Can a Fish Move a Ball in Water?

[Immelmann]

if you put a fish in a plastic ball filled with water, can the fish make the ball roll by swimming.

just like my hamster can make the ball roll by walking.

thanx.

 

[phinds]

if you put a fish in a plastic ball filled with water, can the fish make the ball roll by swimming.

The ball, water, and fish are a closed system. What external force do you think there is that would make the ball move?

 

[metastable]

Suppose the fish has a ballast weight strapped to its back, it rests at the bottom of the ball. swims forward, rests on the bottom. the ball’s center of mass is no longer colocated with the geometric center of the ball... or is my analysis flawed?

 

[A.T.]

Suppose the fish has a ballast weight strapped to its back, it rests at the bottom of the ball. swims forward, rests on the bottom. the ball’s center of mass is no longer colocated with the geometric center of the ball... or is my analysis flawed?

That would work, it's just what the hamster does.

 

[A.T.]

if you put a fish in a plastic ball filled with water, can the fish make the ball roll by swimming.

If the fish has the same mean density as water, the hamster method will not work. But theoretically it could induce a flow that could spin the ball via friction on the inner walls, resulting in rolling.

 

[metastable]

If the fish has the same mean density as water, the hamster method will not work.

What if it has the same mean density as water, and it points its nose at a point on the bottom of the ball slightly to the side of the ball's point of contact with the ground and swims vigorously directly "downwards..." ?

 

[metastable]

...Or what if it starts swimming from the top of the ball, aiming directly towards a point just beside where the ball makes contact with the ground, building up kinetic energy as it swims downwards, and then it collides with the point next to the ball's point of contact with the ground it was aiming towards?

 

[phinds]

If the fish has the same mean density as water, the hamster method will not work. But theoretically it could induce a flow that could spin the ball via friction on the inner walls, resulting in rolling.

Clever. I hadn't thought of that.

 

[phinds]

What if it has the same mean density as water, and it points its nose at a point on the bottom of the ball slightly to the side of the ball's point of contact with the ground and swims vigorously directly "downwards..." ?

Fish swims vigorously downward, causing water to push against ball wall to his rear, ball has slight movement in the direction of his tail, fish hits ball, ball is restored to original position. That's the problem you'll have with any non-rotaing scenario where the fish / water have same density. Whatever force the fish applies at the end of its short trip is offset by a corresponding force when it starts moving.

I think AT's rotating scenario would work though.

 

[pinball1970]

if you put a fish in a plastic ball filled with water, can the fish make the ball roll by swimming.

just like my hamster can make the ball roll by walking.

thanx.

Instinctively I would say yes if the dimensions were favourable, the fish could generate a large enough force. Swimming is uneven it's not a perfect forward force = backwards force so I would say you will get some movement. Put it this way if you put a shark in a plastic ball and it was disciplined enough to swim in a circle as fast as it could at the edges the ball would move due to outward force of the eddy it would create, perhaps a gentle oscillation.

 

[metastable]

Fish swims vigorously downward, causing water to push against ball wall to his rear, ball has slight movement in the direction of his tail, fish hits ball, ball is restored to original position. That's the problem you'll have with any non-rotaing scenario where the fish / water have same density. Whatever force the fish applies at the end of its short trip is offset by a corresponding force when it starts moving.

Wouldn't some of the motion of the water caused by the fish swimming downwards be converted to heat before it can generate an opposite force on the ball, resulting in a very tiny net force in one direction, or is the conversion to heat too minuscule to be considered?

 

[jbriggs444]

motion of the water caused by the fish swimming downwards be converted to heat

Yes, eddies in the water can be damped into heat. But you cannot cancel simply cancel bulk momentum or angular momentum into energy. If the fish is moving one way and the water another then the water has momentum that cannot be lost in the absence of an interaction with either ball or fish.

Edit: Or gravity -- if there is air in the ball along with the water.

 

[metastable]

Yes, eddies in the water can be damped into heat. But you cannot cancel simply cancel bulk momentum or angular momentum into energy. If the fish is moving one way and the water another then the water has momentum that cannot be lost in the absence of an interaction with either ball or fish.

So does it mean that no work would be done between the ground and the ball (as would be done by a wheel) at the time that the fish impacts the point just beside the ball's point of contact with the ground (assuming the ball can't slip across the ground -- ie has to roll)?

