Work energy and power - Class 9 PDF Download

Whenever a force makes a body move, then work is said to be done. But for doing work, energy is required. When work is done by human beings, machines or animals they get energy from the food which they eat and if work is done via machines, then energy is supplied by fuels or by electricity. So, we can say that when work is done an equal amount of energy is used up. This article deals with the summary of Work, Power and Energy, its mechanism, formulas and how they are related to each other in a crux form which not only clear your concepts but also help in your preparation and in solving problems.

Work

Work can be done when a force produces a motion. For e.g. when a person climbs the stairs of an office or a house, work is said to be done as he is moving against the force of gravity.

Basically, Work done by a force depends upon two factors:

(i) Magnitude of the force

(ii) Distance through which the body moves in the direction of force.

Therefore, Work is measured by the product of force and displacement of the body along the direction of force. It is a scalar quantity and its SI unit is joule.

Work = Force X Distance (S) moved in the direction of force

Or Work = F X S

- If a body gets displaced by S when a force F acts on it, then

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Work W = F S Cos θ

Where θ = angle between force and displacement

Note: The condition for a force to do work is that it should produce motion in an object, i.e. if the distance moved is zero, and then the work done on an object is also zero. For example, if a man pushes a wall, but there is no displacement that is wall is stationary it does not move, then, the work done by the man on the wall is zero. But the work done on the body of the man himself is not zero. Because while pushing the wall man consumes energy, his muscles are stretched and he feels tired.

Also, we can take another examplethat if a man stands still at a bus stop with heavy suitcase in his hand, he may get tired soon but he does no work in this situation because suitcase held by the man do not move at all.

So, it is clear now that whenever a force is applied to an object it is not necessary that work is done. Work is done when force able to move the object.

What happens when work is done against gravity?

Whenever work is done against gravity, the amount of work done is equal to the product of weight of the body and the vertical distance through which the body is lifted.

Work done in lifting a body = Weight of body x Vertical distance

W = m x g x h

Where W = work done

    m = mass of the body

    g = acceleration due to gravity

    h = height through which the body is lifted

Motion and Mechanics

Energy

Capacity of doing work by a body is called energy. Energy is a scalar quantity i.e. it has only magnitude but no direction and its unit is joule. The energy required to do 1 joule of work is called 1 joule energy.

1 Kilojoules (KJ) = 1000 joules (J)

The unit joule is named after a British physicist James Prescott Joule.

- Energy developed by a body due to work done on it is called mechanical energy. It is of two types:

(i) Potential Energy

(ii) Kinetic Energy

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Potential Energy: The capacity of doing work developed in a body due to its position or configuration or we can say that the energy of a body due to its position or change in shape. For e.g. Energy of compressed string, energy of water collected at a height, energy of spring in a watch etc.

- The energy of a body due to its position above the ground is called gravitational potential energy.

- The energy of a body due to change its shape and size is called elastic potential energy.

- Potential energy of a body in the gravitational field of earth is mgh. where m = mass, g = acceleration due to gravity, h = height of the body from surface of the earth.

Kinetic Energy: The energy of a body due to its motion. If a body of mass m is moving with speed v, then Kinetic Energy of the body is 1/2mv².

From the above formula it is clear that:

- If the mass of the body is doubled, its kinetic energy also gets doubled.

- If the mass of the body is halved, its kinetic energy also gets halved.

- If the velocity of a body is doubled, its kinetic energy becomes four times.

- If the velocity of a body is halved, then its kinetic energy becomes one-fourth.

Relation between Momentum and Kinetic Energy

K.E = p²/2m where p = momentum = mv

So, it is clear from above formula that when momentum is doubled, kinetic energy becomes four times.

Power

Power is defined as the rate of doing work. It is scalar quantity.

Power = Work done/ time taken

Or P = W/t

where P = Power

   W = work done

    t = time taken

Also, when work is done, an equal amount of energy is consumed. Thus, power is also defined as the rate at which energy is consumed or utilised.

Power = Energy consumed / Time taken

Or P = E/t

where P = Power

    E = energy consumed

    t = time taken

The S.I unit of power is watt (W).One watt is the power of an appliance which does work at the rate of 1 joule per second.

1 watt = 1 joule/ 1 second

Or 1W = 1 J/ 1 s

1 watt = 1 joule per second

1 KW = 10³ watt

1 MW = 10⁶ watt

- Horse power is another unit of power which is equal to 746 watt i.e. 1 horse power is equal to about 0.75 kilowatt (0.75 KW).

1 watt second = 1 watt x 1 second

1 watt hour (Wh) = 3600 joule

1 kilowatt hour (kWh) = 3.6 x 10⁶joule

Principle of Conservation of Energy

Energy can neither be created nor destroyed. Only energy can be transformed from one form to another. Whenever energy is utilised in one form, equal amount of energy is produced in other form. Hence, total energy of the universe remains same.

Some equipments used to transform energy are:

Hence, we can say that many physical situations can be greatly simplifies by understanding about the work. As, it enables us to evaluate force over distance and time. Also, it gives a broader understanding not just the forces acting on a given object, but about what happens to that object over the course of a given journey.