The document discusses work and energy. It defines work as being done when a force causes an object to move, and says work is measured as force times distance. It describes different forms of energy including mechanical, electrical, chemical, and heat. Mechanical energy includes kinetic energy from motion and potential energy from position or condition. The law of conservation of energy is introduced, which states that energy cannot be created or destroyed, only transformed from one form to another. Examples of energy transformations between kinetic and potential energy are provided.

1. What do we see?
Moving parts – gears and
cogs that drive cars,
machines, conveyor belts,
et cetera
Is work done by these
moving parts?
Absolutely yes!

2. What do we see here?
A tractor ploughing or
tilling the soil
Is work done here?
Again, a yes!

3. What do we see here?
A car moving on a stretch
of road
Is work done here?
Again, a yes!
Who/what does the
work here?
The engine and pistons in
the car

4. What does this image
tell us?
A man is about to lift a
weight bar
Will work be done
here when he lifts it?
Again, a yes!

5. What do we see now?
A man pushing against a wall
Is work done here?
We would have to say no!
Why so?
Because the wall does not
move and therefore no work
is done

6. What do we see here?
A man pushing a cart with
fruits
Is work done here?
Again, a yes!
By whom or by what?
By the man

7. ENERGY
INTRODUCTION TO WORK AND ENERGY
CLASSVIII

8. WHAT IS WORK?
• When a force is applied to
an object due to which the
object is set in motion, then
we say that work is done
• In this image, the force is applied
by the little boy as he kicks the
football

9. FACTS ABOUT WORK
Work is only done when a force acts on an object and causes it to
move some distance
So two conditions that need to be fulfilled for work to be done are:
• A force should be applied on the object
• The applied force should produce a motion of the object in the direction of the
applied force

10. MEASUREMENT OF WORK
Work = force × distance moved in the direction of the force
or W = F × S, where
W = work done
F = applied force
S = magnitude of displacement in the direction of the force

11. UNITS OF WORK
Work = Force × Displacement
SI unit of force is newton (N) and that of displacement is metre
Therefore, SI unit of work is newtonmetre (Nm)
Nm is called joule (J) in honour of the British scientist James Prescott Joule
1 J is the work done when the point where the force of 1 N is applied moves
through a distance of 1 m

12. UNITS OF WORK
Larger units of work are kilojoules (kJ) and megajoules (MJ)
1kJ = 1000 J
1 MJ = 1,000,000 J = 106 J

13. WORKED PROBLEMS ON WORK
Example 1
A boy pushes a box through a distance of 6 m with a force of 50 N. Calculate
the work done by him.
Given force (F) = 50 N and the distance moved is 6 m, then
Work done = F × S = 50 × 6 = 300 J
∴ Work done = 300 J

14. WORKED PROBLEMS ON WORK
Example 2
A man applies a force of 15 N to move a toy car. If the work done by him is
180 J, then calculate the distance through which the car has moved.
Given force (F) = 15 N and the work done by him is 180 J, then using the
formula, Work done = F × S
180 = 15 × S or S = 180/15 = 12 m
∴ Distance through which the car is moved = 12 m

15. FACTORS AFFECTING WORK
• There are 2 factors on which the
amount of work done depends
on:
• The magnitude (size) of the
force that is applied to produce
the motion
• The distance travelled by the
body in the direction of the
force

16. ILLUSTRATION
In the accompanying image,
in the first instance – the boy
pushes a box weighing 40 kg
through 5 m
In the second one he pushes the
same box weighing 40 kg
through 10 m
Where is more work done?
In the second case, as the same object is pushed through a greater
distance

17. CAN WORK DONE BE ZERO OR NEGATIVE?
• Yes , there are conditions when
work done is zero or negative
• When pushing a wall and the wall
does not move, then work done is
zero
• When force is applied on a object
and it moves, but returns to its
original position, then work done
is zero (circular motion)
• When a force is applied and an
object moves, but work done by
friction is negative (as it acts in
the opposite direction)

18. ILLUSTRATION
Now look at the two images – what do we have in
the first image?What do we have in the second?

19. ILLUSTRATION
• Food pyramid
• People running
• Can you think of any relation between the two?

20. INTRODUCTION
• The food we eat helps us do work, workout, run, play,
et cetera
• What does food provide us?
•ENERGY!!!
• So how do we define energy?
• Energy is defined as the capacity to do work

21. DIFFERENT FORMS OF ENERGY
• We have different forms of energy around us
• Flowing water, falling water, blowing wind, sun’s light
and heat, et cetera
• We have one form of energy being changed into
another
• To account for these changes, we can broadly
categorize all forms of energy into 4 types

22. DIFFERENT FORMS OF ENERGY
• Mechanical energy
• Electrical energy
• Chemical energy
• Heat energy

23. DIFFERENT FORMS OF ENERGY
• Mechanical energy
Energy possessed by a body due to its state of rest,
position or motion is called mechanical energy
• We have 2 forms of mechanical energy:
o Kinetic energy
o Potential energy

24. TYPES OF MECHANICAL ENERGY
• Kinetic energy
Energy possessed by a body
due to its motion is called
kinetic energy
• Examples: a flying aeroplane,
a rolling ball, a speeding train
• Expression for kinetic energy
Kinetic energy (KE) = ½ mv2

25. TYPES OF MECHANICAL ENERGY
• Potential energy
Energy possessed by a body
due to its position or
condition is called potential
energy
• Examples: a stretched rubber
band, a wound up spring of a
clock, a stretched bow
• Expression for kinetic energy
Potential energy (KE) = mgh

26. DIFFERENT FORMS OF ENERGY
• Electrical energy
The energy possessed by a charged
body is known as electrical energy
• Electrical energy is used to run
various appliances such as fans,
refrigerators,TVs, et cetera
• Electrical energy is generated in
large electrical power plants –
thermal, hydro, solar or nuclear

27. DIFFERENT FORMS OF ENERGY
• Chemical energy
The energy stored by every
substance and released during
certain chemical reactions is
known as chemical energy
• Examples of chemical energy
1. Food that we eat
2. Food prepared by green plants
3. Energy from battery cells

28. DIFFERENT FORMS OF ENERGY
• Heat energy
The energy released when we burn
fuels such as coal, wood, and oil is
known as chemical energy
• Heat energy always flows from a
hot object to a cold object
• Heat energy causes a change in the
temperature of any form of matter

29. LAW OF CONSERVATION OF ENERGY

30. EXAMPLE OF LAW OF
CONSERVATION OF ENERGY
Energy is transferred from the ball to the pin. No energy is lost!

31. What do we have here?
A juggler’s act with balls
He throws one ball up,
and catches another
What is the
transformation of energy?
KE to PE and back again to
KE

32. What do we have here?
A simple pendulum
with to and fro motion
Max. KE in the mean
position
Max. PE in the extreme
positions

33. CONVERSIONS OF ENERGY
During change of state, temperature remains constant

34. CONVERSIONS OF ENERGY
During change of state, temperature remains constant