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Comment: Re:The TSA's math is real wrong. (Score 1) 199

by JaWiB (#35467846) Attached to: TSA To Retest Full Body Scanners For Radiation

Newton's third law doesn't necessarily mean the same amount of work is done on both objects. For instance, consider a block attached to a spring attached to a wall. With the spring initially compressed, it does work on the block as it moves away from the wall, but the spring does negligible work on the wall because the wall is not moving. Substitute pressurized gas for spring, bullet for block, and rifle for wall. An added complication is that the force on the bullet is actually due to gases in the barrel, so if the gases are accelerating then the rifle will feel a different force than the bullet.

The best way to analyze the situation is using conservation of momentum. Assuming the bullet and rifle are both at rest before firing, total momentum must be zero. Take .01kg for the bullet mass, 1kg for the rifle mass, and 1000 m/s for the muzzle velocity. Then .01kg * 1000 m/s = 1 kg * V_rifle --> V_rifle = 10 m/s. In contrast, the kinetic energy of the bullet is .01kg * (1000 m/s)^2 = 10^4 J versus 1kg * (10 m/s)^2 = 100 J, so I would expect the bullet to have around two orders of magnitude greater energy.

You could check wikipedia for "Free Recoil" and "Muzzle Energy" if you want to see how accurate my approximation is.

Comment: Re:The TSA's math is real wrong. (Score 1) 199

by JaWiB (#35466832) Attached to: TSA To Retest Full Body Scanners For Radiation
The two major flaws in your argument are that radiation doses (in rem or Sv) are calculated using a "quality" factor that takes into account the type of radiation and the tissue that absorbs it, and the fact that the units are in energy/mass so the calculations _should_ already take into account the frequency dependence of photon energy. But I do agree that people should be more clear when they say "radiation"--I get the feeling that people hear x-rays and they think of nuclear decay involving protons and neutrons, when x-rays are just high energy light. And what does "background radiation" really mean? Everything around me is radiating but it's mostly infrared light and I'm not too worried about my exposure to infrared, even cumulatively over a long time period.

Comment: Re:Just because the math works doesn't mean it's t (Score 3, Insightful) 650

by JaWiB (#30717122) Attached to: The End Of Gravity As a Fundamental Force
My understanding of an "entropic force" is that it can be described in terms of fundamental forces. The pressure in an ideal gas, for instance, can be derived by looking at the impulse created by a single molecule, and then extending that to a collection of N molecules. This guy seems to be saying that gravity is an entropic force and therefore NOT a fundamental force, but it seems to me that entropic forces are just an abstraction that allows us to ignore the underlying fundamental forces. Of course, I didn't read the whole article, but what I read was poorly written and that doesn't inspire a lot of confidence. Maybe I'll take another look if it gets published.

Comment: Re:Sugar cane not corn (Score 1) 894

by JaWiB (#28090555) Attached to: The Great Ethanol Scam

According to Wikipedia, as of 2008-04-30, ethanol production is no longer subsidized. Care to provide a source countering that?

And to the people who say that more energy is consumed in producing corn I'd also ask for a source. According to the DOE, has a positive energy balance. (Granted, the source is the government, so I guess they're only saying that because of corn lobbyists)

I'm honestly willing to hear claims to the contrary, but all I see here on slashdot is a bunch of people saying that ethanol is bad without citing any research (even TFA didn't cite any original research)

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