Work Energy and Power

Published by: BhumiRaj Timalsina

Published Date: 24 Jan 2022

Work Energy and Power in Grade-7, Reference Note

We perform various types of work in our daily life. To perform work, we need energy. We cannot do work without energy. Work is said to be done when the force acts upon an object to cause a displacement. Energy is the capacity of doing work. Energy and work are measured in terms of a joule (J). Power is defined as the rate at which work is done upon an object. Power is related to how fast a work is done. The unit of power is watt (W) which is joule/second.

Energy

We need the energy to perform daily activities in our life. Food and respiration provide us energy. Energy is the capacity or ability to do work. The SI unit of energy is joule and in CGS system, it is erg. It has the only magnitude so it is a scalar quantity. Kinetic energy, potential energy, heat energy etc are some of the examples of energy. To calculate the amount of energy, we use the formula of work as the magnitude of energy is equal to the magnitude of work done.

There are various types of energy, which are as follows,

Mechanical Energy

The energy possessed by a body when the body is at rest or in motion is called mechanical energy. There are two forms of mechanical energy. They are given below,

Kinetic Energy

Kinetic energy can be defined as the energy possessed by a body due to its motion. It is a scalar quantity. All the objects in motion have kinetic energy whether it is in vertical or horizontal motion. There are various forms of kinetic energy like vibrational energy, rotational energy etc. Flying an aeroplane, travelling car, flowing river, falling asteroid etc. possesses kinetic energy.
If ‘m’ is the mass of the moving body and ‘v’ is the velocity at which the body is moving then kinetic energy of the moving body is given by,
Kinetic energy (KE) = 1/2 mass × (velocity)²
Or, KE = 1/2 m × v²
This equation reveals that kinetic energy of a moving body is directly proportional to the product of mass and square of the velocity of a body.

Potential energy

Potential energy is the energy stored in a body at rest. Chemical energy from a battery is a potential form of energy, elastic energy in a stretched rubber band is a form of potential energy, but the most commonly referred to the form of potential energy in physics is that of gravitational potential energy. This energy is stored due to the object’s position.
Potential energy of a body can be calculated by the following formula,
Potential energy (PE) = m × g × h
Where,
m = mass of the body
g = acceleration due to gravity whose value is 9.8m/s2
h = height of the object
∴ PE = mgh

Heat energy

Heat is the form of energy which gives the sensation of warmth to us. It is obtained from hot bodies. Sun is the main source of heat energy. Heat energy can also be obtained from the burning of fuels like petrol, diesel, etc.

Sound energy

A sound is a form of energy which gives the sensation of hearing. It is produced due to the vibration of a body. It cannot travel through the vacuum. Some examples of sources of sound energy are a loudspeaker, radio, drum, noise etc.

Light Energy

Light is a form of energy which gives the sensation of vision. Sunlight is the main source of light energy which is used by all the living organisms of the earth. The burning of fuel like coal, petrol, etc also produce light energy.

Chemical Energy

Chemical energy is the energy stored in chemicals. It is produced due to the chemical change of a substance. Since chemical energy is a stored energy, it is also the form of potential energy. Battery, food, petroleum, biomass, etc. are some of the examples of stored chemical energy.

Nuclear Energy

The energy which is obtained through the nuclear reaction whether through nuclear fission or nuclear fusion is called nuclear energy. The energy obtained through the sun is a type of nuclear energy. Nuclear energy produces a large amount of heat, light and sound energy. When a nuclear bomb is exploded then it produces a large amount of heat, light and sound energy.

Electrical energy

Electrical energy is a form of energy produced by the moving electric charges. It can be easily converted to the other form of energy like heat energy, light energy, sound energy etc. Electrical energy is used in various types of work. In a car battery, the chemical reaction creates an electron which has the energy to move in an electric current. These moving charges provide electrical energy to the circuits in the car.

 

Magnetic energy

Source: roberts0910.wikis.birmingham.k12.mi.us Fig: Magnetic Energy

 

Magnetic energy is the energy possessed by a magnet. It is the energy within a magnetic field. Magnetic energy is used to produce electrical energy. It is used in various electrical devices like a loudspeaker, telephone, radio etc. It is used in various factories to lift the loads made of magnetic substances.

 

Work

We perform various types of activities in our daily life. These all activities are called work. Some of the examples of work are writing, reading, driving, etc. Work is said to be done if the force applied on a body changes the position of the body in the direction of force applied. The SI unit of work is Joule. Since work has the only magnitude without direction, so it is a scalar quantity. Work done is the product of force and displacement that is,
Work done (W) = Force ×× Displacement (s)
∴ W = F ××s
When 1N force displaces a body through 1m in the direction of the force then the work done is said to be one-joule work.

Work done

Work done by a force is zero when force is applied perpendicular to the displacement. Sometimes in some cases, work done by a gravity becomes zero. Like, when you move on the ground carrying a load of 5 kg then work done by gravity is zero. But the work done by you is not zero. There are two types of work which are as follow,

Work done against gravity

Gravity is defined as the force of attraction exerted by the earth which pulls all objects towards its centre. When work is done against the earth’s gravity then it is called work done against gravity. Throwing ball upward, lifting water from well, etc. are some examples of work done against gravity.

Work done against friction

The force, which opposes the motion of one body over the other when they are in close contact, is called frictional force. When work is done against friction then it is called work done against friction. Some of the examples of work done against friction are pushing a box of certain mass across the rough surface, pushing vehicles in roads, etc.

Example 1

If a boy applied a force of 100N to displace a box at a distance of 20m then what is the work done by the boy?
Solutions:
We have,
Force applied (F) = 100N
Displacement (s) = 20m
Work done (w) = ?
According to the formula,
w = F × s
= 100 × 20
= 20000J
∴ The work done = 20000J

Power

Power is defined as the rate of doing work. The SI unit of power is watt (W) which is joule per second. It is a scalar quantity. If ‘w’ be the work done by the body at ‘t’ time then the rate of doing work or power is given by,
Power = Workdone(w)/Timetaken(t)
∴P = w/t

If a body takes less time to do some work then the rate of doing work is said to be more and if the body takes more time to do work then the rate of doing work is said to be less.

When 1-joule work is done in 1 second then the rate of doing work is said to be one-watt power.

Horsepower

A horsepower is a unit of measurement of power. The electrical equivalent of one horsepower is 746 watts in the international Systems of Unit (SI). It is mainly used in engineering and is denoted by H.P.

Relationship between watt (W), kilowatt (KW) and megawatt (MW)
1000 W = 1 kW
1000 kW = 1 MW
1 MW = 1000000 W

Example 2

A man does 40J of work in a certain time. If the power of the man is 10 watt then what time does it take to complete work?
Solution:
Given,
Work done (w) = 40J
Power (P) = 10watt
Time taken = ?
By using formula,
P = wt
Or, 10 = 40t
Or, 10t = 40
Or, t = 40/10
Or, t = 4s
∴∴ The time of work done is 4 seconds.

Example 3

A man lifts a stone of 100N to a height of 5 metres in 2 seconds. Calculate the work done and power of the man to lift a stone.
Solution:
Given,
Force of stone (F) = 100N
Displacement (s) = 5 metre
Time taken (t) = 2 seconds
Work done (w) = ?
Power (P) = ?
According to the formula,
w = F × s
= 10 × 5
= 50 joule
∴∴ Work done by a man to lift a stone is 50 joule.
Now,
P = w/t
= 50/2
= 25 watt
∴ The power of man is 25 joule per second

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