Question

Anyone know why you tend to sway back and forth while standing still when you are on the last floor of a very tall building?

Answer 1

I guess, by last floor you mean topmost floor.

It is because the building is oscillating.
Read - http://en.wikipedia.org/wiki/Mechanical_resonance#Examples

Answer 2

Conservation of momentum while building is swaying due to winds.

Answer 3

@Garm
How can momentum be conserved when the building is attached to the ground??

Answer 4

The building is moving, while your body tries to stay in place using earth as point of reference. If you use building as point of reference then it would feel like you are moving back and forth inside stable building

Answer 5

@Garm
As the building is firmly fixed to the ground, there would always be a normal reaction (in horizontal direction) from the ground. Hence you cannot conserve momentum.

Also I think the body sways like it is a part of the building. The fact that the head sways for a larger distance (with greater velocity and experience a larger acceleration) than feet is the reason for feeling of nausea.

Answer 6

@Isaiah.Feynman
Also read - http://en.wikipedia.org/wiki/Tuned_mass_damper

Answer 7

Thanks guys. I experienced this phenomena yesterday and couldn't think of a good physical explanation.

Answer 8

@LastDayWork body is not part of the building, it will try to remain motionless unless building acts upon it. the only place building can easily act on the body is feet. Person in a oscillating skyscraper would experience similar situation as riding a bus. Only difference is inside the building a person cannot see the building move like he can see the bus move as such our brain mistakes building as actual point of reference and static object.

Answer 9

By the phrase "the body sways like it is a part of the building" I was comparing the the relation of the body to the angular motion of the building with the relation of a body to the angular motion of earth (only difference is that the earth is not changing its direction of angular motion like the building).

Apparently you're saying the same thing using a different approach.

Answer 10

@LastDayWork frankly at the scale of skyscraper to human angular motion can be ignored. From human perspective it might as well be flying room, which moves a bit horizontally.

When this "room" accelerates in any direction there are essentially two forces acting on a human:

Gravity applied at center of mass
and Friction applied at soles of feet.

Due to different application points these two forces will try to topple human body down in similar fashion as it happens to a standing passenger of an accelerating bus.

Body will only have same angular with building while both are not accelerating, As soon as building starts to accelerate or decelerate the human body will lag behind a bit similarly to the mass damper link posted above.

Answer 11

^^ Completely agreed.