MadSci Network: Physics
Query:

Re: CAN ANY TYPE OF ENERGY CARRY MATTER?

Area: Physics
Posted By: Dave Dixon, Assistant Research Physicist,University of California
Date: Fri Sep 5 12:25:17 1997
Area of science: Physics
ID: 871688817.Ph
Message:
The short answer here is perhaps that energy doesn't carry matter, but
matter carries energy.  But in order to see this, we need to understand
something about what is meant when people talk about "energy".

In physics, we usually think about two types of energy: kinetic energy and
potential energy.  That's a bit of a simplification, but an amazingly large
chunk of what we know about physics is gotten simply from knowing the
kinetic and potential energy functions of a system.  
Kinetic energy is the energy of motion.  When an object moves relative 
to you, it has kinetic energy.  The faster something moves, the more 
kinetic energy it has.  Also, if two objects are moving with the same 
speed, the one with larger mass has more kinetic energy.  An object 
which is not moving relative to you has zero kinetic energy.

As you probably know, the total energy of any system is conserved, that is,
it does not change.  If you push on something, and make it go faster, you
have increased that objects kinetic energy.  Since energy is conserved,
the increased energy of the object (appearing as kinetic energy) must have
come from some other place.  This kind of "stored" energy is called
potential energy, since it has the potential to create motion (note that
this isn't the only way to create motion, since you can transfer kinetic
energy from one object to another by colliding them together; the total
energy still remains the same).

Potential energy may appear in many different forms.  In the example above,
where you push on something to make it move, the potential energy is stored
in chemicals in your muscles.  Another simple example is a spring.  If you
stretch a spring, it now contains potential energy, for as soon as you
let it go, it starts to move.  Gasoline contains potential energy, which
is converted to heat when it burns in your car, which your engine then
changes into the kinetic energy of the car's motion.  The Earth's gravity
also has potential energy.  When you hold up a ball, and then let it go,
the gravitational potential energy is converted into the kinetic energy
of the ball, as it falls faster and faster.

Let's look at the examples you gave.  Rather than say "wind is energy", it
is more correct to say "wind carries energy".  Wind is simply the motion
of air, so it's energy is an example of kinetic energy, in this case,
carried
by the molecules of the air.  Waves in water form as a combination of
kinetic energy of the water and potential energy from gravity.  So roughly
speaking, we can say that matter carries energy, rather than the other way
around.  Obviously you need matter in order to have kinetic energy, and
most of the examples above (muscles, gasoline, spring) also have kinetic
energy somehow stored in matter.  If you want to throw a monkey wrench into
this nice scheme, you can bring up gravity, but that's a question
physicists
are still debating today.

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