 

[jbriggs444]

So does it mean that no work would be done between the ground and the ball (as would be done by a wheel) at the time that the fish impacts the point just beside the ball's point of contact with the ground (assuming the ball can't slip across the ground -- ie has to roll)?

You seem to have changed the subject completely and are now talking about work done by the contact force of friction between ball and ground. Let us assume that you want to talk about the work done by the ground on the ball.

In order to answer that question, we need to know what sort of work you are talking about. Are you talking about real work (force multiplied by motion of the point of contact on the target body) or about center-of-mass work (force multiplied by motion of the center of mass of the target body)?

Alternately, is it possible that you are talking about the work done by the impulsive force couple: fish on ball plus ground on ball.

 

[Dale]

But theoretically it could induce a flow that could spin the ball via friction on the inner walls, resulting in rolling.

Hmm, I am not sure about that. I don’t see how that gets an external torque to happen.

 

[jbriggs444]

Hmm, I am not sure about that. I don’t see how that gets an external torque to happen.

If fish is rotating one way then in order to conserve momentum, water is rotating the other.
If water is rotating, friction exists.
If friction exists, torque exists.
QED

 

[Dale]

If fish is rotating one way then in order to conserve momentum, water is rotating the other.
If water is rotating, friction exists.
If friction exists, torque exists.
QED

Yes, but it is an internal torque.

 

[jbriggs444]

Yes, but it is an internal torque.

Depends on where you draw the system boundaries. It is an internal torque to the ball+fish+water system, sure. But it is an external torque to the ball system. By inspection, there is no torque on the ball from the fish. That leaves a torque on the ball from the environment. If the ball does not move, that torque must be equal and opposite to the torque on the ball from the water. (And if the ball does move, we've answered the original question).

That torque is unambiguously external to the ball+fish+water system.

 

[Dale]

Yes, I see now. In the absence of an external torque the water and the ball would counter-rotate to conserve angular momentum. The rotation of the ball would lead to slipping at the ball/floor contact patch. Static friction at the patch opposes slipping and leads to the required external torque.

 

[metastable]

Alternately, is it possible that you are talking about the work done by the impulsive force couple: fish on ball plus ground on ball.

I'm not 100% certain but this sounds like what I was referring to. The fish swims downwards, impacts the ball next to where it contacts the ground, the ball doesn't slip...

 

[jbriggs444]

I'm not 100% certain but this sounds like what I was referring to. The fish swims downwards, impacts the ball next to where it contacts the ground, the ball doesn't slip...

It is a difficult situation. Our expectation is that if a rigid object bumps into something that a high impulsive force results. But the water complicates things. In this case we have zero net momentum both before and after the collision. Does a high impulsive force actually result? I am having a hard time wrapping an intuition around the situation.

We have an upward flow of water prior to the collision. So a good question would be what happens to that flow. One answer is that it should stop. But if it stops, there has to be a force making it stop. That force is negative pressure. Negative pressure that should be centered on the impact point and that should negate the impulse from the collision. I think we're going to have to get some cavitation going before we can impart much net force. And even then, it would only be temporary.

[Negative pressure is not unreasonable. If the water is under atmospheric pressure we can have up to 15 psi of negative gauge pressure before we hit zero absolute pressure. Even negative absolute pressure is physically reasonable. Water has surface tension. In the absence of nucleation sites, it will resist forming voids. A quick trip to Google yields http://discovermagazine.com/2003/mar/featscienceof which is less than authoritative, but quite readable. This hit is more authoritative]

 

[metastable]

 

[DaveC426913]

I think we're going to have to get some cavitation going before we can impart much net force.

That would have to be a pretty impressive fish. In a pretty large ball...

 

[DaveC426913]

View attachment 247336

Not sure if this is meant to verify or refute the initial supposition.

Either way, it's a faulty analogy.
1] That is not a closed system.
2] It does not have a homogenous density.

 

[metastable]

I thought the only relevant instant is when the wheel strikes the pan. A mass (the skateboard wheel) has kinetic energy, transferring some of the energy to the pan. The pan has to roll.

 

[DaveC426913]

I thought the only relevant instant is when the wheel strikes the pan. A mass has kinetic energy, transferring some of the energy to the pan. The pan has to roll.

