Allancw's World

[ka9q]

Right up there with pizza boxes.

I can't tell if this is parody or  not. Poe's Law is complete.

[raven]

It was meant to terrorize the population into submission and not merely to destroy military infrastructure. I'd call that terrorism.

Well, yes. But in the language of the Allies that was called "weakening enemy morale" -- at least when they did it to the Germans and Japanese. It was Joseph Goebbels who first used the term "terror bombing" to describe aerial strategic bombing. Seriously.

No denying World War II, despite being pretty much the closest we've had to a just war, had its dirty, no, downright horrific, moments on both sides. Still, if Hitler couldn't do destroy that resolve with the full force of night after night of legions of bombers and rockets who could kill you before you even heard them coming, I doubt anyone could.

[ka9q]

At least not with the technology available in 1940.

[gwiz]

Strictly, dynamic pressure is half the air density times the speed squared.

I don't know much about aeronautics, so let me work out the physical meaning of these units. This looks a lot like the classic kinetic energy equation, 1/2 * m * v², with air density substituted for mass to give units of energy per volume. So dynamic pressure is just the kinetic energy density of the air in joules (J) per cubic meter.

The basic units of J/m³ are the same as the pascal (Pa), the SI unit of pressure. And multiplying that pressure by an area (like the effective cross section of an airplane fuselage) gives the dynamic force, which in SI units would be newtons (N).

Did I get all that right?

Yes.  You can think of pressure as a measure of energy stored up in the air.  Release the air into a vacuum and that stored energy becomes kinetic energy.

[Bob B.]

Strictly, dynamic pressure is half the air density times the speed squared.

I don't know much about aeronautics, so let me work out the physical meaning of these units. This looks a lot like the classic kinetic energy equation, 1/2 * m * v², with air density substituted for mass to give units of energy per volume. So dynamic pressure is just the kinetic energy density of the air in joules (J) per cubic meter.

The basic units of J/m³ are the same as the pascal (Pa), the SI unit of pressure. And multiplying that pressure by an area (like the effective cross section of an airplane fuselage) gives the dynamic force, which in SI units would be newtons (N).

Did I get all that right?

I've always seen the equation for dynamic pressure as resembling the momentum equation, m*v, rather than the kinetic energy equation.  If we take the air density times the velocity, we have the mass flow rate per square meter.  That is,

rho * v² = (rho * v) * v = m-dot * v

So the equation gives us momentum per second per square meter.  Since momentum per second is force, we have force per square meter, or pressure.

[ka9q]

I've always seen the equation for dynamic pressure as resembling the momentum equation, m*v, rather than the kinetic energy equation.

But how do you account for the 1/2 factor that shows up in the KE equation?

[Allan F]

It's probably from sharing the energy between the air and the vehicle.

[cjameshuff]

It's from integrating a value that starts as zero and rises proportionally with velocity. Analogously to kinetic energy, energy content of a capacitor, tank of compressed air, tank of liquid, etc.

Various attempts to explain this (to a particularly innumerate "new physics" kook, but the math is good despite his denial of it) here:
http://cosmoquest.org/forum/showthread.php?126254-Doubts-About-quot-Modern-Physics-quot&p=1976237#post1976237