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

jbriggs444
Homework Helper
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.

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

I think it partly depends upon what you mean by rotate.

See here

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jbriggs444