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What is Energy?What is energy?
It's not an easy question to answer  energy is such a fundamental
thing in the universe that it's difficult to state exactly what it is,
or why it exists  it just does. So I won't answer that question explicitly in this article.
For starters, I'll just tell you this: heavy objects, moving fast, have more energy than lightweight objects moving slowly.
Q  So, what's so special about energy? Why is it important?  A  Think about this one: What happens when a moving object hits an obstacle and stops moving. What happens to its energy? Energy is important because it is conserved. In other words, it cannot be created or destroyed.
It can only be transferred between forms. So, when an object hits an
obstacle, its energy goes into that obstacle  the obstacle might start
moving, some of the energy will go into upsetting the obstacle's
internal molecules and causing them to vibrate (which we feel as heat),
some of the shockwaves running through the molecules will collide with
air molecules, distributing energy around the room (which we hear as
sound) etc. The energy doesn't disappear. It is conserved! (And then there's "potential energy," which is what caused the object to start moving in the first place.) 
Energy comes in different forms  in my statement above, I was referring to kinetic energy, but energy can be described in other ways too. Allow me to explain the differences between them:
Kinetic Energy
Kinetic energy is what objects have when they're in motion. It's defined by the following formula:
Where K stands for the Kinetic energy of an object or particle (for example, 15 joules),
p is its momentum m is its mass (for example, 5 kilograms)
For most objects which you encounter in everyday life (e.g solids, liquids and gases), the momenum, p, is defined by:
...that's just the mass of the object, multiplied by it's velocity (speed)
All movement
comes under kinetic energy, for example; what we know as "heat" is
really microscopic vibratory movement of the molecules that make up an
object. That's kinetic energy, since each molecule has its own mass and
average momentum. When you throw a ball to someone, you're giving the
ball kinetic energy via your arm.
Rest Mass EnergyEveryone knows Einstein's famous equation:
(where
m is still the object's mass, and c is the speed of light  not because
this equation has anything to do with light, just because that number
has a special role in the structure of the universe.)
It means
that even just standing still, an object has energy other than its
kinetic energy, which comes simply from it having a mass.
Q  So, what's so special about the mass of an object then? What is mass, anyway?  A  What
we know as mass, is just kinetic energy in another form. In other
words, kinetic energy and mass are really one and the same thing  they
just appear different to us! Don't believe me? Consider this: Hold an object in your hand. That object is made of atoms. An atom consists of electrons
which are tiny particles (with very little mass) that orbit around the
center of the atom at high speed. The center of the atom is made up of
protons and neutrons. That's all that the object in your hand is made
up of. That's it. Now weigh the object in your hand. Then take all
the protons, neutrons and electrons in your object, and add up their
weights. The weights should be the same, right? But no! What you'll
find is that you can't quite match the full weight of the object
by separately adding up the weights of the stuff in it. Why not? Because the electrons are moving! And fast! The kinetic energy of that movement is where that extra mass comes from! What
about the mass of the protons and neutrons? Well, those particles are
themselves made up of fast moving particles, whose individual masses
don't add up to the mass of a proton or neutron. And so on... So when you weigh an object on a scale, what you're actually measuring is a whole lot of kinetic energy! 
Potential EnergyThis
is the most mysterious form of energy. It's not the same of kinetic
energy (perhaps noone understands entirely what it is!) But it's a measure of how much kinetic energy an object will gain if left to its own devices. You
could say that potential energy predicts the future  it is the reason
why a ball in mid air will always end up on the ground, why electric
current flows from one terminal of a battery to another, and why things
happen on much grander scales.
Potential energy has the symbol V.
This
is where the idea of "forces" comes in. Take the force of gravity. When
you throw a ball straight up into the air, it doesn't keep going up
forever. It slows down, then stops in mid air for a moment, before
falling back down. As it slows down and eventually stops, where is the
kinetic energy going to? The answer is that it's being transformed into
potential
energy. Then when the ball falls back down again, all the kinetic
energy that it lost on the way up, will change back from potential
energy into kinetic energy, and you'll see the ball speed up as it
approaches the ground.
Q  OK,
so how about I drop an object onto a table. When it lands and comes to
rest on the table, does that mean it doesn't have any potential energy
anymore? Because then what happens if it falls off the table and hits
the floor  things wouldn't fall off tables if they didn't have
potential energy! And then what happens if the floor rots and the
object hits the ground?  At what point can you claim that something has "zero" potential energy?  A  Actually, It doesn't matter! When you deal with potential energy, you only ever need to worry about the potential difference, in other words, how much an object's potential energy changes between two points. You can decide where "zero potential energy" is (then if the object drops further it will have negative potential energy)
and it won't make any difference! So if an object gains 50 joules of
energy in falling from a table to the floor, that means it's potential
energy decreases by 50 joules. So that might correspond to the
potential energy dropping from 99 joules to 49 joules, or dropping from
0 joules to 50 joules. Whatever convention you use, the potential
difference will always be 50 joules! 
Total EnergyThe
total energy of any system (for example, a bouncing ball, or even the
Solar system), is simply equal to its kinetic and potential energy
added together. You can write this as:
So
for an "isolated system" (i.e. a scenario where you have something
sealed off so that no energy gets transferred in or out), the total
energy never changes.
Q  Why didn't you include the rest mass energy? Isn't that part of the total energy?  A  Of course it is! And if you want to be absolutely correct, add it in by all means!
But,
most of the time you won't need to. Unless you're doing a really
dangerous experiment, the total mass of your system will remain
constant, so adding it in won't help matters (as I said above, you can
define zero potential energy to be anywhere you want, so adding a
constant to the total energy is really no different to just changing
the position of "zero" potential energy in your system).

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