Mechanical energy

The most common form of energy is mechanical energy. The energy that is stored in a body due to its position or motion is called mechanical energy. In this lesson we will discuss two forms of mechanical energy-kinetic energy is produced due to motion and potential energy is produced due to the position of object.

Kinetic Energy

We sometimes notice that cricket ball hits the stamp and strikes it down. If anything hard hits the glass of window the glass breaks down. If we throw stones at mango or jujube it may fall down.

From the above example we can see energy exists in the body in motion. The capacity of doing work acquired by a moving body due to its motion is called kinetic energy.

Why does the object displace from its initial position? This is because the hit makes the pen move and the pen obtains the ability to work that is kinetic energy is produced in it. So it could displace the object.

Creating velocity in a body in rest or increasing the velocity of a moving object means to produce acceleration in it. For this force has to be applied. As a result work will be done on the body. For this the body will obtain the ability to work and this work will be stored in the object as kinetic energy. This is why all moving objects are in possession of kinetic energy. The body will be able to perform this amount of work before it rests.

Let a force F be applied on a body of mass m at rest. The body attains a velocity v. Suppose the body moves a distance s in the direction of the force. The work done to produce this velocity of the body is its kinetic energy.

Therefore,

Kinetic energy = Work done

= Force × displacement

= F × s

Or, E_k = mas; [as, F = ma]

Kinetic energy

But, v² = u² + 2as

Or, as = v²/ 2

2; [since, initial velocity, u = 0]

∴ E_k = 1 / 2 mv²

∴ Kinetic energy = 1 / 2 × mass × (velocity)²

Kinetic energy depends on the mass of object. The more is the mass the higher is the kinetic energy. With the same velocity a light tennis ball and a heavy cricket ball are thrown at you. The hit will be more by the cricket ball than that of tennis ball.

Kinetic energy also depends on the velocity of objects. The more is the velocity the higher will be the kinetic energy. The damage will be less if a truck hits a wall with less velocity but the damage will be more in case of higher velocity.

Potential Energy

If a piece of stone or brick falls on a body from the roof of a building it may flatten or break the body. When the stone or brick was in rest on the roof potential energy was stored in it but the potential energy works when it falls down. The energy was stored in the stone because it was above the ground.

What will happen when the two ends of a spring is stretched and tied to two objects and then released? The objects will move fast and collide with each other. The stretched spring was at rest but potential energy was there in it. If it is released it may work. The energy was stored in the stretched spring because it was strained.

The ability of a body to do work when its normal position or configuration is changed to some other position or configuration is called potential energy.

Potential Energy 1

Expanded Activities: Take a pulley and place a rope on it. Tie a heavy object A with its one end and a light object B with the other end. Such that, A remains above the ground but B remains on the ground [Fig: Potential Energy 1]. Remove your hand.

What did you see? The object A goes down and object B goes up. The potential energy was stored in the object A as it was above the ground from its normal position and gained ability to work. It can work till it reaches the ground that is it can raise the object B.

Potential Energy 2

Experiment: Take a spring and tie its one end with a strong support and a block to its other end. Place them on a smooth surface. Now apply force on the block and contract the spring and keep another object in front of the block [Fig: Potential Energy 2] then remove your hand.

Why did the object move fast? Spring could work while regaining its original configuration and was able to displace the other body. This ability of spring to work for the change of its normal configuration is its potential energy. If some work is done against the force at the time of changing from normal position or configuration to some other position or configuration then the body obtains the capacity of doing work that is same amount of energy is stored in it. This principle is applicable within the sphere of influence of conservative force such as electric force, magnetic force, spring force etc. This sphere of influence is called the field of that force such as gravitational field, electric field etc. We work against the force of gravity when we lift anything higher from the ground. As a result the object obtains some amount of potential energy. It can perform the same amount of work when it falls on the ground.

Potential Energy 3

If a body of mass m is raised to a height h (fig: Potential Energy 3) above the surface of the earth, the work done in such a process is a measure of potential energy stored in the body. In this case, the work done is the product of the applied gravitational force i.e. the weight of the body and the vertical height.

∴ Potential energy = weight of the body × vertical height

= mgh

∴ E_p = mgh

I.E. potential energy = mass of the body × acceleration due to gravity × vertical height Potential energy depends on the vertical height of the object from the surface of the earth. The more is the height, the higher will be the potential energy. Potential energy also depends on the mass of the object. The more the mass the more will be the potential energy of the object.

To use the potential energy stored in a body it is necessary to transform it into other form of energy. For example, a piece of stone is not dangerous as long as it is on the roof and its potential energy transforms into kinetic energy i.e. it starts to fall.

End

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