An air density puzzle

[old man emu]

1 hour ago, facthunter said:

More correctly it's the decelerative force you are subject to that does the damage to you.

Yes and no. In an impact, there are a couple of things that cause injury.

 

Let's look at the vehicle itself.

Prior to the impact, the vehicle, of mass m, has a velocity, v1. As a result, it has momentum, p = (m x v1). After the collision its mass remains the same, but its velocity will change to v2. The change in momentum due to the collision is m(v1 - v2). Since it takes time for a collision to start, then end, Time, t, is involved. This gives the component {m(v1 - v2)}/t.

 

(v1 - v2)/t. is change in acceleration,  Delta a, or, in terms of collisions, rate of change of momentum.

 

We know that Force = m.a

or Force = m.(v1 - v2)/t.

Therefore, Force.t = m(v1 - v2).

The application of a Force for a period of time is called Implulse. So Impulse in this case is equal to the magnitude of the change in momentum.

 

Here are two examples of the injurious effects of Impulse.

Imaging two pallets of goods dropped in an air drop operation. If the the first one is kicked out without a parachute, it will reach a terminal velocity, and collide with the ground at that velocity. What will be its terminal velocity?

 

 

Since we are comparing two pallets of the same projected area, falling through air of the same density, then we can set p and A both at unity. This means the terminal velocity of two identically loaded pallets is only dependent on their drag coefficients. The parachute will add to the pallet's drag coefficient, so it will reach a lower terminal velocity. When the pallet collides with the ground, it will suffer a much greater change in Momentum than the pallet with the chute.

 

Now, switching terms around we can get

v = p/m

But we also know that Kinetic Energy = 1/2 m.(v^2). This allows us to work out the Kinetic Energy of the pallets as they hit the ground.

K.E. = 1/2.m.(p/m)^2

 

OK. That deals with the Forces involved in a collision. The other cause of damage, especially to the our bodies, is their Inertia.

 

Before the collision, our internal organs are sort of floating about in boney cages, and everything is moving in straight line at constant velocity (Newton No.1). When everything around our bodies stops due to a collision, the inertia of our skeletons, brains and organs makes them keep going at the same speed until they either hit something, or reach the limits of their natural movement.

 

Heads go forward, but reach their limit of movement because the torso is connected to the vehicle by restraining belts. The neck and spinal column can snap from over-extension. Similarly the aorta, coming out of the heart can rupture because the heart is better restrained in the chest cavity. The brain suffers most because it is floating in fluid and when the skull stops moving, the brain keeps going, crashing into the skull bones. Injury ranges from concussion to severe trauma to the brain tissue. 

[Bruce Tuncks]

The correspondence between weight ( the force exerted by gravity on a mass ) and inertial change ( F=MA ) was so amazing that it led Einstein to relativity.

There is NO WAY to distinguish between weight and force due to acceleration. So Einstein concluded that they were indeed the same thing and that the fabric of space-time had to be re-thought, in such a way that space and time were warped by a mass to make this so.

Gosh, this is even harder than Jabiru maintenance.

The Einstein thing led to different math and things like the " aberrations" of the orbit of Mercury were explained and measured to be true. Wow, I wish I were smart enough to understand the math.

 

[RossK]

So we started talking about sailing ships and we're onto the orbit of Mercury 🤪

[old man emu]

Per ardua ad Astra

[Bruce Tuncks]

Just imagine that the force of gravity was different from inertial force. We could have a much better artificial horizon and cheaper too.

[APenNameAndThatA]

On 03/12/2020 at 8:52 PM, old man emu said:

 

You messed up because you don't know the meaning of Newton's First and Second Laws. Inertia is a concept. Inertia is simply the tendency of an object to follow a straight line trajectory at a constant speed, unless acted upon by an external force. The tendency of an object to resist changes in its state of motion varies with mass. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion. The sense of the word "inertia" comes from the Latin "iners", which means "lazy"

 

The Momentum of a body is something that it possesses due to the product of its mass and its velocity. We can measure both these quantities to calculate the momentum the body possesses.

 

NO. Momentum is the thing that changes due to acceleration (positive or negative). You can only change the inertia of a body by adding or removing mass.

