# Moon: Apollo 15 Flag Waving

• B
Hi Everyone,

Can someone please explain to me why the flag is moving as if wind is blowing on it during the Apollo 13 mission?

I am well aware that it's not actually wind blowing on the the flag causing it to move, but I still would love to know why this occurs.

Note: I am very uneducated on this subject, a simple answer would be appreciated.

Best Wishes, and Thank you.
Physics345

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fresh_42
Mentor
Nice try. There is no Apollo 13 flag on the moon.

russ_watters and Physics345
Nice try. There is no Apollo 13 flag on the moon.
So that video with the astronauts placing a USA flag on the surface is fake?
Edit: Oops, I may have mixed up Apollo 13 and 15.

CWatters
Homework Helper
Gold Member
Hi Everyone,

Can someone please explain to me why the flag is moving as if wind is blowing on it during the Apollo 13 mission?

Physics345
Like I said, I'm very uneducated on this topic, that's why I'm here to get a better understanding of it.

CWatters
Homework Helper
Gold Member
If you mean this video...

It only moves when it's handled or an astronaut knocks it when going past at the end. That sets it swinging but it soon stops and becomes motionless.

russ_watters, Greg Bernhardt and Physics345
If you mean this video...

It only moves when it's handled or an astronaut knocks it when going past at the end. That sets it swinging but it soon stops and becomes motionless.
Oh okay, so basically a force is acting upon it causing it to move and eventually the force dissipates causing it to become motionless.

Considering the gravity on the moon, the small margin of friction in this scenario, and the low amount of gravity (1.62 m/s^2) present on the moon wouldn't it start to become motionless over a longer period of time?

Is there any math that proves the amount of time it would take for it to become motionless? (Math that takes all the factors into account, such as the force acting upon it, gravity, and etc)

Also I am aware that I sound very ignorant on the topic, but learning has to start somewhere, and I'd rather get some information from experienced/knowledgeable people on here, rather than reading crackpot articles spread over the internet.

Greg Bernhardt
fresh_42
Mentor
Is there any math that proves the amount of time it would take for it to become motionless?
The math is loss by fiction, but the actual calculation is practically impossible, as it depends on too many unknowns, resp. constants which can only be measured by experiment. I think they had a stiff part at the top to keep the flag open and the movements are due to moving the whole thing, either as they put it there or afterwards when touched. The continuous friction by the material will stop it, as it applies a negative force to the system. It will only move again if hit by something.

russ_watters, Greg Bernhardt and Physics345
The math is loss by fiction, but the actual calculation is practically impossible, as it depends on too many unknowns, resp. constants which can only be measured by experiment. I think they had a stiff part at the top to keep the flag open and the movements are due to moving the whole thing, either as they put it there or afterwards when touched. The continuous friction by the material will stop it, as it applies a negative force to the system. It will only move again if hit by something.
Hmm, that definitely makes sense, there are many unknown measurements in this case. I guess in the future, when we create a base on the moon we will be able to conduct more practical experiments such as these.
Is there any way to make a rough estimate, of the unknown variables leading to a vague hypothetical mathematical answer?

I would like to also state that, my line of questioning behind the math, stems from the origin of having an answer to disprove peoples ideas of the moon landing being fake due to the flags motion.

fresh_42
Mentor
Is there any way to make a rough estimate, of the unknown variables leading to a vague hypothetical mathematical answer?
Well, "flag+moon" is certainly a bad idea to search for on the internet. Maybe someone once did a calculation for fun, but the chances to find it are basically zero. I don't even know, what material the flags are made of - maybe it's aluminium. However, this could probably be found out, e.g. on the NASA page, but then it's getting difficult: How long does a flag wave in the air without wind? It's probably far easier to rebuild such a flag, shut windows and doors. stop moving and wait until it stops moving. In this case it stopped mainly due to air resistance. The formula for pressure resistance is ##F_p= \Delta p \cdot A_p## with pressure ##p## and area ##A_p##. This is something we can compute. So ##F_0=m\cdot a - F_p## with the initial push ##F_0## and the mass ##m## of the flag give us a deceleration ##a##. So all we need is an estimate about the initial velocity of movement which is stopped by ##a## after some time.

