## Dolphin in a hamster ball

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GreedyAlgorithm
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### Dolphin in a hamster ball

If you put a dolphin in a giant ball with water inside, would that work?
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Charlie!
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### Re: Dolphin in a hamster ball

GreedyAlgorithm wrote:If you put a dolphin in a giant ball with water inside, would that work?

No, the dolphin would die, since dolphins need to breathe air

Oh, do you mean would it roll the ball? Probably - imagine the dolphin being off-center and swimming - where does the force go? Where does it go from there?
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Tass
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### Re: Dolphin in a hamster ball

No it would not be able to move the ball (and it would drown).

p1t1o
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### Re: Dolphin in a hamster ball

Well since a hamster moves the ball with the aid of gravity and its likely that the dolphin would have neutral bouyancy if it were swimming/floating inside the ball, then this could not work.

If the ball were on the sea-floor, the dolphin could "swim up" the forward wall of the ball and, if it could make itself sink (which I gather, they probably can) it could drive the ball to roll.

But by swimming along and bashing the wall with its nose it couldn't, no.

AvatarIII
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### Re: Dolphin in a hamster ball

assuming the dolphin were neutally bouyant, and there were absolutley no friction between the water and the inside of the sphere i guess no matter how much it moves, the ball itself would not move at all,
perhaps if there were baffles on the interior and the dolphin was not neutrally bouyant, i suppose it could create some sort of movement from the ball.

bigglesworth
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### Re: Dolphin in a hamster ball

Depends on the size of the ball (I'm also assuming that the ball is not completely full of water.

Since the ball is not completely full of water, it can either propel water forward, or propel itself forward, past the centre of gravity of the ball, causing it to begin rolling.
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### Re: Dolphin in a hamster ball

GreedyAlgorithm wrote:If you put a dolphin in a giant ball with water inside, would that work?

Work in what sense? Put on a suit and pick up a briefcase and perform meaningful tasks in a cubicle landscape?
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Charlie!
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### Re: Dolphin in a hamster ball

AvatarIII wrote:perhaps if there were baffles on the interior and the dolphin was not neutrally bouyant, i suppose it could create some sort of movement from the ball.

You can do away with baffles if you just say there's friction between the water and the ball, and you can do away with the buoyancy requirement by letting the dolphin swim in a loop. - I admit that wouldn't move the ball very well though. Being heavier than water would probably make things easiest (so the dolphin could face forward).
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Tass
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### Re: Dolphin in a hamster ball

Charlie! wrote:
AvatarIII wrote:perhaps if there were baffles on the interior and the dolphin was not neutrally bouyant, i suppose it could create some sort of movement from the ball.

You can do away with baffles if you just say there's friction between the water and the ball, and you can do away with the buoyancy requirement by letting the dolphin swim in a loop. - I admit that wouldn't move the ball very well though. Being heavier than water would probably make things easiest (so the dolphin could face forward).

How? I still don't see an external force.

Charlie!
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### Re: Dolphin in a hamster ball

Tass wrote:
Charlie! wrote:
AvatarIII wrote:perhaps if there were baffles on the interior and the dolphin was not neutrally bouyant, i suppose it could create some sort of movement from the ball.

You can do away with baffles if you just say there's friction between the water and the ball, and you can do away with the buoyancy requirement by letting the dolphin swim in a loop. - I admit that wouldn't move the ball very well though. Being heavier than water would probably make things easiest (so the dolphin could face forward).

How? I still don't see an external force.

The external force is provided by the floor, same as a regular hamster ball. Let's say the dolphin is heavier than water. It positions itself at the front of the ball of water and keeps itself in the same position by pushing water downward and around in a current. The water's friction with the walls creates a torque on the ball itself, which then rolls against the ground.
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p1t1o
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### Re: Dolphin in a hamster ball

If we are now assuming that the ball is on the sea bed, the Dolphin would still need to be heavier-than-water and resting on or near the bottom of the ball, otherwise there would be very little friction between the ball and the seabed.

A hamster ball is useful because it protects the hamster from casual harm and is too big to sneak into small spaces, we could build a "dolphin ball" of similar nature but it would have to have openings at the front and back to allow water to flow through (and something would have to be done about getting air to the dolphin too, obvs).

AvatarIII
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### Re: Dolphin in a hamster ball

Charlie! wrote:
AvatarIII wrote:perhaps if there were baffles on the interior and the dolphin was not neutrally bouyant, i suppose it could create some sort of movement from the ball.

You can do away with baffles if you just say there's friction between the water and the ball, and you can do away with the buoyancy requirement by letting the dolphin swim in a loop. - I admit that wouldn't move the ball very well though. Being heavier than water would probably make things easiest (so the dolphin could face forward).

true, but i figured having baffles would help immensely, and the bouyancy thing would help the dolphin actually move the centre of gravity of the ball, all helping the ball move more.

Tass
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### Re: Dolphin in a hamster ball

Charlie! wrote:The external force is provided by the floor, same as a regular hamster ball. Let's say the dolphin is heavier than water. It positions itself at the front of the ball of water and keeps itself in the same position by pushing water downward and around in a current. The water's friction with the walls creates a torque on the ball itself, which then rolls against the ground.

Okay, yes, if it is rolling on the sea floor it is of course possible.

