|
An object has kinetic energy if it is
moving. If there are some constrained or pent up forces
preventing the object to move, it is said to have
potential energy. There are various forms of energy,
including those stated by Einstein's Equations, but they
all ultimately relate to a moving object.
Questions you may have include:
Kinetic energy
The standard textbook definition of
energy is the "ability to do work." Unfortunately, this
definition does not really give a good picture of what
energy is all about.
We normally think of an object having
energy as one that is moving. The energy of a moving
object is called kinetic energy (KE). The greater the
mass of a moving object, the greater its energy will be.
Also, the faster it goes, the greater its energy. That
energy is proportional to the square of the velocity.
The equation for calculating the kinetic
energy of an object is:
KE = �
m*v2
This means that if two objects were
going at the same velocity, but one object had twice the
mass of the other, then the energy of the heavier object
would be 2 times the energy of the lighter object.
If m = 5
and M = 10, then 2 (�* m*v2) = �* M*v2
Now suppose the lighter object was going
twice as fast as the heavier object. Say the lighter
object was going 4 m/s and the heavy one was going 2
m/s. Then the energies would be:
KE of
light-weight object is: KE1 = �* m*v2 = � * 5 * 4 * 4 =
40
KE of
heavy-weight object is: KE2 = �* M*v2 = � * 10 * 2 * 2 =
20
Thus, the effect of higher velocity is
greater than that of higher mass.
Potential Energy
There are situations when an object has
the potential to start moving and gain kinetic energy.
Often there are forces acting on the object, but the
forces aren't yet sufficient to move the object.
If you hold an object a distance from
the floor, it has the potential to start moving, once
you let it go. The force of gravity is pulling on the
object, giving it potential energy (PE). The equation
is:
PE = m*g*h
where "m" is its mass, and "g" is the
acceleration of gravity (32 ft/s2 or 9.8 m/s2). Of
course, m*g = the weight of the object. "h" is the
height of the object from the floor or ground.
If you drop the object, its potential
energy will become the kinetic energy of motion (PE = KE).
Since PE = m*g*h and KE = �* m*v2, then
m*g*h = �* m*v2
You can determine the speed it will be
traveling after falling a height "h" by solving the
equation for "v":
v2 = 2*g*h or
v = SQRT(2*g*h)
SQRT(2*g*h) means the square root of
2*g*h. (Unfortunately, there is no good square root sign
in web pages.) Note that the mass m cancels out of the
equation, meaning that all objects fall at the same
rate.
Thus, if h = 1 ft, and since g = 32
ft/s2, then v2 = 64 or v = 8 ft/s.
Other examples of potential energy that
could cause motion include explosive chemical compounds
and a coiled spring, ready to be released. A stretched
rubber band, also has potential energy.
With chemical explosives it is difficult
to calculate the potential energy without experimenting
to see how must kinetic energy is released in an
explosion.
With a compressed spring, there are
calculations that can determine its strength and
potential energy.
Other forms of energy
Often, you will hear about other forms
of energy, such as heat and electrical energy. In
reality, they are also kinetic energy.
Heat is the movement of molecules. It is
the sum of their kinetic energy. In many physics
textbooks, they look at heat as some sort of substance
and heat energy as something independent of kinetic
energy. In our lessons, it is just one other form of
kinetic energy.
Electrical energy is the movement of
electrons. That is kinetic energy. The voltage in an
electrical circuit is the potential energy that can
start electrons moving. Electrical forces cause the
movement to occur.
Chemical energy is potential energy
until the chemical reaction puts atoms and molecules in
motion. Heat energy (KE) is often the result of a
chemical reaction.
Light is the movement of waves and/or
light particles (photons). It is usually formed when
atoms gain so much kinetic energy form being heated that
they give off radiation. This is often from electrons
jumping orbits and emitting moving photons.
When you feel full of energy, it means
that the potential chemical energy of the food (or
candy) you ate has been converted into kinetic energy of
your motion.
In his Theory of Relativity, scientist
Albert Einstein derived the famous equation:
E = m*c2
(Energy equals mass times the speed of
light squared).
Potential energy great
What this equation says is that the
potential energy of an object of mass m is very great
and would be as if it were moving at the speed of light
(186,000 miles per second or 300,000 kilometer per
second�very fast!).
Pure energy
Many scientists say that this also means
that matter can be converted into pure energy.
In other words, if a gram of some
substance like Uranium would explode, some of its matter
would disappear and cause the rest of the matter to have
tremendous kinetic energy. This is the main idea behind
atomic energy�converting some matter into energy and
heat.
Another viewpoint is that the matter is
converted into radiation energy, which actually still
has mass.
Moving objects have kinetic energy.
Constrained objects may have potential energy. There are
various forms of energy, including those stated by
Einstein's Equations, but they all ultimately relate to
a moving object. |
