Displacement & weight

[angelsson]

Re: The reason for the name ...

For those of you who like brain teasers here is one connected to understanding the displacement and weight etc so well explained in these replies.

Imagine a large tank of water, in which a boat is floating, and in the boat is a block of concrete weighing 2 tons.

A crane lifts the block from the boat and places it in the water at the bottom of the tank.
Q. Does the water level in the tank rise, fall or stay the same?

No prizes offered, just for the fun of it.

 

[aitchw]

Not dumb, I hope, because I asked pretty much the same question last year. I was hoping for a clear cut statement reflecting some convention within the industry only to find, as now, there isn't one.

As far as I can see unless the displacement figure given is qualified in some way by a description of what boat state it reflects it is next to useless. As has already been stated, it will change with what's on board at any particular time.

 

[VicS]

Re: The reason for the name ...

The level falls because while in the boat the block is responsible for displacing 2 tons of water but when in the bottom of the tank it displaces only its own volume, which is equal to the weight of the block divided by the density of concrete.
.
.
I think.

 

[DirkJ]

Another displacement question

While reading about displacement here I come to think about one more thing. The characteristics of my boat (Jeanneau Rush) read weight of the boat 3200 KG, weight of ballast 1140 KG. Does that mean that the displacement is 4340 KG or is the weight of ballast already included in the weight og the boat?

Fair winds
Dirk

 

[Strathglass]

Re: Another displacement question

No

The weight of your boat including the ballast is 3200KG.

This would be the manufacturers weight for the bare boat as originally supplied with the minimum of extra bits.

Now it may weigh a lot more than that it is loaded with your own extra equipment. Unless you are a very keen racer.

Iain

 

[angelsson]

Re: The reason for the name ...

Spot on, 2 tons of dispacement water far excedes the volume of the block displacement so the level falls.
I thought it might give a visual picture of Archimedes principle to think about it this way and possibly be of assistance- not sure if he is politicaly correct in schools now!
Mike