Subject: You were a little off


   It is a popular misconception that mass = weight. This is not so. If it
were true, then everyone living in a gravitational field would slowly
disappear as the field strength decreased. Mass is an intrinsic property of a
particle. Weight is a force and a force is a derived quantity based on the
fundamental units of mass length and time.


   Ok, kids. The bell has rung and its time for Mr. Physics to begin the fun.
Today we are going to talk about photons. Photons are little bundles of energy
that physicists like to call a gauge boson. This gauge boson is the mediator
of the electromagnetic interaction between two charged objects. Photons arise
as a consequence of quantizing the classical electromagnetic field equations
in a relativistically covariant way.

  Does the photon have mass? Can you weigh it?

   Well according to Dr J. D. Jackson, the most loved physicist in the world,
the limits on the mass of the photon currently is less than 10e-48.  


 On the other hand, weight is a force and not an intrinsic property of a gauge
boson such as the photon. Photons do indeed carry momentum and thus photons
can exert a force and hence a pressure on a cross-sectional area. It is this
pressure for instance that produces one of the two tails seen on a comet and
why the tails always point away from the sun. EVERYONE who has had Phys 221
and Phys 222 should remember these facts ( Ha Ha Ha ).  Consider the
following:

       F = dp / dt   	f=force p=momentum t=time
       I = P / A	I=intensity P=power A=area
       P = E / t	E=energy  T=time


   Thus 

    I = ( E / t ) / A

  
   Now the crucial connection. According to Einstein's special theory of
relativity, the energy and momentum of a relativistic particle are related as
follows:

   E^2 = ( c^2p^2 + m^2c^4 )   where the symbol ^ refers to exponentiation


    Thus even massless particles such as the photon are allowed to have
momentum and energy. The energy and momentum of the photon are thus related by:

   E = cp

  Putting this in the equation for I we have

   I = cp / tA

  or 

   p / t   =   IA / c   =   F

  Using I(sun) = 1.36e6 ergs/cm^2/sec = 1360 J / m^2 /sec
  R(earth) = 6.37e6 metres
  A = 4*pi*r^2
  and c = 3e08 m/s

   then

   F = 5.78e08 Newtons = 64955 tons


   Therefore, if the amount of energy collected by the Earths surface during
the course of a day could be concentrated on the area of a scale, it would
exert a force equal to 65,000 Tons.


    Next week, we will talk about how to blow up the moon. Bye Kids!!


    Dave ( aka pre-doctor Physics )
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