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Poo vs Gravity
May 20, 2014 2:07 PM   Subscribe

As a by-product of this FPP about a very large water slide, have just been involved in an inconclusive and heated argument about the physics aspects. Hypothetically, what would happen if, halfway down this very fast waterslide in a raft, you (accidentally or deliberately, it doesn't matter) defecated?

Would either, by the end of your ride:

1) The poo travel at the same momentum / speed as yourself, and therefore you would never leave it behind?
2) The action of defecation would add a negative acceleration to the poo relative to yourself, so you would, for a few seconds, increasingly gain distance over the poo. However, when the raft slows, your poo would catch up and be reunited with you?
3) As 2), but the poo never catches up with you as it never reached your peak non-poo velocity?
4) Other?

Assume (you can tell we have argued this out in detail):

1. You are naked i.e. the poo under consideration is not stopped or hindered by clothing.
2. Note the waterslide involves people sitting in a four person raft, simila to a logflume with a net overhead. You are at the very rear of the raft, and therefore the poo cannot simply fall into the same raft and accompany yourself for the remainder of the ride in that manner.
3. The poo is not affected / deaccelerated by the netting over the slide, it remains solid at all times, and is acted on by the same forces that act on the raft and yourself i.e. gravity and slide water.
posted by Wordshore to Science & Nature (13 answers total) 10 users marked this as a favorite
 
While a creative example, this reminds me of the same conundrum in space.

In space, can you throw a wrench away from yourself and change your momentum?
posted by St. Peepsburg at 2:20 PM on May 20 [1 favorite]


1) You're more-or-less in freefall. The reaction in (2) will be too small to observe with the naked eye.

Related: what do you expect to happen if you released a small coin from head height as you're zooming along? In your frame of reference, which direction does it fall in?

How about a helium balloon -- which way will it float? (See recent FPP on balloons inside accelerating cars.)
posted by phliar at 2:21 PM on May 20


One would hope you don't become aware of the poop and struggle to get away from it.
posted by bleep at 2:37 PM on May 20 [2 favorites]


While I'm okay with accepting a spherical cow for the sake of argument, as a practical matter your butt, and the associated poo emitted therefrom, will be in the raft.

Source: I have not been on the "you're fucked" in KC, but I have ridden a number of Bahn rides in the original park in Texas and your ass does not hang out of those rafts.
posted by immlass at 2:42 PM on May 20


the poo is precisely akin to the exhaust from a rocket engine.
posted by bruce at 2:44 PM on May 20


In this kind of water slide, you move faster than the water. I would guess this is because the water is slowed by friction with the slide, but you are only slowed by the water going up your shorts. But you're not floating in the water, you're skimming across the surface, basically hydroplaning. If you WERE floating in the water, you would pretty rapidly slow down to the speed of the water.

So something small enough to fully float would slow down to the speed of the water pretty quickly once it left the effects of the raft. You could probably test this without getting put on a "never allowed back here again" list by "accidentally" dropping your sunglasses or hat or something while on the ride. Of course it would eventually catch up with you since you stop at the end and water continues to flow past you.
posted by aubilenon at 3:09 PM on May 20


the poo is precisely akin to the exhaust from a rocket engine

I'd posit that applies only if the fecal matter is of an explosive diarrhea-type consistency. If it's a solid piece, isn't it still technically connected to your frame of reference until the very last moment where it detaches from the body?



(Yes I just realized I typed this. On the internet. I'm going home now.)
posted by JoeZydeco at 3:31 PM on May 20 [10 favorites]


I think that this is actually a challenging problem, with different results for at least six phases in the slide. At all times, your acceleration relative to your poo (as you talk about in case 2) will be so small as to be unnoticeable - you have much more mass than any turd you produce.

Phase 1: Initial descent
- For the first part of the descent, you and the water are both accelerating at roughly the same speed, so it is your case 1, where the turd remains with you. If you watch YouTube GoPro waterslide videos of slower slides, you see that the waves in the water are travelling at the same speed you are.
Phase 2: Hydroplaning - In the second phase, after a second or two, you begin to speed up relative to the water, as aubilenon pointed out, it is slowed by friction more than you are. If you watch YouTube videos of GoPro cameras steep waterslides, after a couple of seconds a bunch of water starts to spray up at the camera, because the person filming is going faster than the water, so it is getting kicked up. A turd dropped in a waterslide in phase 2 will experience more resistance from the slower water (less mass per surface area, basically), so it will be slowed relative to you; you will leave your poo behind.
Phase 3: Uphill - There is a camelback on the slide - however, they are using their Master Blaster technology, which is a series of water jets that push the raft along. Once you enter this section, as you are travelling uphill you will quickly slow to the point that the jets are going faster than you and pushing you along; in this case, your poo will drop into the water and (ignoring that the raft would block it) would be propelled ahead of you.
Phase 4 and 5: Final drop - Once you start descending from the camelback, the same thing as phase 1 and 2 will take place; first you, your poo and the water will accelerate at roughly the same rate, then you will outpace the water and your poo will trail you. There is, therefore, a section at the end of phase 3 when your poo will be carried away from you, then you will pass it again in these phases.
Phase 6: End pool - At this point, the poo will remain in the pool that decelerates the rafts; you will be asked to leave the park.

Note that phases 1 and 4 might be very short in actual practice, due to the extreme slope of the slide.

