Liquid Displaced by submerged vs floating object

[supras2kracer]

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I'm confused about why a submerged object displaces less water than the same object but floating.

This is question 113 in EK Physics Lecture 5, 30 minute exam.

A brick sits on a massless piece of styrofoam floating in a large bucket of water. If the styrofoam is removed and the brick is allowed to sin to the bottom:

A. The water level will remain the same
B. The water level will fall
C. The water level will rise
D. density of brick must be known to solve

The book says that the fluid displaced will be equal to the object's weight. Why doesn't it displace the same amount of fluid submerged vs floating?

 

[loveoforganic]

That's only the case if the object in question is denser than water (which is implied as it says 'submerged in water' was one possibility). In a "boat" it is able to displace its weight in water. Submerged, it just displaces its volume.

 

[JFK90787]

if the brick is floating on the styrofoam, a ton of water will need to be displaced to create enough bouyant force to keep it floating (since bricks are heavy, obviously). And it only floats in the first because styrofoam is less dense than water.

Remove the styrofoam and the object will sink, so will displace only its own volume

My feeling is these 'brick on top of styrofoam questions' are just conceptual. Just remember something heavy on top of styrofoam displaces more water than the heavy object would by itself

 

[lDanny]

just to add to it, when an object is denser that means more mass is packed into a smaller volume so when that brick is submerged, the water level is going to be lower.