It is actualy also possible if the ball is half-filled and buoyant. In this case the dolphin doesn't even need to have a different density than the water, since it would be able to push part of its body out of the water in the ball - and it would be able to breathe for a while.

bigglesworth
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### Re: Dolphin in a hamster ball

Clearly the only interesting permutation of this problem is the one with the ball being permeable to gas but not liquid, the sphere having a fair amount of air in it, and the dolphin-ball rolling around in a park chasing people.
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Tass
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### Re: Dolphin in a hamster ball

bigglesworth wrote:Clearly the only interesting permutation of this problem is the one with the ball being permeable to gas but not liquid, the sphere having a fair amount of air in it, and the dolphin-ball rolling around in a park chasing people.

The park would have to be tremendously flat though.

WarDaft
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### Re: Dolphin in a hamster ball

This question is more complicated than it appears on the surface.

Consider a man in a box car. Can the man move the box car by running back and forth in side it?

The answer depends on the friction the box car experiences.

If the man walks slowly across the car, the friction of the wheels on the track will likely prevent the car from moving, momentum is conserved but only in the entire man+box car+Earth reference frame. The man can run back, and depending on the friction of the wheels, may actually be able to move the box car in the opposite direction they are running. Slowly walking back will not move the car all the way back to where it started, because of friction.

The question is then: Can the Dolphin do anything similar?
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GreedyAlgorithm
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### Re: Dolphin in a hamster ball

bigglesworth wrote:Clearly the only interesting permutation of this problem is the one with the ball being permeable to gas but not liquid, the sphere having a fair amount of air in it, and the dolphin-ball rolling around in a park chasing people.

Yes, this was the motivating scenario.
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### Re: Dolphin in a hamster ball

GreedyAlgorithm wrote:
bigglesworth wrote:Clearly the only interesting permutation of this problem is the one with the ball being permeable to gas but not liquid, the sphere having a fair amount of air in it, and the dolphin-ball rolling around in a park chasing people.

Yes, this was the motivating scenario.

Funny that it took me so long to start thinking of the ball on land (or even on the seafloor). In the beginning I just had it floating free in the water.

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### Re: Dolphin in a hamster ball

If the dolphin was by the edge of the ball and pushing water down this would put more force than just its weight-buoyancy. This would roll the ball the same as a hamster. Although the ball would be quite heavy with obvious problems.
Or on the edge facing away from the way of travel creating a buldge of water behind it.

LaserGuy
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### Re: Dolphin in a hamster ball

There are two ways that I think could work. It depends on the friction of the water relative to the surface of the ball, I think. It's possible that the dolphin may be able to move the ball simply by putting its nose against the edge of the ball and swimming forward. The other possibility I see as maybe working is if you have a ball that's let's say half full of air and half full of water, you could create an imbalanced cycle by having the dolphin swim through the water in one direction, then jump through the air to get back to the starting point. Because of the differences in the friction between the dolphin/air and dolphin/water, I would think that you should be able to generate some forward momentum. I'm trying to think of a nice way to work this out mathematically.

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### Re: Dolphin in a hamster ball

LaserGuy wrote:There are two ways that I think could work. It depends on the friction of the water relative to the surface of the ball, I think. It's possible that the dolphin may be able to move the ball simply by putting its nose against the edge of the ball and swimming forward.

And why would the force on the ball from the dolphin's nose be larger than the force on the ball from the back-flowing water?
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### Re: Dolphin in a hamster ball

LaserGuy wrote:There are two ways that I think could work. It depends on the friction of the water relative to the surface of the ball, I think. It's possible that the dolphin may be able to move the ball simply by putting its nose against the edge of the ball and swimming forward. The other possibility I see as maybe working is if you have a ball that's let's say half full of air and half full of water, you could create an imbalanced cycle by having the dolphin swim through the water in one direction, then jump through the air to get back to the starting point. Because of the differences in the friction between the dolphin/air and dolphin/water, I would think that you should be able to generate some forward momentum. I'm trying to think of a nice way to work this out mathematically.

The dolphin could just swim "upwards", strongly treading water so that it is constantly half-raised from the pool.

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### Re: Dolphin in a hamster ball

Silly new-to-physics-person question:
If the dolphin is accelerating / decelerating inside the ball, wouldn't that cause the ball to change its acceleration? As in, ball is floating in frictionless vacuum, so any movement will cause the ball to move?

I might have this backwards...
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Zamfir
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### Re: Dolphin in a hamster ball

Yes, but when the dolphin decelerates to stay in the bubble, the bubbles stops as well. Their combined centre of gravity doesn't move without forces from the outside.

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### Re: Dolphin in a hamster ball

It's possible for humans in a hampster ball in water to make themselves go forward and even change direction: http://www.youtube.com/watch?v=jMnJ-eCX ... re=related

Not sure how adding water to the inside of the ball would change the dynamics or how well a dolphin could make the ball spin, but I wouldn't be surprised if it's technically possible.

On a side note, I just noticed that my spell check says "hampster" is wrong, which puzzled me. I'd always thought people were being lazy for forgetting the "p", so I had to check a few places online... and apparently without the "p" isn't just "preferred", but with the "p" is wrong?! I object!
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### Re: Dolphin in a hamster ball

If the One half of the ball had Studs "A bit like the classic sea mine shape" then if the dolphin swam in a horizontal circle the ball would roll in a circle. If the dolphin times this right he can either travel in sets of half circles or balance it so it travels in a stright line. (I think)