Re: questions about light

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2 total messages Started by Edward Van Nostrand Wed, 27 Apr 1994 11:04

#1573

Author: Edward Van Nostrand
Date: Wed, 27 Apr 1994 11:04

20 lines
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Just a comment about your answer to 2. below


> >2. If light is a particle, when it leaves a flashlight why doesn't it
> >have enough momentum to push the flashlight backward?
>
> No mass, no momentum, no recoil.
>
Though light doesn't have mass it still has momentum.  This is  the
principal behind the light sail for spacecraft.  The momentum is given by

p=h x frequency /c where h = Planck's constant and c is the speed of light.

If the flashlight were not affected by forces such as gravity and friction
at its location it would indeed move backwards under the influence of the
weak force exerted by the photons.

Ted van Nostrand
Holy Heart High School
St. John's, NF, Canada

Re: questions about light

#1497

Author: Rick Matthews
Date: Mon, 02 May 1994 17:48

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Edward Van Nostrand (evannost@CALVIN.STEMNET.NF.CA) wrote:
: Just a comment about your answer to 2. below


: > >2. If light is a particle, when it leaves a flashlight why doesn't it
: > >have enough momentum to push the flashlight backward?
: >
: > No mass, no momentum, no recoil.
: >
: Though light doesn't have mass it still has momentum.  This is  the

I picked this discussion up in the middle, but...

Light does have mass.  The mass of a photon is  its energy divided by
c squared, or 

	Plank's constant * frequency / c^2.

Anything with energy has mass.  Photons have no *rest* mass, but
photons are never at rest anyway.  As the previous poster noted, the
flashlight does recoil.

--
Rick Matthews                     matthews@wfunet.wfu.edu     Ham radio:
Wake Forest University            910-759-5340   (Voice)      WA4GSP
Winston-Salem, NC 27109-7507      910-759-6142   (FAX)

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