Originally Posted by Wizofoz
(Post 10216696)
We are all on the same page, but just someth8ng for the poitive_negative guys to mull- you are defining positive ground speed as being in the direction the aircaft is ponting- well, if there is any cross wind, the aircraft isn' t tracking that way- so is a teack 30 degrees off the nose " moslty positove "
answering the question on a 50 knot airspeed in a 60 knot headwind- how far will the aircaft be from its starting position after one hour? |
Said what for you? Speed is distance over time. Velocity is distance over time with a direction component, a vector. The ground speed of this slow flying Cessna is 10 knots; it may not be making any progress towards its intended target, but its speed over the ground is 10 knots. Its velocity vector is 10 knots in the direction of the wind. There's no 'negative' speed, there's speed in the direction opposite to the intended travel. If you want to plot this on an X -Y axis then you could certainly make the direction it ends up traveling as -X, the speed (magnitude) is still 10 knots, the direction is negative. |
Originally Posted by bluepost22
(Post 10216703)
Can I propose a gedanken (thought experiment)?
A Cessna is flying into a 10 kt headwind and has ground speed 90 kts. Instantaneously the headwind increases to 20 kts. How long does it take for the Cessna's ground speed to attain 80 kts? I've always wanted to know. |
Originally Posted by Winemaker
(Post 10216715)
If you want to plot this on an X -Y axis then you could certainly make the direction it ends up traveling as -X, the speed (magnitude) is still 10 knots, the direction is negative.
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Originally Posted by Winemaker
(Post 10216715)
Absolute value.
Speed is distance over time. Velocity is distance over time with a direction component, a vector. The ground speed of this slow flying Cessna is 10 knots; it may not be making any progress towards its intended target, but its speed over the ground is 10 knots. Its velocity vector is 10 knots in the direction of the wind. There's no 'negative' speed, there's speed in the direction opposite to the intended travel. If you want to plot this on an X -Y axis then you could certainly make the direction it ends up traveling as -X, the speed (magnitude) is still 10 knots, the direction is negative. |
Originally Posted by bluepost22
(Post 10216703)
Can I propose a gedanken (thought experiment)?
A Cessna is flying into a 10 kt headwind and has ground speed 90 kts. Instantaneously the headwind increases to 20 kts. How long does it take for the Cessna's ground speed to attain 80 kts? I've always wanted to know. |
Aka the speed is negative 10 When you are discussing ground speed I assume you are thinking in aviation terms, but this whole thread is not about aviation terms, it's about somehow magically gaining energy when turning windward in a steady wind environment. This seems to me to indicate that we need to treat this as a physics problem, not an interpretation of 'ground speed' in the aviation sense. So call the Cessna's ground speed -10 knots/hr if you wish, and I'll call it 10 knots/hr at angle Θ, but I think we mean the same thing. |
Originally Posted by Jet_Fan
(Post 10216711)
The idea that GS can’t fall into a negative value is really quite stupid. No need to report GS as mainly positive as its regarded as positive unless stated otherwise.
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Originally Posted by bluepost22
(Post 10216703)
Can I propose a gedanken (thought experiment)?
A Cessna is flying into a 10 kt headwind and has ground speed 90 kts. Instantaneously the headwind increases to 20 kts. How long does it take for the Cessna's ground speed to attain 80 kts? I've always wanted to know. This would be the same as flying at 100 kts, then quickly reducing throttle to the thrust you know will sustain 90 knots. From experience I would say a matter of 5-10ish seconds. |
Originally Posted by Jet_Fan
(Post 10216611)
As you love wiki so much, let's see what it has to say about groundspeed:
What is 50 - 60? https://en.wikipedia.org/wiki/Speed in everyday use and in kinematics, the speed of an object is the magnitude of its velocity (the rate of change of its position); it is thus a scalar quantity. A position can't have a negative rate of change. |
Originally Posted by Wizofoz
(Post 10216816)
So, if you have a groundspeed of negative thirty knots for an hour is your displacement negative thirty miles from your origin?
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Originally Posted by Vessbot
(Post 10216825)
Yes!! When defined as positive one way, negative the other. You know, like that nonsensical useless system called longitude.
The distance between two paces can't be negative thirty miles. |
Originally Posted by Wizofoz
(Post 10216826)
Thus, you have added direction to magnitude and are talking velocity not speed.
text filler |
OK,
Scenario B2 (really fast running person inside airliner. He runs at 501 knots and can turn around on a dime.) final groundspeed minus initial groundspeed (airspeed + tailwind) - (airspeed - headwind) or, if you like: (walk + tailwind) - (walk - headwind) (501+500)-(501-500) 1001-(1) 1000 Scenario B3 (really really fast running person inside airliner. He runs at 1000 knots and can turn around on a dime.) final groundspeed minus initial groundspeed (airspeed + tailwind) - (airspeed - headwind) or, if you like: (walk + tailwind) - (walk - headwind) (1000+500)-(1000-500) 1500-(500) 1000 Holy schnike, it turns out that if you substitute any number for the airspeed, the groundspeed difference is 1000, which is twice the wind speed when going from upwind to downwind. Unless that number is less than the wind speed, then it becomes a special case for some reason that wraps people around the axle over scalars and quantum chromodynamics and whatever else. |
Originally Posted by Vessbot
(Post 10216827)
OK, and?
text filler |
Originally Posted by Wizofoz
(Post 10216831)
We were talking about ground speed, not ground velocity.
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Originally Posted by Vessbot
(Post 10216833)
Get real. You know the two are collquially interchangable, and you know exactly what I meant. The point isn't just flying by you, you're actively dodging it. Look one post up.
Your one post up is just silly- turning your back does not change your velocity. |
Originally Posted by Wizofoz
(Post 10216835)
No, speed and velocity are completely different things and are useful; in different circumstances.- and if your whole argument has been that they are the same, you have been sadly mistaken.
Your one post up is just silly- turning your back does not change your velocity. Do you apply that same criticism to scenario A? If not, why not? What's the difference? Scenario A (Cessna) final groundspeed minus initial groundspeed (airspeed + tailwind) - (airspeed - headwind) (100+10)-(100-10) 110-90 20 |
Originally Posted by Vessbot
(Post 10216837)
Do you think there's an error in the math? If so, what is it?
Do you apply that same criticism to scenario A? If not, why not? What's the difference? Scenario A (Cessna) final groundspeed minus initial groundspeed (airspeed + tailwind) - (airspeed - headwind) (100+10)-(100-10) 110-90 20 |
To make sure you haven't lost the plot -- what do you think my (attempted) point is?
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