# Work and energy class 9 notes

work done definition:

Any work is said to be done if it satisfies the following condition:

i) A amount of force should be act on a particular body/object.

ii) And that particular object/ body must be displaced by the applied force.

The amount of work done by the body is equal to the product of the force applied and the displacement.

The work done by a body may be positive or negative depending on the direction of displacement. When the force acts opposite to the direction of displacement then the work done is negative. On the other hand when the force acts in the direction of displacement,then the work done is positive.

work done formula :

Work done= Force x displacement

or

W= FS

The work done unit is Newton meter(N m) or joule(J).

work done examples:

Let us understand the concept by illuminating a simple example.

* Suppose a football(object) is placed on the ground at rest position.

* Consider a player is coming next to the ball and (applied some amount of force)kicked it towards the goal point.Due to which a displacement takes place by the football.

*Then the displaced football goes to the goal point.

From this we can say that, the  work is done by the body(football) due to applied force by the player.

# Energy:

Any kind of ability by an object or body to do any kinds of work is known as energy.The unit of energy is Joule(J).

# Energy is present around us in various forms which includes mechanical energy which includes mechanical energy(kinetic energy and mechanical energy),heat energy, chemical energy, electrical energy and light energy.

kinetic energy definition:

The kinetic energy of a body is the type of mechanical energy, which increasing with increase in motion of that particular body, i.e. it is the energy posses by an particular object due to its motion.It is denoted as Ek. The  kinetic energy unit is given by Joule(J).

kinetic energy with example :

 Running

# Potential energy definition:

The potential energy of a body is the energy  possessed by that particular body due to virtue of its position.It is denoted as Ep.

The potential energy unit is same as kinetic energy i.e. Joule(J)

potential energy examples :

 Television

# To understand the phenomena more deeply, lets try to understand it  by taking the example of television.

When the television is turned off, the electrical energy comes to the television and remains stored in it, until we start the television. Once we start it, the stored energy, runs the television. In this scenario the stored energy is the potential energy, which causes the television to run.

# Law of conservation of energy definition:

According to this law "energy can neither be created nor be destroyed in a chemical reaction.It can only be converted from one form to another form".

Law of conservation of energy example:

 ten-pin bowling

To understand the phenomenon  more deeply, let us consider an example of ten-pin bowling game, in which, when a ball is thrown by a person, it aquires some amount  of kinetic energy, and after colliding with the pins, this particular energy is then transferred from the ball to the pins which cause them to move & those pins also transferred  their energy with one another through collisions,

# Power:

The power of a body may be defined as the rate of doing some amount of  work.The unit of power is Watt (W)& its formula is given by:

Power=Work/time

or

P=W/T

# we know that Joule is the unit of energy,but its demerits is that it is too small to express large quantity of energy.For bigger energy, we use bigger unit of energy known as "kilowatt hour (KWh)".

1KWh is the amount of energy consumed in 1hr at a rate of 1000 joule/sec.

i.e.

1KWh = 1KW x 1h

=1000W x 3600s

=3600000J

=3.6 x 106    J

The kilowatt hour unit is used to calculate the electrical energy in our household, industries.

Notes for work energy class 9:

1) A force of 7N act on an object. The displacement is,say 8m, in the direction of force.Let us take it that the force acts on the object through the displacement.What is the work done in this case? {page no-148}

Ans:  Given,

Force = 7N

displacement = 8m

By applying work done formula, we get

W = F x S

= (7 x 8 ) Nm

= 56 Nm or J.

2) when do we say that work is done? {page no-149}

Ans: An work is said to be done, when a force displaces an object.

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

Ans: The required expression is given by:

Work done = Force x Displacement

W = F x S

4)Define 1 joule of work.

Ans: It can be defined as the work done by a body of 1N force to displace an object at 1m.

5) A pair of bullocks exerts a force of 140N on a plough.The  filed being ploughed is 15m long. how much work is done in ploughing the length of the field ?

Ans: Given,

force = 14N

displacement = 15m

W = ?

By applying work done formula, we get

W = F x S

= (14 x 15 ) Nm

= 210Nm or J.

6)What is the kinetic energy of an object ?

Ans: The kinetic energy of  an object is defined  as a  type of mechanical energy, which increasing with increase in motion of that particular body, i.e. it is the energy posses by an particular object due to its motion.

7) Write an expression for the kinetic energy of an object.

Ans: The expression for kinetic energy is given by:

Ek = 1/2 mv2

8) The kinetic energy  of an object of mass, m moving with a velocity of 5m/s is 25J. What will be its kinetic energy, when its velocity is doubled ? What will be its kinetic energy, when its velocity is increased three times ?

Ans: Given,

Kinetic energy( Ek) =25J

Velocity of the body (V) = 5 m/s

CASE I :

If the value of  velocity is doubled, then the value of  Ek  =  1/2mv4

= m x 54/2

= (m x 625)/2    Nm or J.

CASE II :

If the value of  velocity is doubled, then the value of  Ek 3 x (1/2 mv2)

= 3 x (mx25 )/2 Nm or J

8) what is power ?

Ans: The power of a body may be defined as the rate of doing some amount of  work.

9) Define 1watt power.

Ans: 1 watt power can be defined as the power  of  a body to do any amount of work at a rate of 1joule per second.

10) A lamp consumes 1000 J of electrical energy in 10s. What is its power?

Ans: We have,

workdone= 1000 J

& time = 10sec

By applying:

power= work/times

p = 1000/10

P = 100 watt.

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