# Tag Info

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If you are familiar with electric circuits in a quite loose analogy you can look at your windows as impedances for the air current. Since current is turbulent (take a look at the Reynolds number if you do not know it) the air does not directly go all the way down through all the windows, but each of them will create some impedance to the flow. The bigger the ...

17

Each window represents a restriction to the air flow. The greater the pressure difference across the aperture, the greater the flow. An electrical analogy: each window is a resistor. The current through the resistor is proportional to the voltage across it - but when you have two resistors in series they must carry the same current (air that enters through ...

-1

A bird's wings have muscles, unlike the wings of a plane. It's true that we can control them, but they're metal(\m/). Human mechanisms are not as flexible as those of nature. Compare driving a car with running in your imagination: If you are walking and someone is going to crash into you and you see him, you could dodge it easily. But if you are driving a ...

0

The point you're asking about considers only the matter shovelled out of the way to let you move, ignoring the structure of the matter. That's not all there is to it, but it's enough to make a difference between air and water. Air is less dense than water. Therefore at a given speed the mass of air that you're pushing out of the way to accommodate your ...

1

If you were restrained, and subjected to a sufficiently powerful stream of air, it would be fatal. However, this does not happen during a jump, because you reach terminal velocity, which is typically somewhere between only 100 km/h and 200 km/h. Exposure to a wind of this velocity (which, let us say, consists of clean air that is free of debris such as ...

0

I don't have exact statistics for any of this. The fall is not what kills you, it is the sudden deceleration at the end. The only thing that can cause a change in speed is another force being applied to you. During the fall, the 2 forces of air resistance and gravity are acting on you constantly in opposite directions, with gravity causing more and more ...

2

It depends what speed you are moving relative to the air (or water). If you start a jump at zero velocity relative to the air, your speed will be limited to the terminal velocity of about 125 miles/hours (at least for the density of air near ground level). An estimate of the fatal velocity relative to air is 300 miles/hour (again for the density of air ...

22

It's not the falling that's fatal, it's the deceleration at the end that kills you. Something like water or concrete does this on a sub-meter distance (which requires extremely high forces). On the other hand a gas is much less dense, so it cannot decelerate a falling object nearly as quick. Sometimes inflatable cushions are used as safety nets (think: ...

11

Let's look at this another way: you're just moving from one fluid to another. Sounds harmless, right? By specification of the problem, we're at terminal velocity when we hit the water. The force of drag (in both mediums) is roughly: $$F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A = \rho \left( \frac{1}{2} v^2 C_D A \right)$$ You can imagine that ...

11

Consider jumping into a swimming pool. Do a barrel-roll (sorry I mean cannon ball, that just kind of slipped out). It's fun, you enter the water nicely and make a huge splash, probably soaking your sister in the process (that'll learn her). Now do a belly flop. Not as fun. You displace exactly the same amount of water in the same time, but this time there is ...

6

I'm not a physicist. So I am treading very carefully attempting to answer a question here... :) A physical example that may help explain this is rock skipping. When you skip a rock, it will 'bounce' off of the water when at high speeds. Eventually it slows enough to no longer bounce but 'sink' into the water. Picture your body doing the same thing. Your ...

11

The ocean surface is not as hard as the ground but if you drop from a plane, you would hit it with such a high velocity that the pressure would most likely kill you or cause very serious damage. Considering air resistance, the terminal velocity of a human, right before reaching the water, would be at most some $150\text{ m/s}$. If you weigh $70\text{ kg}$, ...

29

When you would enter the water, you need to "get the water out of the way". Say you need to get 50 liters of water out of the way. In a very short time you need to move this water by a few centimeters. That means the water needs to be accelerated in this short time first, and accelerating 50 kg of matter with your own body in this very short time will deform ...

0

when you go fast enough the water molecules just can't move out of the way fast enough for a soft landing.

1

In normal subsonic flying, as air gets thinner, it means there is less lift but also less drag, which can be made up for by going at a higher speed. So to get maximum range and speed, planes fly as high as possible. (Other factors may weigh against this, such as headwinds being faster at higher altitude.) Piston engines generally require turbochargers at ...

1

Yes it's called the absolute ceiling. This is the highest altitude at which an aircraft can sustain level flight. When a plane reaches this height the thrust of the engines at full power is equal to the total drag at minimum drag speed. This occurs where the maximum thrust available equals the minimum thrust required, so the altitude where the maximum ...

1

A helicopter is an airplane whose wings happen to go in circles. As an airplane goes higher into thinner air, its wings get less lift and less drag, so with the same power, it just goes faster - bringing its lift and drag up to what it is at lower altitude. That's why jets fly high. Of course with jets there's a limit because as the air gets thinner the ...

3

As altitude increases, density of air decreases. As air density decreases, the lift of the helicopter blades decreases (for a given angle of attack and RPM). As long as the "real aircraft" is open to the atmosphere (doesn't have a pressurized cabin), performance of the toy helicopter would be the same as if it were outside the "real aircraft".

0

Nice question! The answer is yes, it is certainly possible. There is no need to start at the top of the atmosphere though. The air around the earth has a density (and pressure) gradient that increases as you get closer to the surface, see the figure below. If you have an object whose average density is less than the atmosphere at the ground, then it will ...

0

You need to consider the pressure on the cube. The cube would have a certain initial average density (mass of the steel divided by the volume of the cube). The cube would sink to at least a level of its average density. Then the pressure would begin to crush the cube, decreasing the volume and increasing the density. The cube would progressively be ...

0

Calling something aerodynamic is shorthand for calling it aerodynamically efficient. There's nothing wrong with referring to a system as thermodynamically efficient, it's just that this isn't usually shortened to thermodynamic.

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