# How do flags behave on the Moon?

I have seen many theories and question/answers on this site touching upon this subject but no focused question. So I would like to ask this as a two part question.

Is what we saw in the Lunar landings videos, the expected behaviour of a flag on the Moon and how should we expect a flag to behave on Mars - if/when we get there ?

See video below

• yes, that's pretty much the expected behavior for thin plastic on a flexible metal frame. On Mars would be similar due to the very thin atmosphere, except during storms where they would sway a little.
– user20636
Jun 11, 2019 at 13:55
• Imagine a windless earth with 16.3% of the gravity. It was fluttering in the Flag Plant video because of the friction against the astronaut holding the cloth. Also, maybe link what video you're talking about. Jun 11, 2019 at 13:59
• One tag is wrong, there is no atmospheric drag on the moon.
– Uwe
Jun 11, 2019 at 18:45
• There is no atmospheric drag on the Moon because there is no atmosphere.
– Uwe
Jun 12, 2019 at 7:40
• @Uwe I put the atmosphere tag there because the question considers such a possibility implicitly, not because there is or there isn't an atmosphere on the Moon Jun 12, 2019 at 9:57

Since I am sure you're not denying the authenticity of this video, let us turn your question another way: why is the flag behaving this way ?

The flag is roughly subjected to 3 forces :

1. Its weight: 1/6 of the very same force it would have been subjected to on Earth
2. Tension from its attached part on the rod
3. Internal material tension due to the elasticity of the fabric

Pushing the flag into the ground, Armstrong gave it an initial momentum, and the flag was then only subjected to those three forces in the nearly vacuum of space. Please note it is a very classical physical situation: what we have here is a pendulum with a mass (the fabric) a tension (the fabric is attached to the rod), a fixed part (the rod), and an initial velocity. What many have wrongly interpreted as some wind proving the scene would have been recorded on Earth is just the oscillation of this pendulum. Such a scene is impossible on Earth since with such a low mass as a pendulum, friction forces from the atmosphere added to the other three force would impede any oscillation movement.

Now about Mars. Friction forces on the flag from the atmosphere mentioned above are proportional to the mass density of the fluid and the square of the speed of the flow. On Mars, the pressure and therefore the mass density of the atmosphere is so low that friction forces on the flag would be almost zero, except with an extremely fast flow, for example during a storm. So it would be pretty much like on the Moon. On the other hand gravity is 2.3 times that on the Moon, which would amplify the oscillation phenomenon. All in al, the flag would behave approximately the same way.

For a more casual answer to this question--The Mythbusters built a replica of the lunar flag assembly. In a vacuum chamber they moved it around to see how the flag behaved in a vacuum.
Dailymotion video

Time index 11:30