Class 9 Science

# Work and Energy

## Work

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

text(Work) = text(Force) xx \text(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 xx s = 0

If displacement, s = 0

Therefore, Work done, W = F xx 0 = 0

Thus, there are two conditions for work is considered done.

• 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.

### 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 xx s we get

W = N xx m

Thus, unit force is Nm.

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

The 1 joule of work done is equal to 1N xx 1 m.

Or, 1 joule = Nm

### Direction of Force Positive and negative work

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

i.e. W = F xx s

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

i.e. W = – F xx s = – Fs

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

## Energy

Energy is the capacity of doing work.

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