Suppose a one hundred kilogram person abruptly ejects one kilogram at two meters per second, his vertical velocity would only be two centimeters a second, which would only bounce him off the toilet for one third of a second, which would not be noticeable.
Even Ceres gravity does not allow rocket propulsion by excretion.
Physiology is not
quite physics.
In defecating (particularly with formed stool in the rectal ampulla), increased thoracoabdominal pressure is entirely responsible for the expression of waste. In comparison with the urinary bladder, the large bowel is almost bereft of intrinsic muscle that might serve effectively in expelling solid stool.
The Valsalva maneuver is a sequence of voluntary actions in which the individual inhales, closes the glottis, and by exhaling forcefully against the closed glottis, increases pressure within the thorax and abdomen. Among other things, this serves to assist with the evacuation of both the urinary bladder and the rectosigmoid colon, the latter further facilitated by the individual assuming the seated position which the usual commode is specifically designed to facilitate, the torso flexed (leaning forward) so that within the distal large bowel, the sigmoid segment to some considerable extent "folds" over the top of the rectal ampulla to make of it a chamber open only at the anus.
Think "toothpaste tube."
The value of this seated position is not to be underestimated. Anyone who has ever had to employ a bedpan for defection can tell much about how damnably difficult it is to move formed stool while supine.
If he or she can overcome the embarrassment. There are reasons why nurses and physicians do not laugh when the expression "digital disimpaction" comes up in clinical discussions.
"Body cavity search," sure. "Digital rectal examination"? Big funny. But a bed-bound patient with a left hemicolon stacked full of hard stool represents a genuine hazard and real pain for the patient - in neither aspect trivial - as well as an exacting and thoroughly unpleasant intervention for the caregiver. No friggin' joke.
Got all this clearly pictured? Preadolescent sniggering set firmly aside, let us proceed to consider what else happens in the human body while taking care of the necessary function of ridding the digestive system of waste, particularly in what are called the "core" muscles of the abdomen and lower extremities, the muscles responsible for upright posture and bipedal ambulation in a one-gravity field.
In bowel movements, these muscles are being exerted as well, and bear in mind that on Ceres the insufficiently cautious exertion of the foot's plantar flexor muscles - just the muscles in the calves, the gastrocnemius and soleus - has been depicted correctly as sending the newcomer literally crashing against the ceiling.
The "lift-off" potential while straining at stool ought to be apprehensible to anyone.
While I would counsel diets high in both soluble and insoluble fiber (as well as oral consumption of surfactants like dioctyl sodium sulfosuccinate) for anyone considering a life in microgravity, no matter how soft and bulky the colonic contents are kept, it is necessary to engineer bathrooms to handle human physiological functions that must cope with formed bowel movements and constipation.
Anybody who wants to get cute about that they
think are the physics of defecation without taking into consideration the biomechanics involved in coping with this inescapable requirement of human life is invited to come along on hospital or nursing home rounds with any average geriatrician.
Like I said, no friggin' joke.
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