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spacexplain

Last status update:
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Gender: male
Date Signed Up:4/01/2014
Location:Huygens Crater on Mars
FunnyJunk Career Stats
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Highest Content Rank:#1723
Highest Comment Rank:#3694
Content Thumbs: 759 total,  795 ,  36
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Level 57 Content: Sammich eater → Level 58 Content: Sammich eater
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Content Views:37477
Times Content Favorited:70 times
Total Comments Made:586
FJ Points:2300
Something cool happen in a black endless void, and you wanna know what it is? A mention will get you all the knowledge you need!

latest user's comments

#77 - Space horror, you say?  [+] (3 new replies) 18 hours ago on I am making a Space... +17
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#163 - harion (14 hours ago) [-]
I know. But I enjoy the challange.
#220 - anon (13 hours ago) [-]
psst, not that kind of hard my friend, the expansion lord has other things in mind with that picture
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#229 - harion (13 hours ago) [-]
~Uhuhuhu...<3
Mysterious <3 ;3
#29 - Dibs on a useless, migrant cheese maker that dies to a carp. 07/26/2016 on Fortress announcement +2
#71 - Exactly, so as soon as the plane breaks even with the treadmil…  [+] (1 new reply) 07/23/2016 on Plane almost stops mid air... 0
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#74 - therealtjthemedic (07/23/2016) [-]
Of course.
#69 - The way I understand the situation, the treadmill moves at a c…  [+] (3 new replies) 07/23/2016 on Plane almost stops mid air... 0
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#70 - therealtjthemedic (07/23/2016) [-]
well yeah, all you have to do is ignore the treadmill and look at the airspeed
if the plane could move faster than the treadmill and have an actual airspeed then definitely, the plane could take off
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#71 - spacexplain (07/23/2016) [-]
Exactly, so as soon as the plane breaks even with the treadmill, the takeoff process is as normal.
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#74 - therealtjthemedic (07/23/2016) [-]
Of course.
#54 - It's not about the lifting area of the body behind the propell… 07/23/2016 on Plane almost stops mid air... 0
#51 - The prop does provide the thrust. It's not just some…  [+] (16 new replies) 07/23/2016 on Plane almost stops mid air... 0
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#57 - therealtjthemedic (07/23/2016) [-]
>>#57
if the plane is on a treadmill, then it's not moving relative to the air
if you stood next to the treadmill you'd see a plane with the propeller on full not moving at all
no air would run over the wings
the wings would provide no lift
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#82 - lean (07/25/2016) [-]
Thrust > rolling tire friction bud.
The thrust will pull it forward regardless of the track speed.
#62 - anon (07/23/2016) [-]
Let's analyze your assumption. Why do you think the plane will not move if it's on a treadmill? You know the impetus for forward motion is not provided by the wheels, but by the prop creating thrust. You know the prop does not interact with the ground. You know the wheels are not powered. You know (or should know, at least) that the prop's thrust is strong enough to overcome any friction generated by the wheels unless the brakes are on (in a traditional prop plane, at least). As seen in the GIF, you also know air speed and ground speed are not necessarily the same thing. So, why do you assume the plane will not move if it's on a treadmill?
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#63 - therealtjthemedic (07/23/2016) [-]
...
Because, the air speed is zero.
The plane is not moving relative to the air if it's just on a treadmill moving forwards as the treadmill moves backwards..
That's it.
#65 - anon (07/23/2016) [-]
You keep saying the same thing. If the plane is not moving if it's on a treadmill, obviously its airspeed is zero. I ask, "Why do you think the plane will not move if it's on a treadmill?" and you respond with, "The plane is not moving relative to the air if it's just on a treadmill moving forwards as the treadmill moves backwards.." I understand that's your assumption. I want you to tell me why.
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#66 - therealtjthemedic (07/23/2016) [-]
Isn't that the base of the question? Whether a plane on a treadmill going the same speed as the treadmill in the opposite direction could take off?
That's how it was told to me.
#72 - anon (07/23/2016) [-]
I'm specifically looking for what physical laws/phenomena you believe are in effect that prevent the plane from lifting off.