I'm now thinking of using my AskMe to find out why I can't seem to motivate myself to finish my technical documentation; apparently there are some things even shittier than this topic.
posted by Homeboy Trouble at 3:48 PM on May 20 [11 favorites]


The same thing that would happen if you threw something fairly light behind you. Ignore that you're moving for a moment; imagine you're on sitting on a flat, effectively no friction slippery surface in a vacuum.

You're sitting still, so there's no unbalanced forces acting on you - Newton's first law is that an object at rest tends to stay at rest, and a moving object will keep moving at the same velocity. The force exerted on you by gravity is balanced by an equal and opposite force from the surface, as it must for you to be stationary; so by definition, there are no net forces acting on you.

So you throw something away from you, the weight of a standard poo, say to the south. Newton's third law is that every action has an equal and opposite reaction. That means that by exerting a force on the poo, it also exerts a force on you. Since neither of you are constrained horizontally, you will move to the north, and the poo weight to the south.

However, you will move much slower to the north than the poo heads south.
Newton's 2nd law is that f=ma, or force = mass x acceleration. Since the force exerted is equal on you and the poo weight in the horizontal direction, we look at mass - your mass is much greater than the poo weight, therefore you will accelerate a small amount, while the poo weight goes quite fast.

Of course, once the initial force 'push' is over, what then? Well, the gravity will pull the poo weight downwards until it hits the ground, at which point the forces vertically will balance out (we assume it doesn't bounce) and it will continue sliding away to the south, while you continue north at constant acceleration, and you move apart forever. Of course, it's very unlikely you've found a truly zero friction surface to sit on. Even a low friction surface will start to exert a small force on you in the horizontal direction, and that force will exert an acceleration in the opposite direction - a deceleration from your point of view. Eventually that deceleration will cause you to come to a stop, as the deceleration from friction will cause the poo to come to a stop. At that point, you will be a small distance from your starting point to the north, and the poo weight will be a large distance south from the start. What final distance it achieves will depend upon what angle you threw the poo weight, i.e. how long it was able to continue to move freely before friction with the surface stopped it.

Now, we return to the original question. Since the boat is moving, we can assume that some portion of the force of gravity is being applied to you and the boat. Since you're at an angle, the force from the surface of the slide will cancel out some of it (the slide is attached to the earth, and that's a BIG mass), and friction from the water will cancel some more. You're gliding on the surface of the water, which means that friction is low. Since the water can't 'grip' the slide, or your boat as its low friction, you both fall towards the earth. You will fall faster than the water, as the water is in contact with the relatively high friction slide, while you are only in contact with low friction water surface - in effect, you skate on the surface somewhat.

The important thing is, since the poo separates from you, at the point of separation, you are both experiencing the same force from gravity in the same way - so the only net forces acting in reference between you and the poo are the force you put on it - and it puts on you in equal measure.

So you will accelerate away from the direction you defecated; in this case, faster down the slide. However, you plus 3 people plus boat is a much bigger mass, so you'll only accelerate a tiny bit. The friction with the water will be high; you may end up bringing a bit of the water too, making the accel even slower. Given the disparity of the masses, you may well not notice any change at all on you and the boat.

So what for the force of the poo? Well, it will head backwards while in the air. The air is very low friction at that kind of speed, so we can pretty much ignore it. It will continue to accelerate backwards until it hits the water. How far will depend how hard you poop it out, and at what angle. In practise, it won't accelerate much. Both of you are of course still falling down the water slide due to gravity, so relative to each other you'll be at the same rate.

Then the poo hits the water. Depending upon its reaction with the water, it could do a few things.

At this point, the poo is behind you in the water, and floating - you have separated due to the force you put on it. If it floats high on the water, and 'skips' over the surface, same as your boat is falling faster than the water, then you will reach the bottom with the same distance between you - quite an airy poo!

However, poo is quite sticky, and it will be partially or fully submerged as you said it was solid; it will probably end up going at the same rate as the water. So you will continue to move away from it, as gravity is countered less on you than it is on the poo, and your floater will arrive a much later time than you.

Ultimately, this is how rockets work. They throw something away (combustion gases from burning rocket fuel) in one direction, and the rocket goes the other. The harder they throw the gases (bigger bang), the faster the rocket goes. Your rocket exhaust is a muscle and gas expelled poo, which doesn't really compare in force.

In reality, you'd probably just end up shitting all down the side of the boat, which would be no fun for anyone.
posted by ArkhanJG at 4:01 PM on May 20 [3 favorites]


It would squish (poo is squishy) against the boat, you, or your neighbor, or come apart while being pushed through the mess by the water.
posted by myselfasme at 4:29 PM on May 20


[Guys, you know I love a good poo joke but please remember this is askme and stick to concrete answers.]
posted by cortex at 4:49 PM on May 20 [2 favorites]


no joezydeco, it's matter expelled out the rear, doesn't matter if it's solid, liquid or gaseous, newtonian physics still applies, no need to consider frames of reference. don't feel bad, you're in good company; at the dawn of the rocket age when robert goddard was doing his thing, the new york times opined that rocket propulsion in a vacuum was impossible because there was nothing against which to push. it only recently issued a correction.
posted by bruce at 5:15 PM on May 20


Been asked to clarify, so another assumption:

4. Assume, for simplicity, that the poo is a floater (in both moving and still water) and not a sinker. Therefore, it won't be acted on by friction from the floor of the waterslide.
posted by Wordshore at 6:09 AM on May 21


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