# B Can the fish move the ball?

#### 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.

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#### phinds

Gold Member
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

Gold Member
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

Gold Member
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

Gold Member
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

Homework Helper
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

Homework Helper
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

Mentor
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

Homework Helper
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

Mentor
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

Homework Helper
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

Mentor
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

Homework Helper
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 this article which is less than authoritative, but quite readable. This hit is more authoritative]

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#### DaveC426913

Gold Member
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

Gold Member
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.

"Can the fish move the ball?"

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