I recently came across this video on youtube. A woman with her elbows pointed directly at the ground was lifted easily. As she moved her elbows slightly forward making an angle with the vertical, it turned out be really difficult to lift her. The video explained the trick saying that centre of gravity was shifted backwards which made the lifting more strenuous. But I have difficulty in understanding the explanation given in the video. Could you explain the physics behind this trick, preferably with a free-body diagram?
closed as unclear what you're asking by Kyle Kanos, Jon Custer, glS, AccidentalFourierTransform, ZeroTheHero Jun 13 '18 at 2:21
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The top sequence of four images is when the mad could lift the woman and the bottom three images is when the man could not lift the woman.
The important position is that of the centre of mass of the woman and the man.
For the lift to happen the vertical line through the combined centre of mass must pass through the feet of the man.
In the first sequence the mad moves his backside to the right to move his centre os mass to the right and he brings the woman as close as he can to himself to move her centre of mass to the right.
Once the combined centre of mass is above his feet the man can fift the woman.
In the second sequence the woman makes it as difficult as possible for the man to move her centre of mass to the right by first angling there arm and secondly by allowing her legs to be inclined to the left.
This results in the man being unable to position their combined centre of mass above his feet.
What the man needed to do in the inclined arm case is to push forward onto the woman so that her legs were not inclined and position then underneath his thighs and also try and have his torso in a more vertical position.
Do try this at home!
It is clever but simple. And the words used are misleading. It isn't that they can not lift her weight; they can not lift without falling forward.
When the woman moves her elbows (the lifting points) forward, most of the weight the men try to lift is moved away from them just a few inches. This is enough that they will fall forward if they assume they can lift straight upward.
If they had to lift her and had a few minutes, they would figure out they have to stand with their feet apart and lean backwards so that the woman's legs hang between their thighs. Or lean back until she is resting against them, which is awkward with a stranger. The trick relies on that social awkwardness.
The first case works mainly because the men's feet are long enough to keep the total center of mass within their balance. They could lift her wither way if their feet were longer.
The diagram in the video is misleading because it isn't the woman's center of mass that is important so much as the combined center of mass of the man and woman. It must be over the men's feet or they will fall over.