Thread: Who knows the answer to this?

I read somewhere a while back (can not for the life of me find the correct book to foind the answer) that if the QEII or a similar large ship was berthed at a wharf and not tied up and it was a perfectly still day, you could push it out with one finger.

Now, I stupidly brought this up in a discussion and now have to prove I am right. Anybody?

Originally Posted by Oakman

I read somewhere a while back (can not for the life of me find the correct book to foind the answer) that if the QEII or a similar large ship was berthed at a wharf and not tied up and it was a perfectly still day, you could push it out with one finger.

Now, I stupidly brought this up in a discussion and now have to prove I am right. Anybody?

Sure. If nothing is holding it in place, not even wind. But it would take a VERY long time for it to start moving.

For example, the domes on some observatories are so well balanced and have bearing of extremely high quality that they can be made to rotate with a single very small electric motor. And most of those things weigh many tons.

Is there any way I can PROVE this?

Know of any physics laws or anything that might make the arguement a little more believable?

Originally Posted by Oakman

Is there any way I can PROVE this?

Know of any physics laws or anything that might make the arguement a little more believable?

Yes. :wink: All that needed to move ANY object is enough force to overcome it's resistance to whatever is holding it in place. In the case of a floating object with no other force acting upon it (like wind, moorings, etc.) the resistance presented by the water is neglible. And the only reason it would take so long to get it started moving is it's inertia. But a push with a single finger will be more than enough force given a long amount of time.

Old Geezer, you have just made tomorrow morning at work a bit more interesting and fun for me.

Thanks heaps!

Originally Posted by Oakman

Old Geezer, you have just made tomorrow morning at work a bit more interesting and fun for me.

Thanks heaps!

You're very welcome, Oakman.

Now get to bed and get some sleep. :-D

yeah, in New Zealand, so it is only 9pm Thursday night here.

Off to bed soon though

Originally Posted by Oakman

yeah, in New Zealand, so it is only 9pm Thursday night here.

Off to bed soon though

Ahhhh - gotcha! It's just after 5am here on the East coast of the states but I'm retired and a bit of a night owl. :wink: I don't have to keep any kind of schedule. But, of course, it took a lot of years to GET to this. Heh-heh!

Ahh, which part of the states? Would be nice not to have any schedule to adhere to. I am a NZ Customs Officer and tend to have schedules running my life.
Don't drink to much "cwoffee" now or you will never get to sleep.

Right, speaking of which, I am off to bed now. Enjoy your Thursday. Thanks for your help mate.

You will first have to apply enough force to overcome static friction. I doubt you will be able to do this. Therefore I believe you are mistaken in your claim.

Originally Posted by Ophiolite

You will first have to apply enough force to overcome static friction. I doubt you will be able to do this. Therefore I believe you are mistaken in your claim.

Hello, Ophiolite,

I do understand what you mean but I think you can overcome it. However, how does on calculate the value of static friction for a free-floating body?

Originally Posted by Ophiolite

You will first have to apply enough force to overcome static friction. I doubt you will be able to do this. Therefore I believe you are mistaken in your claim.

Good point. The kinetic resitance or inertia might in theory be minimilized and thus overcome but static friction (electromagnetic) remains.

Static friction applies to two solid surfaces in contact. The friction between a solid and a fluid is called drag and is proportional to the speed, so there is no drag until you get the ship moving.

That's a good point, since water is pretty much a good Newtonian fluid and does not behave like, say, a Bingham Plastic, or Herschel-Buckley fluid, so there is no yield point, which would be the equivalent of the static friction value.

Originally Posted by Oakman

I read somewhere a while back (can not for the life of me find the correct book to foind the answer) that if the QEII or a similar large ship was berthed at a wharf and not tied up and it was a perfectly still day, you could push it out with one finger.

Now, I stupidly brought this up in a discussion and now have to prove I am right. Anybody?

Not entirely true.

There's a lot of factors involved. Mainly, the water being displaced by the vessel.

In order to move the ship, you have to move the water out of the way (displacement). The ship's curved hull makes this easier, but you still will require a considerable amount of force to move the vessel.

Chances are, you'd never find a perfectly sterile environment to try this, except may in a lab. So there's always other actions playing against you. Wind, currents, wave action, hull shape and depth, etc.

Of course, the other side of the coin is that your pressing your finger on the bow of the ship does apply some minuscule amount of force on the ship. Although your force is likely too small to make any difference against the much larger forces at play, you could in fact be moving the ship. You might only be moving a few milliliters of water at a time, but eventually, over maybe 30 years, you might get it to move a foot or so.

Originally Posted by Wolf

Originally Posted by Oakman

I read somewhere a while back (can not for the life of me find the correct book to foind the answer) that if the QEII or a similar large ship was berthed at a wharf and not tied up and it was a perfectly still day, you could push it out with one finger.

Now, I stupidly brought this up in a discussion and now have to prove I am right. Anybody?

Not entirely true.

There's a lot of factors involved. Mainly, the water being displaced by the vessel.

In order to move the ship, you have to move the water out of the way (displacement). The ship's curved hull makes this easier, but you still will require a considerable amount of force to move the vessel.

Chances are, you'd never find a perfectly sterile environment to try this, except may in a lab. So there's always other actions playing against you. Wind, currents, wave action, hull shape and depth, etc.

Of course, the other side of the coin is that your pressing your finger on the bow of the ship does apply some minuscule amount of force on the ship. Although your force is likely too small to make any difference against the much larger forces at play, you could in fact be moving the ship. You might only be moving a few milliliters of water at a time, but eventually, over maybe 30 years, you might get it to move a foot or so.

Consider that he more or less stated the question in such a way as to remove all the miscellaneous forces - I believe that was his intent when he said "not tied up, perfectly still day (meaning no wind). And one should assume (I think) that he would have included waves, currents, etc. if asked.