# NCERT Solutions for Class 9 Science Chapter 11 Work and Energy

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NCERT Solutions for Class 9 Science Chapter 11 Work and Energy: When a person is holding a bucket of water for 10 minutes we can tell he is physically doing work but according to Physics he is not doing work because bucket has no displacement. The NCERT Solutions for Class 9 Science Chapter 11 starts with the concept of work in physics and the condition to say that a work has been done by a force. Other topics in the NCERT Solutions for Class 9 Science are kinetic energy, potential energy, the law of conservation of energy power and commercial unit of energy.

When work is said to be done?

Work is said to be done when a force is applied and due to the force, the objects have a displacement.

If F is the constant force acting on the object and S is the displacement then work W= FS. The unit of work is Joule and 1 Joule= 1N m.

An important topic of NCERT Solutions for Class 9 Science Chapter 11 is Energy: Energy is the capability of doing work. The unit of energy is the same as that of energy.

Kinetic Energy: The kinetic energy of an object moving with a certain velocity is the work done on the object to acquire that velocity. Suppose initial velocity of an object is u, by applying a constant force we changed the velocity to v. Let the acceleration due to force be a and S be the displacement then work done.

$\\W=FS\\s=\frac{v^2-u^2}{2a}\\W=mas\\W=ma\frac{v^2-u^2}{2a}\\W=\frac{1}{2}m(v^2-u^2)$

This work done is known as kinetic energy

If the body starts from rest then u=0 then the kinetic energy $=\frac{1}{2}mv^2$

Potential Energy: Potential energy of an object is the energy possessed by an object due to its position.The gravitational potential energy. The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity. Suppose the mass of the object raised is m and the height to which object is raised is h then,

Potential Energy = mgh

One of the main topics of  NCERT Solutions for Class 9 Science Chapter 11 Work and Energy is given below:

Work-Energy Theorem: When energy gets transformed total energy gets conserved. That is energy can neither be created nor be destroyed. For example, When we open a tape the potential energy stored in the water tank at height is converted to the kinetic energy of the water. When an electric fan is working electrical energy is converted to mechanical energy and certain energy are lost in the form of heat.

The main topics of  NCERT Solutions for Class 9 Science Chapter 11 Work and Energy are:

11.1 Work

11.1.2 Scientific Conception Of Work

11.1.3 Work Done By A Constant Force

11.2 Energy

11.2.1 Forms Of Energy

11.2.4 Potential Energy Of An Object At A Height

11.2.5 Are Various Energy Forms Interconvertible?

11.2.6 Law Of Conservation Of Energy

11.3 Rate Of Doing Work

11.3.1 Commercial Unit Of Energy

Power: The rate of doing work is Power. Power measures the speed of work done, that is, how fast or slow work is done. If an agent does a work W for time t, then power

$P=\frac{W}{t}$

The unit of power is watt.

Commercially electric bills are calculated in terms of unit and 1 unit= 1Kilo Watt-hour

$1KWH=1\times1000W\times60\times60sec=3.6\times10^6\ joule$

That is if equipment with power rating 1 KW is operated for 1 hour then it consumes 1 unit of energy.

There are many questions mentioned in the NCERT book, you will get all the NCERT solutions here. Doing all the questions in the book are mandatory for exams.

## NCERT Solutions for Class 9 Science- Chapter Wise

 Chapter No. Chapter Name Chapter 1 Matter in Our Surroundings Chapter 2 Is Matter Around Us Pure Chapter 3 Atoms and Molecules Chapter 4 Structure of The Atom Chapter 5 The Fundamental Unit of Life Chapter 6 Tissues Chapter 7 Diversity in Living Organisms Chapter 8 Motion Chapter 9 Force and Laws of Motion Chapter 10 Gravitation Chapter 12 Sound Chapter 13 Why Do We Fall ill? Chapter 14 Natural Resources Chapter 15 Improvement in Food Resources