I stand corrected. Well done on clawing one back. The issue wasn’t that I don’t know Newton’s laws. The issue was that I didn’t know the definitions. As far as I can tell, inertia = mass. Momentum = mass x velocity, broadly speaking. 

[old man emu]

7 hours ago, APenNameAndThatA said:

Well done on clawing one back.

Listen, mate.

I'm getting well and truly sick of you flaming me. I have yet to see you make a positive contribution to any topic on this forum. You might think that you are being quite smart, but all you are doing is showing what a moron you are. If you can't stop flaming me then F off.

[Scotty 1]

18 minutes ago, old man emu said:

Listen, mate.

I'm getting well and truly sick of you flaming me. I have yet to see you make a positive contribution to any topic on this forum. You might think that you are being quite smart, but all you are doing is showing what a moron you are. If you can't stop flaming me then F off.

This site never fails to amaze me.............

[Geoff13]

On 01/12/2020 at 10:13 AM, old man emu said:

A flaw in my otherwise flawless conditions. However, from here on in, I will specify that the wind speed is measured by a hot-wire anemometer. They take advantage of the fact that air cools a heated object when it flows over it. In a hot-wire anemometer, an electrically heated, thin wire is placed in the wind. The amount of power needed to keep the wire hot is used to calculate the wind speed. The higher the wind speed, the more power is required to keep the wire at a constant temperature.

Of course if you mount two of these side by side (as in the Leopard tank TFCS) you can also tell the direction of the wind.

However if mounted along the centre line of the vessel with the wind blowing from astern, it would not give a reading. 

 

Oh to live in a theoretical world. Sorry OME just playing.

 

[APenNameAndThatA]

On 05/12/2020 at 7:18 AM, old man emu said:

Listen, mate.

I'm getting well and truly sick of you flaming me. I have yet to see you make a positive contribution to any topic on this forum. You might think that you are being quite smart, but all you are doing is showing what a moron you are. If you can't stop flaming me then F off.

OME, I don’t blame you for being angry. This time were going well and looking clever, until you said that something that had a weight of 2.3 kg had a mass of 0.23 kg. 😞

 

With the previous thread about the carburettor ice, I said that a trimmed aircraft that loses power would lose height rather than airspeed. At first I thought that making an important point would be a “positive contribution”, but in the light of your objective I guess my post was very negative. ☹️
 

Earlier on this thread you got upset at someone for answering this question the wrong way. The same thing happened the last thread too. It looks like if you keep posting you are going to feel worse and worse. I suggest that you try blogging. You will have administrator privileges so when someone makes a negative comment, you can change things so you look better. If you write the whole thing in Latin you will be able to keep the riff raff away. 👍  

 

 

[lee-wave]

On 01/12/2020 at 9:06 PM, old man emu said:

I should have simplified the whole bloody thing by not posing the question at all. I pose a serious question and get stupid responses. There are times and places for wit and repartee. One of these is called socialaustralia.com.au. 

 

The point of the question, and, dare I say, the title and location where it exists, indicate that it is something to be contemplated in light of pilots' supposed knowledge of the properties of the atmosphere and the effects those properties have on Man and machine.

There was no intention to ridicule your question....but the heading of your posting was 'an Air density puzzle' and then the question you posed was 'which ship would travel faster in the two different regions assuming both identical ships were sailing (in a steady state motion) directly downwind in a steady 10knot wind'.   

Most people would assume that your question is.. 'will different air densities affect the velocity of the ships ?'.  The answer is a lot more complex then you might think...

Before you can answer the question the velocity of the wind has to be calculated in the same way pilots calculate TAS and CAS. 

Generally TAS and CAS are within a few knots of each other in an aircraft but for ships in different regions of the world not only would the atmospheric pressure be taken into account and measured very accurately but the atmospheric temperature will also have to be measured very accurately to make sure the wind velocities are identical in both regions of the world.  There was some suggestion of using a hot wire anemometer to measure the true wind velocity which is good thinking along the correct lines...

Assuming some very clever person has got the wind velocity absolutely spot on and assuming the density, temperature and salinity of the sea in both regions are identical......... repeat the question.... which ship will travel faster?    and the answer is..... the ship sailing in Alaska will 'go faster' by a very, very small margin.    why ?