The force that causes the plane to move forward does not depend on its interaction with the ground, unlike a car or bipedal motion. If the treadmill is moving back at a rate of 2v (twice the velocity of the airplane), and the airplane is moving forward at 1v, the wheels will spin at a rate proportional to 3v. The plane will move forward at 1v because the prop is thrusting against the air, not the ground. The air (assuming a calm day) is stationary, unlike the ground. In a car on a treadmill, for example, the car is thrusting against the ground, which means the car's velocity relative to stationary ground will have to be greater than that of the treadmill's in order to achieve motion. The same restrictions don't apply to the plane because the force inducing motion is acting on a different medium.
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#73 - therealtjthemedic (07/23/2016) [-]
To counteract the movement of the ground would require a force from the plane. You're assuming the wheels are frictionless, I guess?
#75 - anon (07/23/2016) [-]
That's what I'm saying: there is a force from the plane, provided by the prop. The wheels are just coasting there.

Think of it this way, in a reverse situation. Let's say the wind is blowing, and you're walking into it. You can do this because your forward motion is provided by your feet interacting with the ground. Sure, you'll be resisting the wind, and the plane will be resisting friction from its wheels (a small amount, but it's there nonetheless), but you can still move forward. Similarly, the thrust provided by a prop--which is acting on the air, not the ground--is easily able to overcome the friction from the spinning wheels.

Here's another example. Ocean currents can be dangerous because they'll sweep you out to sea, and they do this because the swimmer is resisting the same medium that's moving. In walking in the wind and taking off from a treadmill, you/the plane are resisting mediums that are stationary, even while a nearby medium is not.
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#76 - therealtjthemedic (07/23/2016) [-]
The wheels aren't really coasting, though. Consider a plane attempting to move from one spot on the ground to another, it has to apply a constant force due to friction from the wheels.
If you take that reference frame and put it in motion, you'd have the treadmill problem.
Wherin, the plane would have to apply thrust to counteract the motion of the treadmill.
#77 - anon (07/23/2016) [-]
And that constant force is provided by the prop. Friction does not increase with speed. The equation for friction force is F=mu*N, where N is the normal force, the Greek letter mu is the coefficient of friction (determined by the materials interacting with one another), and F is the force created by friction. There is no velocity term in the equation. The force provided by the prop does increase with speed: as the prop spins faster, the force increases. The friction from the wheels is going to be small. When I took flying lessons (back when I was able to afford it), the pilot would pull the plane out himself. There's not a lot of resistance there. So, no matter how fast the wheels are spinning, friction remains constant. Obviously, if the wheels spin much too fast, they can heat up, which can create physical changes, thereby changing the coefficient of friction, but that's beyond the scope of this discussion.
#78 - anon (07/23/2016) [-]
This anon is going to head out. If you still have your doubts, there's probably some Khan Academy-esque video about it.
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#69 - spacexplain (07/23/2016) [-]
The way I understand the situation, the treadmill moves at a constant rate. If this treadmill is going let's say 5mph backward relative to the plane, it could still take off, since it's essentially just simulating a very slight tailwind. The plane still picks up speed and takes off no problem, since the treadmill isn't interacting with the prop, just the freely spinning wheels.
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#70 - therealtjthemedic (07/23/2016) [-]
well yeah, all you have to do is ignore the treadmill and look at the airspeed
if the plane could move faster than the treadmill and have an actual airspeed then definitely, the plane could take off
User avatar
#71 - spacexplain (07/23/2016) [-]
Exactly, so as soon as the plane breaks even with the treadmill, the takeoff process is as normal.
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#74 - therealtjthemedic (07/23/2016) [-]
Of course.
#40 - Take a regular Barret M82 Move the trigger assembly in fro… 07/09/2016 on /k/omamando defend's his home +2
#21 - Your name really fits here. 06/23/2016 on >when you're bad at drinking +3
#104 - That one in the picture went for $900  [+] (1 new reply) 06/21/2016 on For when it's needed 0
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#126 - scowler (06/22/2016) [-]
Dammit.
#103 - This thing is pretty much a backup for your backup. Obviously … 06/21/2016 on For when it's needed +2