12. a block has a volume of 0.09 m3 and a density of 4,000 kg/m3. what's the force of gravity acting on the block in water?

If each m³ of the  block has 4,000 kg of mass in it, but the volume of the whole block is only 0.09 m³, then the mass of the whole block is

(4,000 kg/m³) x (0.09 m³) = 360 kg

The force of gravity acting on any mass on Earth is (mass) x (gravity) .

Force of gravity = (360 kg) x (9.8 m/s²)

Force of gravity = 3,528 Newtons

This is the force of gravity on the block.  The environment doesn't matter.  It may be in air, in water, in vacuum, or in chicken soup.  The force of gravity acting on it is 3,528 Newtons, pointing down.

If it happens to be in water, then there's ANOTHER force acting on the block, in the opposite direction, called BUOYANCY.  It's equal to the weight of the water displaced by the block, and it acts UPWARD on the block.  It subtracts from the force of gravity.  This is why some things FLOAT in water.

-- The volume of water displaced by the block is 0.09 m³ .

-- The density of water is 1,000 kg/m³.

-- The mass of the displaced water is (0.09 m³) x (1,000 kg/m³) = 90kg .

-- The WEIGHT of the displaced water is (90kg)x(9.8m/s²) = 882 Newtons.

-- The apparent WEIGHT of the block in the water is (3,528 - 882) =  2,646 Newtons.

-- This is what you'll see on a bathroom scale at the bottom of the pool if the block is sitting on it ... 2,646 Newtons ... but it's NOT the force of gravity on the block.  The force of gravity on the block, whether it's in or out of the water, is 3,528 Newtons.  

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