Class 9 Science

Work and Energy

  • SI unit of work
  • Positive and negative work
  • Forms of energy


When force is exerted on an object and object is displaced, work is said to be done.

Work = Force × Displacement

Or, W = F × s

Where, W is work
F is force and
s is displacement.

If force, F = 0

Therefore, work done, W = 0 × s = 0

If displacement, s = 0

Then, work done, W = F × 0 = 0

Thus, there are two conditions for work to happen.

  • Force should act on the object.
  • Object must be displaced.

In the absence of any one of the above two conditions, work done will be equal to zero, that is work is not considered as done. If no force is applied on an object then work is not possible. If someone applies force but there is zero displacement, work does not happen.

To understand this, let us take some examples.

If you are reading continuously for four to five hours, you are not doing a work because there is no displacement involved. On the other hand, if you are dribbling a football, you are doing some work because football is being displaced.

If a coolie keeps on standing for hours with some luggage one his head, he is not doing work, because no displacement is happening. If the same coolie is moving from one platform to another with even a small luggage, he is doing work because displacement is taking place.

SI unit of work

The SI unit of Force is newton (N) and the SI unit of displacement is meter (m).

Therefore by substituting the SI units of Force and displacement in the expression, W = F × s we get

W = N × m

Thus, unit of work is N m or Newton meter.

The SI unit of work is joule and is denoted as J, which is named after an English physicist James Prescott Joule.

1 joule of work done is equal to 1N × 1 m.

Or, 1 joule = 1 Nm

Positive and Negative work

When force is applied in the direction of displacement, work done is considered as positive.

i.e. W = F × s

When force is applied in opposite direction of displacement, the work done is considered as negative.

i.e. W = – F × s = – Fs

For example, when engine accelerates to move the vehicle forward, work done is positive. But when brakes are applied to stop a moving vehicle, i.e. work is done against the direction of displacement of the vehicle, work done is considered as negative.


The capacity of doing work is called energy.

An object which can do more work is said to have more energy and vice versa. For example, a motorcycle has more energy than a bicycle.

SI unit of energy

Since energy is the capacity of doing work, therefore, the SI unit of energy is same as that of work.

Thus, the SI unit of energy is joule and is denoted by J.

Larger unit of energy is kilo joule and is denoted by kJ.

1kJ = 1000 J

Forms of Energy

There are many forms of energy, such as kinetic energy, potential energy, mechanical energy, chemical energy, electrical energy, etc.

Law of Conservation of Energy

Energy cannot be created and cannot be destroyed. We can only change the form of energy. This law is called the Law of Conservation of Energy.


See Answer

1: (c) A baby pulling a toy car, 2: (b) Force and displacement, 3: (a) Zero N m, 4: (b) Joule, 5: (c) Driver pressing brake pedal, 6: (d) Joule, 7: (d) All of these, 8: (a) A rock on a hill, 9: (b) (1/2) mv2, 10: (d) mgh

Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.


A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.


If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.


Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.


Sound is a type of energy. Sound travels in the form of wave from one place to another.

Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.