You see how many unknowns are within such a calculation and how many estimations have to be made. And I only used the pressure resistance by air here. There is probably also a resistant component from the material itself. However, I have the strong feeling that even a flag manufacturer on earth doesn't know the resistance coefficient of his tissue.

Physics345
Well, "flag+moon" is certainly a bad idea to search for on the internet. Maybe someone once did a calculation for fun, but the chances to find it are basically zero. I don't even know, what material the flags are made of - maybe it's aluminium. However, this could probably be found out, e.g. on the NASA page, but then it's getting difficult: How long does a flag wave in the air without wind? It's probably far easier to rebuild such a flag, shut windows and doors. stop moving and wait until it stops moving. In this case it stopped mainly due to air resistance. The formula for pressure resistance is ##F_p= \Delta p \cdot A_p## with pressure ##p## and area ##A_p##. This is something we can compute. So ##F_0=m\cdot a - F_p## with the initial push ##F_0## and the mass ##m## of the flag give us a deceleration ##a##. So all we need is an estimate about the initial velocity of movement which is stopped by ##a## after some time.

You see how many unknowns are within such a calculation and how many estimations have to be made. And I only used the pressure resistance by air here. There is probably also a resistant component from the material itself. However, I have the strong feeling that even a flag manufacturer on earth doesn't know the resistance coefficient of his tissue.
That is a very logical and unbiased answer. This shows and explains what i meant about crackpot articles on the internet vs the information I can gather here about this topic. Anyways, based on the unknown factors present in this topic/question we can conclude that it would be a timely process to derive such calculations based on the known factors and possible hypothetical unknowns, where the hypothetical variables are the most challenging to re-create and/or come to a logical assumption, which would allow a relatively accurate mathematical answer. Overall the math could be done, but it is not worth the time it would take to disprove crack pot hypothesis, because there is no benefit of doing so, since people will always believe what they want to believe.

CWatters
Homework Helper
Gold Member
Oh okay, so basically a force is acting upon it causing it to move and eventually the force dissipates causing it to become motionless.

Considering the gravity on the moon, the small margin of friction in this scenario, and the low amount of gravity (1.62 m/s^2) present on the moon wouldn't it start to become motionless over a longer period of time?
Lower gravity changes the period of a pendulum making it swing slower. It doesn't make it stop quicker. The energy will dissipate at a rate that depends on the stiffness of the material and how its suspended. Difficult to quantify.

Is there any math that proves the amount of time it would take for it to become motionless? (Math that takes all the factors into account, such as the force acting upon it, gravity, and etc)
Not really and anyway no calculation would ever satisfy some people.

Physics345
Lower gravity changes the period of a pendulum making it swing slower. It doesn't make it stop quicker. The energy will dissipate at a rate that depends on the stiffness of the material and how its suspended. Difficult to quantify.
Good to know, you learn something new everyday =).

Not really and anyway no calculation would ever satisfy some people.
I totally agree, no matter what you do people will believe what they want to believe and they have every right to believe what they want.

https://www.hq.nasa.gov/alsj/ApolloFlags-Condition.html

Shadows on photos show they were still "flying" in 2011.
That's interesting. Thanks for taking your time to find that out for me. It could very well be the first step to figuring out hypothetical calculations, if I ever do decide to venture into that route as a personal project for fun, but that will have to wait until I have acquired a wider range of mathematics knowledge.

The simple answer is that parts of the flag were moving when it was first placed by the astronaut.
Residual momentum acquired by the simple act of it being moved by the astronaut.
It only persisted for a few seconds

Physics345
stefan r

https://www.hq.nasa.gov/alsj/ApolloFlags-Condition.html

Shadows on photos show they were still "flying" in 2011.
(LRO Camera Principal Investigator, Dr. Mark) Robinson is skeptical that the flags are intact, if they are still there.

jtbell
Mentor
Consider a simple pendulum. After you set it swinging, its amplitude of motion decreases slowly, and it eventually comes to a stop. Air friction is the first thing that comes to mind as a cause, but there are also frictional losses inside the string, especially at the upper end where it flexes back and forth. Here's a video that compares the motion of pendulums in air and in vacuum:

Physics345
The simple answer is that parts of the flag were moving when it was first placed by the astronaut.
Residual momentum acquired by the simple act of it being moved by the astronaut.
It only persisted for a few seconds
Basically in physics terms work was applied then the energy dissipated, in turn leading to a motionless flag correct?

