# 4 person chair trick

by rvgene
Tags: chair, person, trick
 P: 3 Ok, so the trick is shown in the following link... http://www.youtube.com/watch?v=CL-CHenUooY could anyone explain how this is accomplished?
 Emeritus Sci Advisor Thanks PF Gold P: 15,867 Just get 4 friends and try it??
 PF Gold P: 427 As far as I can tell they are just transferring the weight of their upper body from the chair to the legs of the guy next to them. The only reason you need the chairs is because there is no way to get in that load balanced configuration initially without some support.
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P: 5,300

## 4 person chair trick

Ive done this with friends before. It's not easy but not too hard either. All you're doing is transferring your weight to the next person's legs. So long as they hold them straight and you stay still you balance.
 P: 3 hmm, does it have to do with concepts such as centre of gravity and moments? for example, the pivot is at the head, and there is a anticlockwise moment due to the centre of gravity and a clockwise moment due to the normal force at the feet?
P: 5,610
 Quote by rvgene hmm, does it have to do with concepts such as centre of gravity and moments? for example, the pivot is at the head, and there is a anticlockwise moment due to the centre of gravity and a clockwise moment due to the normal force at the feet?
Consider each person individually as an object resting on two supports. The weight of each person is supported by 1.) their own legs from the knees down, and 2.) the legs, from the knees down, of the person on whose knees their back is resting. The center of gravity of each person is somewhere between their knees and the point where their back touches the next persons knees, though it is much closer to the latter. I just experimented with getting myself into this position with my back resting on the seat of a chair. If you try that you'll see there's much more pressure on your back than on your feet. In the group situation each person's knees are supporting a little bit of their own weight and most of another person's weight. If you could determine the center of gravity of each person in this position you could work out how much of their weight is supported by their own knees and how much by the knees of the next person.
 P: 103 Okay, that was amusing.
 P: n/a our teacher made me and 3 others do this demonstration in class back in high school. It really isnt as hard as it looks.

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