Does a floating object always rotate about its centre of mass?

DaveC426913

This is an textbook example of committing the very crime you accuse others of doing. You criticize others for not answering, but then do not contribute yourself.


There are many ways to interpret the OP's question, which is why there are so many answers. But the upshot is that there are certainly examples involving drag that will cause a floating object to rotate about an axis other than its CoM.

Steve Zissou

Dave,

Thanks for the reply. However I think my point is valid. Let's have a look. The OP said,
The guy in question starts off with,
Imagine? I imagined what the guy said, which was a "large, stable floating object placed in the ocean, free floating." Where is the sea anchor in the guy's question? He didn't say "damping," he didn't say "asymmetric," he didn't say "tethered."

I personally find it to be a pet peeve when people act wise and imperious, and write in a manner that seems like talking down. "Imagine, little person, a situation in which..." the guy didn't say "help me imagine," he said "what's the answer."

Anyways that's my 2 cents. Peace

sophiecentaur

Glad you know about nautical.matters. The contents of some of the posts on the thread implied that not everyone knew.
My point is that the CM is not relevant to how the boat interacts with the water in this case. A drogue or a towed tender need hardly change the CM at all but can affect the pivot point position greatly.

Studiot

Now that we have started to seriously engage our physics brains about the subject perhaps we can discuss how the forces imposed by water movement can cause rotation.

Can pressure forces, which are normal forces, cause rotation under any circumstances?
The pressure on a surface can be repleced by a single force acting on the centre of pressure. It is clear that the moment of this force must be zero about its own line of action.

What about tangential forces?
Is there any condition on the flow regime of the water required. For instance can a parallel velocity field lead to rotation?
Even the whirlpool example I offered earlier has rotationless flow.

It is a good question, worthy of treating with respect.

jbriggs444

Imagine a hose sprayed onto the bowsprit of a boat from the starboard side.

Yes, pressure forces can cause rotation under some circumstances.

Yes, the moment of that force is zero along its own line of action. But that line of action need not intersect any particular axis of rotation.

Studiot

Yes indeed there are many mechanisms that could force rotation about any desired axis, including the finger of God.

But the OP specified that the rotation was to be effected by the tides and currents.

When you were on the Titanic did you see the icebergs go spinning gaily past?

jbriggs444

The stream from a hose is a current.

Studiot

That's just dodging the issue.

Let us see your proof that natural water movements can rotate a floating rigid body about a vertical axis other than the COM of the body.

Bear in mind that we don't know if this body has any motive power or is under way.
Also that a boat plus sea anchor is not a rigid body.

jbriggs444

The question you asked, Studiot was:

"Can pressure forces, which are normal forces, cause rotation under any circumstances?"

You did not specify that these pressure forces were required to originate as the result of a sub-surface, naturally originated current. The stream of water from a hose happens to be an above-surface, artificially originated current.

If you wish you can instead imagine a boat nosing from calm ocean water into the outflow of a river from the side.

The outflowing river water will pile up against the prow of the boat, increasing pressure. This will rotate the boat. If this boat has a movable centerboard then I expect that you will agree that the axis of rotation will depend on the location of the centerboard. The center of gravity of the boat will not vary much with the centerboard location.

Studiot

Why would the side stream not just push the whole boat sideways?

Edit

Let us put this on a more logical basis. Let us say the side stream comes from left to right.

You draw a line from the stern through the bow and measure the heading.

After the side stream has moved the boat you do this again and find the the heading has increased.

How do you determine the axis of rotation? The boat will have both rotated and translated

The issue also goes back to my original statement

See here

http://www.youtube.com/watch?v=hPvssJPAx-E

jbriggs444

Because the bow of the boat enters the side-stream first.

I've done the experiment many times over many years. Usually in a canoe.