Ask and answer stupid questions here! - Page 708

On October 08 2018 09:11 Dangermousecatdog wrote:
None of these sentences make any sort of actual sense.

1) Kinetic energy wtf? You are concerned with mechanical forces, not energy. Energy shouldn't even come into it.
2) Energy is not created nor destroyed. If the drone hits the roof of the box, forces will act upon it and energy will be transfered, but energy does not simply dissapitate.

On October 14 2018 02:48 Yanokabo wrote:
Since nukes are primarily designed as human to human combat weapons, I.e point to point on earth lunch and detonation, if you shot even 1 nuke at the moon, would it be able to even reach the distance and detonate? Wouldn’t the vacuum in space destroy the nuke long before it arrives at the moon? Even if you then shot “every nuke the Russian aresenal posesses” into space it would still result the same?

On October 14 2018 03:38 Sent. wrote:
I did ask to assume the missiles have what it takes to reach the moon. Was mostly curious if all those explosions could lead to anything spectacular like making a crater easily noticeable from Earth, brightening the night or even slightly altering the moon's orbit.

On October 14 2018 02:38 Simberto wrote:
Okay, lets do a few quick calculations there.

The russians apparently have about 1600 strategic nuclear warheads, and 1500 tactical nukes according to wikipedia. So, since we almost certainly are only interested in orders of magnitude, lets say that rounds to about 3000. I have no idea how strong those are, but the strongest tested nuke the russians had was the Tzar Bomba with 50 Megatons. So lets say they average at 10 Megatons.

That means we are talking about a total energy of 30 Gigatons of TnT. 1 Megaton is 4.182*10^15 Joules, so 30 Gigatons equate about 1.2 * 10^20 Joules. Apparently about 50% of the yield of a nuke is in thermal radiation (Of which visible light is a part).I have no idea if a detonation in a vacuum is different. So we are at 6*10^19 Joules of Thermal radiation. Let's just say that is all visible light, and evenly distributed in all directions.

The earth as seen from the moon has an angular diameter of about 2°. An Angular diameter of 2° means that less than 1/1000 of the directions into which the radiation can go are towards earth. That means that the earth is hit by about 6*10^16Joules of Energy.

As a comparison, the total solar energy which hits the earth is about 1.3*10^17 Watts, which means 1.3*10^17 Joules/second. This means that for about half a second, the sunlight is doubled. Or, if it is done at night, we are talking about half a second of sunlight at midnight.

Now, all of my calculations might be wrong, and there are a lot of assumptions. So i would say that the total effect is definitively less than that, but it would still be a very visible flash of light.

On October 13 2018 02:52 AbouSV wrote:
Going back to that helicopter in a box question, there are some good, and not so good intuition there. In the end the understanding is not very difficult, though the calculation would be much more if you want actual numbers.

First, let's address an important point:

1) Lifting an item requires your forces to work. The work done by your force to do so will be a variation of the energy of your system, and in udual cases, a change in kinetic and potential energy. I will come back to that point latter on.
2) I believe this is more a problem of phrasing and you are both saying the same thing on that point. The energy can absolutely dissipate. It does not mean it disappears. If we are still speaking about kinetic energy for instance, it can turn to potential energy, thermal energy (though one could put this as kinetic energy depending on what you are looking at) and so on.

Now, back to the main problem:
The helicopter/drone/... lifts itself by creating an airflow going downward (opposite to gravity). So in a way it is pushing down to go up, but that does not mean that the box is equally pushed down by this airflow.
Let us take as a first approximation that the air flow as uniform (having the same airflow anywhere under the blades). You would get for a medium transport helicopter (Sikorsky S-92) a pressure of about 50 kg per square meter (about 10 000 kg and a rotor area of 200 m²). This number goes down to 8 kg/m² for a small chopper (Robinson R22, 400 kg, 50 m², damn these rotors are actually huge), and almost nothing for a drone. So let's stick with the big stuff to help us see what is happening.

I believe the question is how much of this 8-50 kg/m² actually "pushes the box down". Even though it is a fair approximation to say that the air is pushed exactly downward and not in a cone (the downward force needs to be at least this, otherwise you would not have enough lift, so if the airflow is in a cone, you could ignore the non-vertical part of the flow) , the answer is impressively little and it decreases quite fast with the height. The reason is that the airflow in question is not 'straight' (i.e. laminar), but turbulent.
Unfortunately, I do not deal with air in my work, so I do not have much number in my head nor by the hand to give more quantitative info on that. If you desperately need more, I may try to, but otherwise, the general picture would have to suffice :p

The key word here is turbulence.
Your vehicle blades generate high speed airflow, and as it turns out that a faster fluid is more likely to become turbulent (see Reynold stress for more info). As a result, you will have vortices created not far bellow the blades. These vortices will then break into smaller ones, which will then break into smaller, which will then... Well, you can guess.
Now, how come these vortices do not push the box down?
If you get to sufficiently small sizes, then they cannot really break into smaller ones: imagine you only have one air particle in a given vortex, it cannot split itself! Although, you don't actually get to this point. When you reach the limit in size, then the energy of your turbulence becomes thermal energy. It heats the air.
Coming back to my first comment about energy, you can sum up as: The drone helicopter is pulled by the force of gravity, and needs to compensate by apply a force down that will work and create kinetic energy in the form of airflow. This kinetic energy will then dissipated through by turbulence to heat.
So the end result is: the scale does not show the flying item weight at all, but the air will gradually heat up. Slowly. Very slowly. Especially compared to your running engine, whether electric or thermal.

The number I cannot give as is, is the actual length at which the flow becomes turbulent, and on which scale does this turbulence breaks down. Though you cannot actually solve the equations for that, you have many simple numerical codes that can simulate it very accurately.

On October 20 2018 22:11 pmh wrote:



Interesting,this means that in theory the drone could eventually lift up the box as well if it would have enough power. This seems very counter intuitive to me to the point where I consider it impossible? By heating a box you will eventually be able to lift it up?
In this situation the box itself has to generate downward trust without any mass getting ejected,seems impossible to me. It would mean that space travel would not depend on ejecting mass,which would be quiet a revolution.

I probably did miss something here but I can not see what it would be,maybe upward vortexed when box comes close to ceiling? could be avoided by placing a large pole on drone to create sufficient distance then?
Not sure what is missed,your explanation does make a lot of sense when i read it.

Have mythbusters not tested this yet?