Understanding Submerged vs Underwater Objects: A Quiz and Explanation

[wavingerwin]

i understand that an object with smaller density than water will float on water
with the same density, it will be submerged and "suspended" in the middle of the water

my question is in the case of an object being denser than water, it will sink and finally touches the ground (underwater).

i did a quiz 6 months ago and try to recall a question (picture attached)
A boat floating on an enclosed pool while a ball of a big density is sinking.
When the ball is finally underwater i.e. touches the pool's bottom,
What will happen to the water's surface?
1.relative to the bank
2.and relative to the boat
I forget what the answer was, can somebody please help?

thank you!

 

[maverick_starstrider]

The water will rise relative to the bank but the boat will rise with it so that relative to the boat there is no difference.

 

[wavingerwin]

The water will rise relative to the bank but the boat will rise with it so that relative to the boat there is no difference.

Can you please explain why?
why would it rise only when the ball finally touches the ground?
since the ball displaces the same amount of water while sinking and while being underwater (at the bottom of the pool), would it be the same surface level when the ball is sinking and underwater?


Thank you very much!

 

[maverick_starstrider]

Sorry, I should have clarified. The water won't rise when the ball hits the bottom, it'll rise when you initially insert the ball. When that happens it displaces its volume worth of water which will lead to the water level rising. However, this will not effect the bouyancy equilibrium of displaced water to bouyancy force for hte boat and thus its level of submersion will remain the same. The life of the iron ball is a little more complicated, however that's not part of the question