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Flying circus of physics

Death by bunk bed

Thursday, August 01, 2013


Death by bunk bed
Jearl Walker
Aug 2013   In a Spanish hotel room, two men went to sleep on bunk beds (one bed above the other) that folded out from the wall. When the higher man awoke the next morning, his friend was nowhere to be seen but his possessions were still in the room. Curiously, the lower bunk bed had been folded back up against the wall. Much later in the day, the room maid arrived to prepare the room for the next guests. However, when she attempt to fold out the lower bed to change the sheets, she could not get it to move. When the hotel manager arrived, they discovered that the missing guest was wedged between the wall and the folded up bed with his right arm dangling onto the floor. The trapped man was dead, and his arm prevented the bed from rotating out to the horizontal position. Firemen had to dismantle the bed to free the body.

Here is a sketch of the situation with the bed horizontal, based on the forensic analysis in the research paper listed below.

The bed can rotate between its vertical and horizontal positions by pivoting around a support rod that runs along its length. When rotated to its horizontal position, an element prevents further rotation downward. The mattress and bed are rectangular, with a combined weight of 1604 newtons. When they are horizontal, we can assign their collective gravitational force to their center of mass, which is located 0.115 meters outward from the pivot.

If a person lies on the bed, we can assign the collective gravitational force on the person to the body’s center of mass. Of course, the person can move somewhat over the bed, so the center of mass can in a variety of locations. But here comes the chilling physics. If the body’s center of mass is closer to the wall than the pivot, then the gravitational force on the body tends to rotate the bed to the vertical position. In my diagram, the force on the bed-mattress center of mass tends to give a clockwise rotation: The torque due to the force is clockwise. If the body’s is near the wall, then the force on it tends to give a counterclockwise rotation and torque.

In this case we can calculate a torque by multiplying the force by the lever arm, that is, the distance between the pivot and the point where the force acts (the center of mass). Here are some numbers: the bed-mattress has a weight of 1604 newtons and a lever arm of 0.115 meter, which gives a torque of 185 newton-meters. The man had a weight of 1030 newtons. If we assume that during the night he rolled over toward the wall, putting its center of mass at 0.20 meter from the pivot, then he produced a torque of 206 newton-meters. That counterclockwise torque overwhelmed the clockwise torque, and the bed rotated to its vertical position, squeezing the man hard against the wall and compressing his chest so that he could not breathe. He died of asphyxiation.

Physics is everywhere, and sometimes it can kill you.

Journal reference style: author, title, journal, volume, pages (date)
· Domenech, M. S., H. M. Alcazar, A. A. Pallares, I. G. Vicente, J. C. Garcia, C. V. Gutierrez, and J. M. Muniz, “The murder is the bed: An unusual case of death by traumatic asphyxia in a hotel folding bunk bed,” Forensic Science International, 220, pages e1-e4 (10 July 2012) 

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