Home » Mechanics » difference between potential and kinetic energy

What’s kinetic energy?

kinetic energy  possessed by a body due to it`s motion . It is the energy of motion . when you are walking or running your body is exhibiting kinetic energy . A bicycle or skateboard in motion possesses kinetic energy .Running water has kinetic energy and it is used to run water mills .Moving air has K.E and is used to derives wind mills and pushes sailing boats , similarly a bullet fired from a gun has kinetic energy and can penetrate into a target because of its K.E . Remember, kinetic energy is the energy of motion . The faster the body move,s the more the kinetic energy is formed as a process of movement . for example , when a bus speeds up going down a hill, the potential energy of bus converts into K.E . There is not much potential energy at the bottom of the hill , but there is a great amount of kinetic energy .Similarly a basket ball shows kinetic energy ,the K.E is proportional  to the balls mass and square of its velocity . to throw the same ball twice as fast , a player does more work and transfers four times the energy . formula of kinetic energy is given by

where m is the mass of the body moving with velocity v . unit of kinetic energy is joule . if the mass is given in kilogram and velocity v is given in meter/second then kinetic energy comes out in joules . one joule is the energy when a force of one newton displaces a body through one meter .

What happens when the velocity is increased by keeping the weight constant?

It mean if we increase the velocity of an object two times its kinetic will increase four times,if we increase the velocity three times its kinetic energy will increase nine times,if we increase the velocity 4 times its kinetic energy will increase sixteen times and so on..

• Heat energy:

Heat is a form of energy given out by hot bodies.Large amount of heat is obtained by burning fuel.Heat is also produced when motion is opposed by frictional forces.The foods we take provide us heat energy.The sun is the main source of heat energy.

• Electrical energy:

Electricity is one of the widely used forms of energy.Electrical energy can be supplied easily to any desired place through wires .We get electrical generators.These electric generators are run by hydro power,thermal power or nuclear power.

• Sound energy:

When you knock at the door,you produce sound.Sound is a form of energy.It is produced when a body vibrates ;such as vibrating diaphragm of a drum,vibrating strings ,of a sitar and vibrating air column of wind instruments such as flute pipe etc..

• Light energy:

Light is a form of energy.The sun is the major source of light for us.Light helps plants to make food in the process of photosynthesis.

Derivation of the formula of k.E:

Let a force F moves a body from

Vi=0 to  vf=v

And a body covers a distance S in its direction.We know that:

W = FS …..(1)

But

F = ma

And we also have a relation:

2as = Vf 2– Vi…..(2)

By putting values of vi=0 and vf=v we get:

2as = (v)2 –(0)2

2as = v2…….(3)

We also know that:

F=ma

a=F/m

put value of a in equation (3) then we will get:

2(F/m)s=V2

By rearranging above equation we will get:

FS = ½ mV2 ….(4)

But we know that FS=W so equation (4) will become as:

W=1/2 mV2

Because this work done on the body of mass m will set the body moving with velocity v from its state of rest,it is equal to kinetic energy of the body.So:

K.E=1/2 mV2

Video of  Derivation of K.E:

Forms of energy:

Energy is found in different forms such as light,heat,chemical,motion, etc.

We put all the forms of energy into two categories:

1. Potential energy
2. Kinetic energy

Potential energy:

“Potential energy is the energy that is stored in an object due to change in its position.For example ,if a body of mass m is raised to height h above the ground ,the potential energy is mgh,where g is the acceleration of free fall.That is ,if the body returns to the ground level ,the gravitational field of the earth earth will do this amount of work on it.It is written as P.E.

Examples of potential energy:

When we stretch a rubber band or lift a stone to a some height,energy is stored in these objects.This energy is called  potential energy.

A brick on the ground cannot do any work.But when we raise the same brick ,energy is stored in these objects.This energy is called potential energy.

The energy in the wound up spring of a toy car is potential energy.This energy can causes the toy car to move.

When we put a stone in the sling of a catapult and stretch  its rubber ,potential energy stores in it.This energy can throw away the stone.Similarly the water stores in the dam has potential energy.

There are several different forms of potential energy.

Chemical Energy:

Chemical energy is a form of potential energy produced during chemical reactions.It is stored in food,batteries and fuels such as coal ,petrol and natural gas.Food ,fuels and batteries releases chemical energy as result of chemical reactions.Electric energy is obtained from electric cells and batteries as a result of chemical reaction between various substances present in them.Animals get heat and muscular energy from the food they eat.

Stored Mechanical Energy:

Mechanical energy is energy stored in the objects by the application of force.Compressed springs and stretched rubber bands possess stored mechanical energy.

Gravitational Energy:

Gravitational energy is energy stored in an object due to its height.When we raise a brick up to some height ,it possesses gravitational energy.

Derivation of gravitational potential energy:

Let a body of mass m as shown in figure is raised up through height h from the ground.The body will acquire potential energy equal to the work done is lifting it to height h.

So potential energy P.E = (F)(h)

P.E=(W)(h) …….(1)

Where  weight of the body is W= mg

By putting value of W in equation (1) we have:

P.E =mgh