should be converted to SI units, and then calculations should be done.
- Use Power = Work done / Time if power is asked.
- Draw a diagram when necessary in order to observe forces and displacements.
- Do the calculations stepwise, and indicate all calculations.
- Review the final answer for correctness and units.
Benefits of NCERT Class 9 Science Work and Energy Solutions
The NCERT Solutions of Class 9 Science Chapter Work and Energy help the students get a clear picture of the relation between energy and work in everyday life. These Work and Energy class 9 question answers offer step-wise answers, hence difficult subjects such as kinetic and potential energy are unproblematic to understand. They also enhance problem-solving skills, hence students are prepared well for exams and clarity in understanding and proper explanations.
- Concept Clarity- Concepts such as work, energy, power and conservation of energy have been explained simply and thus are easy to grasp, as shown by the solutions.
- Step-by-step Solutions- Solutions to each question are clearly described and presented step by step in a style that helps a student learn how to write the answer in the exam.
- Exam Preparation- The solutions are strictly done as per the CBSE syllabus, so that students can be assured to confidently prepare themselves to be presented to the board examinations and competitive tests.
- Practice of HOTS & Numerical Questions - Consists of higher-order thinking questions and numerical problems to reinforce applications-oriented learning.
- Quick Revision Tool - This is helpful when needing to do last-minute revision since its contents list formulas, definitions, and key concepts in brief form.
- Enhances Confidence - The practice with these solutions enables the student to answer the problems quickly and correctly.
How Can NCERT Solutions for Class 9 Science Chapter 10 Help in Exam Preparation?
Class 9 Science Chapter 10, Work and Energy, NCERT Solutions are useful in letting students get ready to take the exams, as they pay attention to the relation between the work done and the energy. In contrast to the earlier chapters, this chapter focuses on learning how energy is converted between different types of energy, as kinetic energy is converted to potential energy, and covers real-life ideas of power and energy conservation. The class 9 science Work and Energy question answers will also help students to solve special numerical problems and conceptual questions, with the help of which they will develop their abilities to think analytically and apply the knowledge more effectively, in case they want to pass the exams successfully.
NCERT Solutions for Class 9 Science Chapter-wise
The NCERT Solutions of Class 9 Science (Science-Chapter-wise) not only give in-depth solutions to every question in the textbook but also the students develop a firm understanding of concepts. The step-wise answers simplify revision, help with exam practice, and develop problem-solving confidence in Physics, Chemistry and Biology.
NCERT Solutions for Class 9 - Subject Wise
Also Read,
Also Check NCERT Books and NCERT Syllabus here:
Frequently Asked Questions (FAQs)
Q: What is the process for identifying whether the work done in NCERT problems is zero, negative, or positive?
A:
In relation to the displacement direction, the force's direction determines the sign of the work done. The following is how to find it in a problem:
- When the force acts in the same direction as the displacement of the object, the work is said to be positive. Pushing a box over a floor is one example.
- When the force acts against the displacement, the work is negative. Friction work is a typical example of NCERT problems.
- A force acting perpendicular (at a 90° angle) to the direction of displacement, such as a satellite orbiting the Earth, or there is no displacement (s=0) are both cases in which work is zero.
Q: What is the most frequent mistake that students make when attempting to solve Law of Conservation of Energy problems in the NCERT exercises?
A:
One of the most common mistakes is the inability to consider all forms of energy at the starting and ending points. Students tend to forget to add either kinetic energy or potential energy when an object is at ground level or an object is at rest. To prevent this, it is always good to begin with the full equation: Total initial energy = total final energy. Next, replace zero with any term that is not applicable (e.g. KE initial equals zero when the object is initially at rest). This will not leave any component of energy behind.
Q: What is the process for identifying whether the work done in NCERT problems is zero, negative, or positive?
A:
In relation to the displacement direction, the force's direction determines the sign of the work done. The following is how to find it in a problem:
- When the force acts in the same direction as the displacement of the object, the work is said to be positive. Pushing a box over a floor is one example.
- When the force acts against the displacement, the work is negative. Friction work is a typical example of NCERT problems.
- A force acting perpendicular (at a 90° angle) to the direction of displacement, such as a satellite orbiting the Earth, or there is no displacement (s=0) are both cases in which work is zero.
Q: What is the most frequent mistake that students make when attempting to solve Law of Conservation of Energy problems in the NCERT exercises?
A:
One of the most common mistakes is the inability to consider all forms of energy at the starting and ending points. Students tend to forget to add either kinetic energy or potential energy when an object is at ground level or an object is at rest. To prevent this, it is always good to begin with the full equation: Total initial energy = total final energy. Next, replace zero with any term that is not applicable (e.g. KE initial equals zero when the object is initially at rest). This will not leave any component of energy behind.
Q: Why do we feel tired even when holding a heavy object without moving it?
A:
Scientifically, no work is done (as there is no displacement), but our muscles use up energy to maintain tension, which makes us feel tired.
Q: Why is the law of conservation of energy important in daily life?
A:
It helps us understand energy transformations in machines, electricity use, vehicles, and natural processes, ensuring efficient use of resources.
Q: Why should I solve Work And Energy Class 9 NCERT Science Chapter 10 questions?
A:
Solving the NCERT questions will give knowledge of how to apply the concepts studied to solve a numerical question and will give conceptual clarity. To solve more problems refer to NCERT Exemplar questions for Class 9 chapter 10 Work And Energy.
Q: What are some Work and Energy Class 9 important questions and answers to focus on for exams?
A:
Some of the Work and Energy Class 9 important questions and answers include numerical problems on work done, types of energy, power, and conservation of energy. Focusing on these will help you score better in exams and build strong concepts.
Q: Are NCERT Solutions for Class 9 Science Chapter 10 sufficient for exam preparation?
A:
While NCERT Solutions for Class 9 Science Chapter 10 are a good resource for exam preparation, it is recommended that students also practice solving other reference books and previous year's question papers to get a better understanding of the concepts and exam patterns.
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