Regardless, the two above-mentioned factors are relevant.
( 1] That is not a closed system. 2] It does not have a homogenous density. )

 

[metastable]

Even if the fish has the same density, wouldn't the mass of the overall fish relative to the mass of each water molecule at least somewhat factor into the equation? Some of the motion of the water molecules has to turn into heat instead of recoil.

 

[phinds]

View attachment 247336

Now, you're just being silly. Unless you have COMPLETELY changed what we are talking about here.

 

[metastable]

Can we agree if the fish swims directly from the bottom to the top, all of the motion it causes in the water molecules from swimming through the water will eventually be converted to heat and radiated to the environment? Are you saying this process won't also occur if the fish swims downward at a slight angle and forcefully knocks into a point beside where the ball makes ground contact?

 

[metastable]

 

[phinds]

Can we agree if the fish swims directly from the bottom to the top, all of the motion it causes in the water molecules from swimming through the water will eventually be converted to heat and radiated to the environment?

I guess it probably does, but so what? That's a long term result and will have no effect on the short term motion of the closed system.

 

[jbriggs444]

Some of the motion of the water molecules has to turn into heat instead of recoil.

No. Energy and momentum are each conserved separately. They do not convert.

 

[metastable]

I observe in the computational fluid dynamics video of the trout swimming above, to my untrained eye much of the motion of the water could be described as spinning vortices, while the trout appears to derive directional thrust.

 

[jbriggs444]

I observe in the computational fluid dynamics video of the trout swimming above, to my untrained eye much of the motion of the water could be described as spinning vortices, while the trout appears to derive directional thrust.

That impression is incomplete. Momentum is conserved. Any forward momentum of the trout is necessarily mated with an equal and opposite rearward momentum of the water.

You really need to have more faith in conservation laws. They work.

 

[phinds]

I observe in the computational fluid dynamics video of the trout swimming above, to my untrained eye much of the motion of the water could be described as spinning vortices, while the trout appears to derive directional thrust.

I don't understand your issue with that fact. The resulting movement in a fluid once the thrust is imparted is irrelevant to the direction of motion imparted by the thrust. You seem to keep bringing in side issues.

 

[metastable]

I have faith in conservation laws. In space if I "shove off" against the edge of the wheel rather than pushing against its center of mass, with the same amount of force in both cases, in one case the wheel will spin and move away somewhat conserving momentum and the other case the wheel will move away from me conserving momentum (not spinning). By analogy I am asking if the fish is creating vortices which eventually decay into heat, is the force imparted on the ball by the collision of the fish next to the bottom completely offset by these vortices and heat produced?

 

[jbriggs444]

I have faith in conservation laws. In space if I "shove off" against the edge of the wheel rather than pushing against its center of mass, with the same amount of force in both cases, in one case the wheel will spin conserving momentum and the other case the wheel will move away from me conserving momentum. By analogy I am asking if the fish is creating vortices which eventually decay into heat, is the force imparted on the ball by the collision of the fish next to the bottom completely offset by these vortices and heat produced?

I cannot make sense of a question that can only be asked by analogy.

 

[metastable]

Do the water molecules set in motion by the fishes tail have interactions with other water molecules before they traverse to the other side of the ball, and if these interactions produce vortices, can these vortices completely counteract the direct force applied to the ball by the kinetic energy of the fish impacting a point adjacent to the bottom?

 

[jbriggs444]

Do the water molecules set in motion by the fishes tail have interactions with other water molecules before they traverse to the other side of the ball, and if these interactions produce vortices, can these vortices completely counteract the direct force applied to the ball by the kinetic energy of the fish impacting a point adjacent to the bottom?

Again, you are failing to ask a coherent question.

A single force acting in a single direction imparts momentum. That is momentum. Momentum is conserved. It is not convertible into heat.

 

[metastable]

"The figure above shows one of Joule’s most famous experimental setups for demonstrating the mechanical equivalent of heat. It demonstrated that work and heat can produce the same effects, and helped establish the principle of conservation of energy. Gravitational potential energy (PE) (work done by the gravitational force) is converted into kinetic energy (KE), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. His contributions to the field of thermodynamics were so significant that the SI unit of energy was named after him."

https://tophat.com/marketplace/scie...ax-college-physics-openstax-content/666/19511

 

[A.T.]

What if it has the same mean density as water, and it points its nose at a point on the bottom of the ball slightly to the side of the ball's point of contact with the ground and swims vigorously directly "downwards..." ?

