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Laser and Motion
Name: Clarence L.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 9/12/2004
Question:
If you have a laser in space (assuming no friction at
all), and you turn it on, would the laser move backward?
Replies:
Clarence,
I will take a stab at this one. If I understand your question, if you
fire a laser in space (with no friction) will there be a recoil effect,
or will the laser device move opposite the laser beam?
I would say no since the laser is emitting light in the form of photons
which have no mass. F=ma with m=0 gives F=0.
Bob Hartwell
Dear Clarence,
Yes! The light emitted by the laser has energy and momentum. A force is
needed to push the light in the forward direction. Then, by Newton's 2nd
Law, (For every Action, there is an equal and opposite reaction), the light
must exert a reaction force on the laser.
The force, however, is not very large. A 1 watt output laser, which is more
powerful than you would find in student laboratories, emits 1 joule of
light per second. By a calculation using a little relativity and mechanics,
this would exert a force of 3.3 E-9 N = 0,0000000033 N. =
1.2E-8 oz = 0.000000012 oz. After one year, the laser will have accelerated
to a speed of about 0.1 m/s or about 4 inches/second. Not at all like the
Starship Enterprise!
Best, Dick Plano
Absolutely. But it is weak.
A 1-billion-watt laser is a nice size, a bit more power than our biggest
electrical power plants.
By E=mc2, (c =3x10^8 meter/second), it would emit light weighing 10^-8
kilograms/second.
Because that mass is thrown at 3x10^8 m/sec, by F=mv, the thrust is 3
Newtons, or 0.3 kilogram-weight, or 0.7 pound-force.
Not much thrust for a giant nuclear power-plant.
Accelerates itself somewhat slowly.
A solar sail which catches sunlight of equal wattage and reflects it
backwards, would generate twice the thrust.
One unit of thrust for stopping the light, and another for starting it
backwards.
The laser only starts the light. And it pays the full energy price of
generating the photon. A mirror doesn't have to.
Spacecraft propulsion using on-board lasers would only be worth doing
once one has a top-notch energy source, like anti-matter with near-100%
conversion efficiency,
and the need to accelerate to near light-speed, and no need to do it very
fast.
Jim Swenson
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