# NCERT Class 9 Work and Energy Notes pdf

These are NCERT Class 9 Work Energy and Power notes pdf. These notes will help you a lot in securing good marks in your board exams.

### Some Common Terms

**Work**: Work is said to be done by a force acting on a body if it produces displacement in it.

Work done is equal to the product of force and displacement. It is a scalar quantity. It is denoted by W.

If F is the force and S is the displacement, then **W= F x S**. The S. I unit of work is joule. In CGS system its unit is erg. 1J = 10^{7 }erg.

**Joule** : It is the amount of work done on an object when a force of 1 N displaces it through 1m along the line of action of the force.

1J = 1Nm (where 1N = Newton)

**Kinetic Energy**: It is the amount of work done on a body to make it move with a certain velocity.

It is the energy possessed by a body by virtue of its motion. K.E =1/2 mv^{2 }(Where m is the mass of body moving with a velocity of v.

**Gravitational Potential energy**: It is defined as the work done in raising a body from the ground to a particular point against gravity.

It is the energy possessed by a body by virtue of its position or configuration.

**Law of conservation of energy:** According to this law, the total energy before and after transformation remains the same i.e. total energy of the system remains unchanged.

**Kilowatt hour (KWh)**: It is the amount of electric energy consumed in one hour at the rate of 1000 J/S (1kw).

**Acceleration due to gravity(g)**:It is the acceleration produced in a body under the action of gravity.

## Work and Energy Class 9 Textual Questions & Answers

In this section, you will get NCERT Class 09 work and energy textual questions and answers. These notes will help you a lot in preparing you for your board exams.

### A force of 7N acts on an object. The displacement is, say 8m, in the direction of the force.Let the force acts on the object through the displacement. What is the work done in this case?

Here force = 7N & Displacement = 8m

Now Work = force x displacement

=> W = 7N x 8m = 56 J

### When do we say that work is done?

Work is said to be done by a force when acting on a body produces displacement in it.

It is equal to the product of force and displacement, it is denoted by W.

If F is the force and S is the displacement W = (F)(S).

### Write an expression for the work done when a force is acting on an object in the direction of its displacement.

W = (F)(S)

### Define 1 J of work.

We know that W = (F)(S)

If F= 1N and S=1m, then we can say that W = (1N)(1m) =1J

So we can say thay work done on a body is said to be 1J if the force of 1 N acting on it displaces it through 1m in the direction of force.

### A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work done is ploughing the length of the field?

Here force F = 140N, and displacement S =15m.

Therefore W = (f)(s) = 140N x 15m

=> W = 2100 Joule.

### What is the kinetic energy of an object?

The energy possessed by virtue of its motion is called its kinetic energy.

### Write an expression for the kinetic energy of an object?

Kinetic Energy = = Â½ mv^{2}

### The kinetic energy of an object of mass m moving with a velocity of 5 ms^{-1 }is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased threeÂ times?

Here KE = 25 J

=> Â½ mv^{2} = 25 J

=> Â½ m(5)^{2} = 25J

=> m= 2kg

Now when the velocity of the body is doubled, its Kinetic energy is given by;

KE = Â½ mv^{2} = Â½ (2) (10)^{2}

=> KE = 100 J

Now when the velocity of the body is made three times, its Kinetic energy is given by;

KE = Â½ mv^{2} = Â½ (2) (15)^{2}

=> KE = 225J

### What isÂ Power?

Power is defined as the rate of doing work or the rate of transfer of energy.

It is denoted by P and its SI unit of power is watt.

### Define 1 watt of power?

Power = workdone/Time taken

=> P= W/t

Now If W = 1 joule & t = 1S, then,

P= 1 joule/second = 1 watt

Hence power of a body is said to be one watt is it does one joule of work in one second.

### A lamp consumes 100 J of electrical energy in 10 s. What is its power?

Here work done (w) = energy consumed = 100 J

And time taken (t) = 10 second

Now power = work done/ time taken

=> Power = 100 j/10s = 10 watt.

### Define average power?

Since the power of an agent may vary with time. Therefore we use the concept of average power.

We obtain average power by dividing total energy consumed by the total time taken.

=> Average power = Total energy consumed/Total time taken.