Consider a simple pendulum. After you set it swinging, its amplitude of motion decreases slowly, and it eventually comes to a stop. Air friction is the first thing that comes to mind as a cause, but there are also frictional losses inside the string, especially at the upper end where it flexes back and forth. Here's a video that compares the motion of pendulums in air and in vacuum:

That is a very interesting video, so we we could say so far we have two obvious sources of coefficient static friction in this case.
1. Air friction.
2. Tension friction from the string. (which is caused by the motion of the flag leading to the temperature of the string rising)
Okay I understand how the tension friction is occurring, but how is there air friction on the moon? I was under the assumption there is no type of air resistance on the moon, since there is no air on it.

stefan r
Basically in physics terms work was applied then the energy dissipated, in turn leading to a motionless flag correct?
Sunlight will apply between 5 and 9 micro newtons pressure. The solar wind is variable. There are moon quakes. Static electricity can build up on the moons surface.

The fibers should disintegrate and land near the flagpole. Many of the original atoms will have gassed of and moved into deep space. A few molecules may bind with regolith or possibly settle in a crater near the north or south pole.

The flag is also orbiting earth and it is moving ≈370km/s relative to cosmic background.

Physics345
Sunlight will apply between 5 and 9 micro newtons pressure. The solar wind is variable. There are moon quakes. Static electricity can build up on the moons surface.

The fibers should disintegrate and land near the flagpole. Many of the original atoms will have gassed of and moved into deep space. A few molecules may bind with regolith or possibly settle in a crater near the north or south pole.

The flag is also orbiting earth and it is moving ≈370km/s relative to cosmic background.
All of those are definitely variables, but wouldn't the moon quakes be noticed by the astronauts leading to some type of panic, which would be evident in the video?
Next the solar wind. If I'm not mistaken solar wind is extremely hot and if solar wind were to hit the moon while they were there wouldn't their suits melt or wouldn't they be vaporized (not sure if vaporized is the right word for this scenario)? I am asking this because I remember reading that the moon does not have an atmosphere so nothing would protect the astronauts suits from direct contact with the solar wind in this case. If this is true we can factor out solar wind.
Also is the sunlight's pressure a constant occurrence?

Last edited:
I would like to take a moment and thank everyone that has taken their time to input different pieces of information into this discussion. I am having a lot of fun, this has become a very engaging and interesting thread!

stefan r
Also is the sunlight's pressure a constant occurrence?
Sunlight is nearly constant. The pressure on the flag would vary by angle.
Gravity on the moon is 1.62 m/s2 if the flag had 1 gram of mass the force would be 1620 micronewtons. If the flag was positioned north-south so you have full exposure at sunrise and sunset and if the flag is bleached enough a 1 m2 flag could experience 8.1 micronewtons. That is one part in 200. It probably has more mass and probably is not perfectly north-south. The effect would be measured in mm or less.

If I'm not mistaken solar wind is extremely hot and if solar wind were to hit the moon while they were there wouldn't their suits melt or wouldn't they be vaporized (not sure if vaporized is the right word for this scenario)?
Solar wind is hot enough to melt suits. The density of the solar wind is much lower than the density of the suit/astronaut. The suits were painted white to reflect radiation. They emitted thermal radiation which was mostly from sunlight and body heat. You need to radiate heat. If a human body is fully insulated it gets cooked and dies.

All of those are definitely variables, but wouldn't the moon quakes be noticed by the astronauts leading to some type of panic, which would be evident in the video?
I live in Pennsylvania, USA. The seismograph I looked at a few weeks ago says we are constantly having quakes. I have never felt them here.
Moonquakes:
Between 1972 and 1977, the Apollo seismic network saw twenty-eight of them; a few "registered up to 5.5 on the Richter scale,
The lunar lander was on shock absorbers. 5.5 would not be very alarming while walking around. They were on the surface for about 3 days. 5 quakes per year is not likely to coincide with the landing.

Physics345