# Gravity and water slides

I have just returned from a holiday in turkey, and the resort we stayed at had a water slide that ran into the pool. When sunbathing, I noticed that any children who attempted the water slide seemed to descend at at a slower rate than when a larger/fatter adult tried it.

Now, I am firm in my belief that all objects accelerate to the earth at the same rate due to gravity, and that it is gravitational attraction that causes both the child and the larger adults to whiz down the slide into the pool below.

So what is happening? Why do people with bigger masses fall quicker down the slide than people with smaller masses? Is it to do with frictional forces and surface areas? Or is it an illusion, where in fact people of all masses go down the water slide at the same speed?

Drakkith
Staff Emeritus
The acceleration due to gravity that any object here on Earth experiences is the same. IF the children are indeed going down the slide at a slower velocity it is NOT because of gravity, but most likely because of friction. Since they are less massive they could be "sticking" a little bit in different parts of the slides where an adult would not.

yeah, it could be that kids are a bit afraid and stick their feet or something.

as far as a possible illusion, that could be too, even if you were measuring with a chronometer...the thing is that even if we say that both were falling at the same rate, you would think that a larger body fell sooner since it reaches the water a bit sooner...what you would need to do is to pay attention to the center of mass, instead of just the feet..

could it be?

Adults have more surface than children and friction force is directly related to surface area. By means directly I don't mean a linear dependence. It has orders between 2~3. Check out drag equation for more insight ;

F= (1/2).$\rho$.v2.C.A
F is the force of drag,
$\rho$ is the density of the fluid,
v is the speed of the object relative to the fluid,
C is the drag coefficient (scalar),
A is the surface area.

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Pengwuino
Gold Member
Yes you're talking about a very complex system that can't really be answered with a basic point-like particle falling in a gravitational field kinda analysis.

russ_watters
Mentor
Adults have more surface than children and friction force is directly related to surface area.....
However, an increase in surface area would correspond to a larger increase in mass, implying adults should accelerate against drag faster than kids.

rcgldr
Homework Helper
The main issue seems to be related to friction. I recall an article and video showing that a rider could increase his speed by stiffening and trying to arch his body so that most of the force was on his heels and shoulders. I'm not sure if this had a direct effect on the friction, or if instead it affected the way the water would pool up under the rider (perhaps a bit of a plane effect on the riders back and shoulders?). The video showed several runs with another person in the tube taking a chase view, with the rider in front going faster and pulling away when using the arching method. However I haven't been able to find that article or video again, although I think the video was on youtube.

A.T.
The main issue seems to be related to friction. I recall an article and video showing that a rider could increase his speed by stiffening and trying to arch his body so that most of the force was on his heels and shoulders.
I did this on water slides that clock your time. I got twice as fast that way, and was still far away from the slide record. So, yes the speed depends very much on the technique and simple falling models don't apply.

However, an increase in surface area would correspond to a larger increase in mass, implying adults should accelerate against drag faster than kids.

That might not be entirely correct. Weight increases yes but mass is always constant. Neglecting environmental effects and extraordinary cases (puts his feet to somewhere, hold a piece of the slide in the middle of the ride.. etc) will simplify the problem to drag equation. And there is nothing related to mass or weight in the drag equation.

From a microscopic point of view, mass of human body components doesn't vary that much with age. So problem can be reduced to a drag equation.

In the original post author stated that he observed that adults slide faster than kids. Since this is an absolutely true observation statistically speaking it is not that possible that all children did the same trick to slow themselves down where adults didn't.

F= (1/2).ρ.v2.C.A.cos($\theta$) where $\theta$ is the angle of the slide to the floor. Slide can be helicial as well, same rules apply.

On the other hand, due to aging and etc drag coefficient of people might vary. Stratum corneum, epidermis, dermis etc these layers are highly dynamic and can change very widely between individuals even twins. It is obvious that this phenomena will contribute to drag coefficient in the equation.

Friction force is the initial force here (neglecting external forces like pushing yourself etc), water is passing between your body and slide, there will be a limit for the force your body is applying to slide, and till you reach this limit friction force will contribute to your fall. More surface > faster slide. Let's eat fast food.

I'm pretty sure kids falling out of airplanes would also hit the ground much slower than adults. The smaller you are, the greater your surface area compared to your body weight, and the more drag you experience.

Isn't there a difference between surface friction and friction in a medium a.k.a viscous drag? I guess the formula of surface friction applies here, not that of viscous drag. And if I'm not mistaken, the frictional force is given by
f=$\mu$R ; thus one can vary the component of the weight of a person which equals the normal force 'R' in the above described formula to get a change in frictional force. Also, surface friction is independent of the surface area. Having a tough time tryna reason out why the drag equation is being used!

