Class 9 Science

# Work and Energy

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

## Work

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.

## Energy

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.