# Tbr physics bouyancy chapter 7

#### crazy person

##### Full Member
5+ Year Member
example 7.4b page 63:

why is the mg 36 N pointing down? shouldn't be pointing up because its says it weights in the air and 9 newtons should be ponting down because it specifically says 9 newtons in water? did they know it was already sunken before doing any calculation because of the specific gravity ratio?

So do we always use the specific gravity ratio pobject/pwater for sunken objects only?

I really don't like Fluids !

#### caereax

##### Full Member
5+ Year Member
The picture is a free body diagram of all forces acting on the object.

36N downward is the force of the object's weight.
9N upward is the normal force (in water it weighs 9N, thus the ground pushes upward 9N).

If an object still has weight in water that means it has sunk to the bottom. As for the ratio, you can use it whether an object is sunken or floating. If the object is sunken, the density ratio will represent the weight-to-buoyancy ratio. If the object is floating, the density ratio represents the % submerged.

Also, if English is a second language, no problem on this post. If English is your first language, FFS, what are you doing?

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