Correct me if I'm mistaken.

I suspect, based on some experience riding water lubricated slides, that a hydrodynamic bearing is being formed under the slider. My guess is that larger people present a greater area to the water/slide interface and thereby create a more effective hydrodynamic bearing, and therefore a lower resistance.

I've noticed this as well on "race track" type slides where riders race down in parallel lanes on neoprene mats so friction should be more or less similar. Riders generally start out together but by the half way mark the heaviest usally take the lead. I put this down to the larger riders having more momentum/higher inertia to keep them moving where as friction will have a lager effect on the lighter riders with less momentum.

256bits
Gold Member
I suspect, based on some experience riding water lubricated slides, that a hydrodynamic bearing is being formed under the slider. My guess is that larger people present a greater area to the water/slide interface and thereby create a more effective hydrodynamic bearing, and therefore a lower resistance.

I would tend to agree. The water has a more difficult time "squishing out" sideways from underneath the person.

My daughter has just completed her 5th grade science fair project, trying to answer this exact question - should you fill up your raft with your big dad and family, or go down alone to go the fastest? Her hypothesis was heavier meant faster trip since it sure looks that way! She set up a PVC pipe, ran water down it, and sent a sport water bottle down empty and with varying amounts of quaters. I held the hose, dad did the count down to letting go and the stop watch. Oh, and I helped her with the equations of conservation of energy, and as you know, mass falls out (1/2mv2 = mgh). While the math said, wow, they should be the same, none of us believed it - much like this blog. Result: all the times were the same (except for measurement noise), even when she ran the empty one at the end to confirm set up - all about 1.78 seconds (give or take 0.1 seconds). The bottle weighed 5 lbs full and essentially weightless for empty bottle, and did 3 weights inbetween. Facinating. The full one blew out the end where the empty one just sat there at the bottom - perhaps that is what we see at the parks: the massively filled rafts fly out the end and make a big splash....but they are apparently not going any faster than the little people coming down on their own (assuming they don't lose their raft or mat along the way!). Hope she wins a prize! SHe sure learned a lot.

Bobbywhy
Gold Member
sciencemom, Welcome to Physics Forums!

Just a comment: Thank you for sharing the results of your daughter's 5th grade science fair project. Experimental evidence is the foundation upon which science is built.

My 2 cents worth

Adults may tend to use better technique then children in an attempt to go faster. This would not contradict sciencemom jr. experiment since the bottle had the same shape regardless of weight. This would be analogous to an adult and a child using identical technique, which probably doesn't happen very often.

Adults tend to be flabbier then children, yes there are fat children, but an adults outer layers are going to be more loosely attached. This allows them to conform more closely to the shape of the slide. This means that the layer of water they are riding on is more consistent, and smaller portions of them penetrate the fluid to contact the slide.

A good follow-up experiment for the sciencemom family might be to host a back yard slip-'n-slide party. Set up a camera in a window with a view of the slide but don't tell any one it's there (don't want psychological effects skewing results). When all is said and done review the video and catalog slide times while carefully looking for differences in physiology and technique that might explain the differences. Once you have a theory of what affects slide times and how, design an experiment to test it.

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I was just searching the web to find the best technique for sliding down a 100 meter slide with a few turns here and there. An Ipad and Iphones at stake for the winners. I saw and interviewed the winner before the race when he was training. The timing was electronic with optical sensors at start and finish and gave the time within 1/100 second.
He said: Faster runs if he is sitting up (legs straight) rather than lying down.
Faster with shaved *** and legs (easy to understand - buy wow he really wants so win).
He also let me in on a secret that he used his arms on the sides of the tube to push himself forward.
So the question I was looking for is really: Why is he faster when sitting up? and is that really what other sliders experience?
How important is it that the track has turns compared to a straight waterslide?
Clearly sitting up means less surface and thus less drag, bur then the legs should be bent rather than straight...
Sitting up increases the air-drag so I should belive that there is an optimal angle at wich his body should be bent at the middle in order to minimize the total of the to drag effects and still be able to use the arms.
The question of the effect of the mass of the person. It seems that kids and woman run slower - at least from the results of the race witch was divided into categories and the winner slided 105 meters in 17:05 seconds while the fastest kid got a time of 19:81 (19 seconds and 81/100 seconds). This suggests that surface friction is not important but drag is the most important force also responseble for the face that heavier bodies fall faster than lighter bodies if the shapes are equal (just compare a falling ballon and a falling football).
I an gonna enter the competetion next year for sure, but I am not sure I will eat fast food for a year in an attemt to beat the record.

On twists/turns in the slide, do lighter/smaller people go higher up on the bend than heavier/larger people? (in general)

I found a link about a german who has a speed record for water slides, who only makes contact with one heel and with his shoulder blades.