Floating inferred?

[SaintJude]

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If 500-N of weight is placed atop a particular boat at rest in the ocean, the boat sinks 3cm lower in the water.

So the starting equation is 500 N = (rho)gΔV
By setting buyoant force to the weight, they are suggesting that the ship is floating.

Previously I only thought you could use the "weight = bouyant force" when "floating" was specifically written. Are ships always known to float?

If you think this is a dumb question, I'll blame it on my tiredness....

 

[chiddler]

what are you asking, exactly?

do ships always float? lol

 

[SaintJude]

The "weight"=buoyant force can only be applied when an object is floating! And so I didn't use it because I didn't realize the ship is "floating" Couldn't it have been "partially submerged"?

 

[MedPR]

The "weight"=buoyant force can only be applied when an object is floating! And so I didn't use it because I didn't realize the ship is "floating" Couldn't it have been "partially submerged"?

See, this is the kind of confusion I was having early on with buoyancy. There's quite a bit of ambiguity in the terms and reasons/ways to know when you can use specific relationships.

However, in this situation I'm 99.9999999% sure you can assume the boat is floating. Here's why.

1. A boat pre-weight added is probably floating.
2. If you add some weight to the boat and the boat sinks only 3cm, it is unlikely that the boat is now submerged. How many boats do you know of that are <3cm tall?

 

[milski]

All floating ships, submarines excluded, are partially submerged. The alternative to partially submerged is fully submerged which generally is a bad omen for a ship or fully out of the water which is also very impractical for a ship.

If a problem does not say that the boat/ship sunk, I would expect it to be floating. What's the point of saying that it is a ship otherwise?

 

[MedPR]

Also if you're not sure, couldn't you just start with the basic equation and work through it the long way regardless of if it sinks or floats?

 

[milski]

Also if you're not sure, couldn't you just start with the basic equation and work through it the long way regardless of if it sinks or floats?

If it sinks, you don't know the buoyant force, you only know that it's less than the total weight.