# Thoughts on an elevator

So I was taking the elevator down to the lobby as I left work yesterday. I was the only one on the elevator for the entire ride from the 30th floor to the lobby.

I had a clump of dog hair stuck to my coat that I picked up and dropped toward the ground. It just slowly drifted, like a feather, toward the carpet. In fact, the elevator slowed and stopped before the bit of fuzz hit the floor.

It was hard to tell from my vantage point, but - from a relative point of view - would the fuzz speed up as the elevator slowed? Or, since the air resistance was what made it take so long to fall, would the fuzz slow down at the same pace as the air in the elevator slowed?

on a related note, what happens to the air in an elevator as the elevator stops? Does the pressure increase in the direction of movement, i.e. more dense at the bottom when the elevator was going down and vice versa? Or are there enough openings in the elevator to pretty much let air move about as needed? What if the elevator were sealed?

It would. The deceleration of the elevator can be seen as a relative increase in the G-force on the elevator and what it contains. Since the fuzz is somewhat more dense than the air, it sees a greater increase in acceleration than does the air.

Unless the elevator is a wire cage, it probably is sufficiently airtight that the pressure does change as you describe.

You can see an interesting example of this by tethering a helium balloon in a car, so it floats freely just below the ceiling. As the car accelerates, the balloon moves forward; as the car turns left, the balloon leans left. It’s caused by the air density changing due to imposed accelerations, with the lighter-than-air balloon seeking the least dense air it can reach.

One of the tenets of Einstein’s relativity, iIRC, was that you could not distinguish acceleration due to gravity from acceleration due to change in velocity. (Unless you look out the window).

So the effect of the elevator moving is the same as the effect if gravity increased decreased and feels he same too. Since fuzz and air (and you) have mass, a stronger force (apparent pull) will act upon them all during elevator deceleration.

I was so freaked out the first time I tried this. But remember, kids, if you try this at home, it has to be a helium-filled balloon. An air-filled balloon will do exactly what you expect.

Intuitively, we percieve the balloon as real, and the air as nonexistent, so we expect the balloon to go in the opposite direction, just like everything else does. But the reality is that the air has more mass than the balloon does, so it wins, and it pushes the balloon out of the way.

Not sure why but I would have guessed that the balloon would do nothing.

When I was a kid I could not understand why a hot-air balloon floating above in no wind still moved with the earth. I kind of expected it to stay still while the earth rotated under it.

It may depend on the elevator. I ride an elevator every day about a dozen floors. When decending, I can feel the pressure difference in my ear before the doors open up on the ground floor. It is most definately not airtight.

If it were neutrally buoyant, it would do nothing. But most helium balloons (fresh ones, at least) are positively buoyant, and need to be held down with a string.

An elevator is not air tight. Most elevators have air holes around the bottom of the car and an exhaust fan at the top of the car. An elevator car also has open space between the car and the shaft. My swg would be that the pressure increase under the car would be no more than 0.01inches of water pressure.

Another factor would be the pressure difference between the shaft and each floor.