NCERT solutions for class 9 science chapter 11 covers the most essential topics of work, energy, and power. It explains the basic concept behind Work and Energy along with relating them to real-life scenarios. The scientific definition of work is the force applied over a distance. 

The formula for work is Work = Force x Distance.

Force is measured in Newtons (N) and distance in meters (m), so the units of work are Nm or Joules (J). If you lift a one Newton object one meter, you have done one Joule of work.

When work is done by a constant force, the force is always in the same direction and is applied along a line of action that never changes direction. The work done by such a force equals the magnitude of that force multiplied by the distance through which it acts.

So, if you lift a heavyweight up one foot and drop it, it does work on whatever it lands on. And if you push a lightweight through a long distance, that also does work. The same amount of total work is done in both cases.

Positive work is the product of force and distance in the direction in which the force is applied. Such work has been done by a force when it tends to elongate or extend a body, as when one lifts a weight with an upward movement.

Thus, positive work can be given by 

W = F x s

And negative work will be given by

W = F x (-s)

Or, W = -F x s

Negative work is the product of force and distance in the direction opposite to that in which the force is applied. Such work is done by a force when it tends to shorten a body, as when one lifts a weight against gravity, or when one presses down on a spring.

Examples of Work

When forces are exerted upon each other, and motion takes place, there may be positive and negative work done simultaneously; for example, if two men pull at each other's hands with equal force, but in opposite directions, there will be no displacement; but if one man moves forward while the other remains stationary, both have done positive work upon each other.

Energy is the capacity of a physical system to do work.

This is different from the everyday meaning of energy. In ordinary speech, we think of energy as a property of people and objects, like mass. A living thing has energy; so does an object that is heavy and hard to lift. But in physics, energy is not a property of an object; rather, it is a relationship between two objects. 

The SI unit of energy is the joule (J). 

There are various forms of energy in the world. These include mechanical energy, heat energy, etc. Kinetic and potential energy sum up to make mechanical energy. 

The definition of kinetic energy is "the energy possessed by a moving object." It is calculated using the following formula:

KE = 1/2 * m * v²

where KE is the kinetic energy, m is the object's mass and v is its velocity.

Potential energy, on the other hand, is the stored energy of an object due to its position. It can be calculated using this formula:

PE = m* g * h

where PE is the potential energy, m is the mass, g is the acceleration due to gravity and h is the height.

The more mass an object has and the faster it's moving, the more kinetic energy it has.

You can easily see this effect in a car crash when a large car with a lot of kinetic energy crashes into another car. The larger car has more mass and thus more potential to damage the other vehicle.

Another example: you need to add a lot of energy to put an apple in the air, but once it is there and moving, it will take quite a bit of time before it loses all its kinetic energy and falls back to earth.

An example of potential energy is: if you are standing on top of a staircase, you are eight feet closer to the centre of the earth than someone who is standing at the bottom of it. That means that if you jump off, gravity will accelerate you towards the ground faster than it would them. You will take less time to hit the ground than they would; so, because velocity equals distance divided by time, your velocity will be higher when you land than theirs would have been.

The Law of Conservation of Energy says that energy is never created or destroyed; it only changes form. When you push a child on a swing, you do work on it. The child's speed increases and her potential gravitational energy decreases, but her total energy remains the same. This is because she gains kinetic energy as her potential energy decreases. If she started with 15 Joules of potential energy, then she must end up with 15 Joules of kinetic energy.

Thus, KE+PE = constant.

Power is the rate at which you do work. The SI unit of power is the Watt, which is one Joule per second. In other words, a Watt is the rate at which energy is produced or consumed. A 100 Watt light bulb consumes energy at a rate of 100 Joules per second.

The commercial unit of power is kWh or the KiloWatt-hour. It is also referred to as “units”. 1 unit or kWh is 1000 Watts of power exerted in an hour. 

1 kWh = 1000x3600 Joules

1 kWh = 3.6x10^6 Joules

Work and Energy is essential chapter of physics. It is a fundamental concept that cannot be missed in any examination. Work, energy and power are the basic fundamentals of physics. These three concepts are interrelated to each other. 

We have prepared a comprehensive set of NCERT Solutions for Class 9 Physics to help you become familiar with all types of questions that might come into the subject. This will help you to practise more effectively. 

It is often seen that students find this chapter difficult and confusing. This is because of unclear concepts and a lack of practice. To help students overcome this problem, we have provided a detailed explanation of each and every concept involved in the chapter along with examples. We have also provided a plethora of solved and unsolved questions that will help you get a deeper understanding of the topics covered in this chapter. All our study materials are prepared by top teachers and are completely free.

    1.  What is Work and Energy Class 9th?

Work done can be defined as the product of the force applied on an object and the displacement experienced by the object. Energy is defined as the capacity of doing work. When the displacement of an object is zero, then the work done is also zero. The SI unit of Work and Energy is Joules.

     2. How many types of work are there according to NCERT Class 9 chapter 11?

There are three types of work. It is a scalar quantity, thus, the direction is not required to define it. If the displacement of an object is zero, then it is zero work. If the direction of force applied and the direction of resulting displacement of the object are the same, then it is positive work. If the direction of force applied and the direction of the object’s displacement is opposite, then it results in negative work. 

     3. How many types of energy are there?

As per NCERT class 9, Work and Energy, there are two kinds of energy. First is kinetic energy is the energy that is the result of the velocity of an object. The larger the velocity and mass of the object, the more kinetic energy. Another kind is called potential energy. It is the resultant of the position of the object. The higher the position of the object concerning the sea level, the more potential energy. 

     4. What is the commercial unit of power?

The SI unit of power is a Watt. However, as the unit is too small to be used in day-to-day life, the commercial unit of power used is a kiloWatt. To measure energy, “units” are used in everyday life. 1 unit is equal to 1 kWh. That is 1 kilowatt-hour. 

     5. How many questions are there in NCERT science class 9 chapter 11?

In the back exercises provided at the end of chapter Work and Energy, there are 21 questions. These include conceptual questions, numerical questions, and thinking questions. There are several in-text questions provided after every topic as well.