Assume spherical blinds in a vacuum... (very minor physics question)
June 9, 2019 12:19 PM   Subscribe

I have nothing riding on the answer to this, I'm just idly curious about the physics behind the movement of some vertical blinds as they're being closed.

In my apartment there's a set of 7' tall x 3.5" wide vertical blinds mounted across the sliding glass door, attached only at the top with the bottoms hanging free - pretty much like these. This is the scenario that happens each evening when I close up house at bed time:

1. I twist the rod so that the slats are rotated "open", i.e. the widths of the slats are parallel to each other and perpendicular to the glass of the sliding door.

2. I draw the blinds closed from the end of the rack where they're all bunched together to the other end, spanning the doorway.

3. Once they are pulled fully across I rotate the rod so that all the slats are turned along their vertical axis to put the widths of them all roughly on the same plane, blocking the view in or out.

4. As I'm drawing the blinds across in step 2, pulling them from the top, the bottoms trail behind and then start to sway. After I'm done pulling them across the bottoms continue to sway with a diminishing movement over a period of 45-50 seconds...I also timed the swaying with the slats open and it's around 30 seconds longer than when they're closed, I assume due to the friction of the closed slats rubbing against each other.

But for this question let's say the slats were far enough apart that they didn't bump into each other when in the closed position. Does rotating the slats 90° to close them in step 3 affect that swaying momentum? Is there any transfer or modification of momentum when the individual slats are rotated? If the mechanism allowed me, what would happen to the swaying if I continued to rotate the slats 360° and beyond?

Thanks for indulging me!
posted by Greg_Ace to Science & Nature (9 answers total)
Absolutely, in air. The air resistance in the plane of motion is going to result in more or less damping of the oscillation, depending on orientation.

Assuming a vacuum, though, if the slats are attached at one point at the top, and if that point is in line with the centre of mass of the slat, then the slat is just a simple pendulum. Rotation about the axis won't have any effect on the oscillation.
posted by pipeski at 12:52 PM on June 9, 2019 [3 favorites]

In that case, given air resistance, why did I see a longer period of swaying with the slats "open" (parallel to each other) compared to the "closed" orientation in step 4? Can friction be legitimately blamed for reducing swaying so significantly more than air resistance would? In other words, if the slats were separated and never ever touched, would the slats sway for longer in the closed position than in the open position?
posted by Greg_Ace at 5:10 PM on June 9, 2019

Sorry, to be clear I meant "never ever touched each other".
posted by Greg_Ace at 7:36 PM on June 9, 2019

The friction of all of those slats rubbing against each other is going to be very significant, probably quite a lot more of an effect on the total system than the air resistance.
posted by flug at 8:47 PM on June 9, 2019 [1 favorite]

I don't think it's fiction. It would be the same with only one slat. In closed position they can only swing at the point of connection at the top. In open position they can swing there and bend along their whole length, letting the bottom of the slat fall far behind the top, which means it takes longer to catch up and stop moving.
posted by twirlypen at 10:10 PM on June 9, 2019 [1 favorite]

I suggest also asking this over at the physics stack exchange.
posted by klausman at 12:20 AM on June 10, 2019 [1 favorite]

The connection of the slats to the track may also have something to do with it. if the joint up there allows slightly more "wiggle" in the "open" position than in the "closed" position, it could cause a significant difference at the bottom of the blind.
posted by Rock Steady at 7:45 AM on June 10, 2019

This is neat! I'm going to have to try this the next time I'm in a room with these kind of blinds.

My very naive and inexpert guess is that the interactions with the air while pulling the blinds closed is different depending on their orientation and you're starting out with a significantly larger swing if the slats are roughly perpendicular to that motion. Assuming the pendulum swing of the slats is happening at a lower velocity than the initial motion, and given that many dissipation mechanisms scale more quickly than linear with velocity, that initial size of the swing wins.

It sounds like this could be a fun experiment. The questions I'd ask are: (1) is the initial displacement different in the two cases? A video and a ruler might be helpful here. (2) If you take completely stopped blinds in both conditions and displace several of them by exactly the same amount by hand, do you get the same result? (2) If it's possible to temporarily remove a few slats, does an isolated one behave the same if it never touches its neighbors? Perhaps you could just tie some of them up and out of the way.

If you're rotating the slats while closing them, there could be complicated momentum transfer. After they've been *rotated,* though, it sure seems like interactions with air and contact between slats are the only significant differences. Unless the pivot mechanism is complicated, e.g., if each slat was fixed to an axle that allowed free rotation in only one plane that depends on the orientation of the slat. That's not impossible, but I'd think it would go the other way, if you're putting a pin through a planar thing.
posted by eotvos at 10:47 AM on June 10, 2019

When you pull the blinds closed, you've added motion to the left (say) by putting force on the tops, and the bottoms catch up. In the absence of air resistance the whole thing would move at once, but as it is you get drag at the lower tips. If the slats were perfectly rigid, they'd sway like a mathematical pendulum, but actually they're flexing as they move. It's easy to bend the strip along the thin direction, but it doesn't curve sideways (i.e. holding a strip flat in front of you, it can curl toward you but not arc left or right.) So there's a different behavior; if you've got the stiff axis turned parallel to your closing force (blinds closed) that's just a pendulum frequency, but if the blinds are open, the springiness of each blind acts like a tiny restoring force as they swing.
posted by aimedwander at 11:18 AM on June 10, 2019

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