...Or what if it starts swimming from the top of the ball, aiming directly towards a point just beside where the ball makes contact with the ground, building up kinetic energy as it swims downwards, and then it collides with the point next to the ball's point of contact with the ground it was aiming towards?

If we assume that the fish has the same density as the water, the water is in-compressible, and the ball is a rigid sphere, then the center of mass of ball+water+fish is fixed in the center of the ball, no matter what the fish does. Without the possibility to move the CoM relative to the ball you cannot "shove" or "rock" the ball around.

 

[phinds]

"The figure above shows one of Joule’s most famous experimental setups for demonstrating the mechanical equivalent of heat. It demonstrated that work and heat can produce the same effects, and helped establish the principle of conservation of energy. Gravitational potential energy (PE) (work done by the gravitational force) is converted into kinetic energy (KE), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. His contributions to the field of thermodynamics were so significant that the SI unit of energy was named after him."

OK, now you have TOTALLY changed what you are talking about. This thread has become a total mish mosh.

 

[metastable]

"Sharks are denser than water, and must swim continually, using dynamic liftfrom their pectoral fins.
Bone and muscle tissues of fish are denser than water. To maintain depth, bony fish increase buoyancy by means of a gas bladder. Some fish store oils or lipids for this same purpose. Fish without these features use dynamic lift instead. It is done using their pectoral fins in a manner similar to the use of wings by airplanes and birds. As these fish swim, their pectoral fins are positioned to create lift which allows the fish to maintain a certain depth. The two major drawbacks of this method are that these fish must stay moving to stay afloat and that they are incapable of swimming backwards or hovering.[9][10]"

https://en.wikipedia.org/wiki/Fish_locomotion#Dynamic_lift
"In physostomous swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can be removed in a similar manner."

https://en.wikipedia.org/wiki/Swim_bladder

 

[A.T.]

Sharks are denser than water, ...

If the fish is denser than water, then it can just do what the hamster does or what you have described in post #3 (no ballast needed).

 

[1977ub]

If there is no friction, can the hamster succeed ?+

 

[metastable]

Without the possibility to move the CoM relative to the ball you cannot "shove" or "rock" the ball around.

I'm having an extremely difficult time conceptualizing this. My mind keeps visualizing 2 "fish" which are neutrally buoyant swimming downwards towards a "see saw" at the bottom of a pool (or rod balanced on a fulcrum). They both hit at the same time but one fish is traveling downwards at twice the velocity of the other fish. Is the center of mass at time of impact balanced? Will the "see saw" or balanced rod deflect predictably one way or the other upon impact?

 

[A.T.]

If there is no friction, can the hamster succeed ?+

No friction with the ground means no external horizontal force, so the CoM will not move horizontally, but the contact point can slide back and forth.

 

[Vanadium 50]

This thread has become a total mish mosh.

1. True.
2. As has been said before, momentum and energy are different things and one cannot convert one to the other.
3. Metastable, your median time before responding has been something like three minutes. I think you should more carefully read what has already been posted - you shouldn't expect to immediately understand. Firing off replies that complicate the situation isn't going to help you understand nearly as much as thinking hard about what's already been written.

 

[rcgldr]

This is not a closed system, because the container is in contact with the floor. As long as the fish can swim away from the center of the ball and induce a circular flow of water, then the angular momentum of the water and ball have to be offset by the angular momentum of the Earth (the only point of interaction is at the contact point between ball and floor).

If the experiment was tried in space, free from any external forces, then the center of mass of the now closed system could not be moved.

I don't see how the fish in a ball differs that much from any other "self-propelled" vehicle, such as a unicycle, bicycle, motorcycle, car, ..., other than it's inefficient.

update - A better explanation is that the fish can swim forwards in the water to keep the center of mass of ball, fish, and water in front of the contact point. Then the pull of gravity and the upwards force from the floor combine to exert a forwards torque on the ball.

 

[DaveC426913]

This is not a closed system, because the container is in contact with the floor. As long as the fish can swim away from the center of the ball and induce a circular flow of water, then the angular momentum of the water has to be offset by the angular momentum of the ball's container (as well as the very tiny effect on the earth), in which case the ball will roll as long as the angular momentum of the water remains non-zero.

I'm not good at this conservation of angular momentum, but won't the fish's angular momentum in one direction be exactly canceled by the water's momentum in the opposite direction? This isn't like other situations, where a rider could "grab" the machine and manipulate it.