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Gramps,
It is a fundamental law of physics that for every action; there is an
opposite and equal reaction. So
how come when you turn on a flashlight, you don’t fly backwards and
squish yourself on the wall facing your backside?
Also, what would happen if the flashlight shot out gamma rays?
Chris |
Dear
Chris,
If
you were small enough, that’s exactly what would happen. You’d be shoved
back against the wall. But you’d have to be really, really small! The key
word here is EQUAL reaction. In the action/reaction game there is the
conservation of energy and the conservation of momentum. Momentum is mass
times velocity. So when two objects collide, momentum is conserved. Let’s
say that you shot a 20-pound bowling ball with a BB that weighed 5 grams, and
the BB was traveling at 750 feet per second (about 500 mi/hr). It would push
the bowling ball away at a speed of 0.42 ft/sec or a little less than 0.3
mi/hr.
Now,
considering the light from your flashlight. Light does weigh something. It has
what we call relativistic mass. That is found by Einstein’s famous equation,
E = mc squared, where c is the velocity of light. The energy of light is
related to the wavelength of light--the shorter the wavelength, the greater
the energy. So, if we know the energy of light, we can calculate its mass from
Einstein’s equation. Let’s take blue light. Blue light has a wavelength of
.00004 centimeters. Instead of using all those zeros, let’s write it this
way, 4 x 10-5 centimeters. (The exponent of the 10 tells us how
many zeros there are to the left or to the right of the decimal
point--negative to the left, and positive to the right. The velocity of light,
c, is 3 x 1010 cm/sec. The square of the velocity of light is 9 x
1020 cm/sec. Now, the energy of light is defined as Plank’s
constant divided by the wavelength of the light. Plank’s constant is a really
small number-- 6.62 x 10-27 erg sec. So now we can calculate the
mass of a single photon of blue light. That mass comes out to be 1.8 x 10-41
grams!!! That’s less that a billionth of a billionth of a billionth of a
billionth of a gram. So, even if you had a high intensity searchlight in your
hand, you wouldn’t feel a thing.
What
if your flashlight shot out gamma rays? Gamma rays are just like light rays,
but have a shorter wavelength, or a higher energy. But the energy is still in
the same range as that of light, so it still would have a negligible reaction
effect.
But
speaking of equal and opposite reactions, let’s look at something a little
more practical. Think of an automobile sitting in your driveway. If you let some
of the air out of the tires they will get a little flat—more of the tire will
be touching the ground. If you put more air in the tires they will get rounder
on the bottom with less of the tire touching the ground. Now gravity is pulling
the car down with a force proportional to its weight, and the driveway is
pushing back with an equal force; if not, the car would sink into the driveway.
Let’s say that you measure the pressure in your tires and find that you have
30-lbs./sq. inch in each tire. Thus there is 30 pounds of pressure in the tire
for every square inch of the tire that is touching the driveway.
Now, get out a ruler and measure the area that each tire has touching the
ground. Let’s say that the tire is 5 inches wide, and six inches along each
tire is touching the driveway. That’s 5 x 6 = 30 sq. inches per tire, or 120
square inches for the four tires. If you have 30 pounds pressure for each square
inch of the tire that is touching the ground, then you have 3600 pounds pressing
on the ground--the weight of your car! So, you can weigh your car with just a
tire gauge and a ruler.
Gramps