Question

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?

Answer 1

                                       Density = (mass) / (volume)

                                4,000 kg/m³ = (mass) / (0.09 m³)

Multiply each side
by  0.09 m³ :           (4,000 kg/m³) x (0.09 m³) = mass

                                 mass = 360 kg .

Force of gravity = (mass) x (acceleration of gravity)

                           = (360 kg) x (9.8 m/s²)

                           = (360 x 9.8)  kg-m/s²

                           =   3,528 newtons . 

That's the force of gravity on this block, and it doesn't matter
what else is around it.  It could be in a box on the shelf or at
the bottom of a swimming pool . . . it's weight is 3,528 newtons
(about 793.7 pounds).

Now, it won't seem that heavy when it's in the water, because
there's another force acting on it in the upward direction, against
gravity.  That's the buoyant force due to the displaced water.

The block is displacing 0.09 m³ of water.  Water has 1,000 kg of
mass in a m³, so the block displaces 90 kg of water.  The weight
of that water is  (90) x (9.8) = 882 newtons (about 198.4 pounds),
and that force tries to hold the block up, against gravity.

So while it's in the water, the block seems to weigh

       (3,528  -  882) = 2,646 newtons  (about 595.2 pounds) .

But again ... it's not correct to call that the "force of gravity acting
on the block in water".  The force of gravity doesn't change, but
there's another force, working against gravity, in the water.

Answer 2

Density = (mass) / (volume)

                               4,000 kg/m³ = (mass) / (0.09 m³)

Multiply each side

by  0.09 m³ :           (4,000 kg/m³) x (0.09 m³) = mass

                                mass = 360 kg .

Force of gravity = (mass) x (acceleration of gravity)

                          = (360 kg) x (9.8 m/s²)

                          = (360 x 9.8)  kg-m/s²

                          =   3,528 newtons .  

That's the force of gravity on this block, and it doesn't matter

what else is around it.  It could be in a box on the shelf or at

the bottom of a swimming pool . . . it's weight is 3,528 newtons

(about 793.7 pounds).

Now, it won't seem that heavy when it's in the water, because

there's another force acting on it in the upward direction, against

gravity.  That's the buoyant force due to the displaced water.

The block is displacing 0.09 m³ of water.  Water has 1,000 kg of

mass in a m³, so the block displaces 90 kg of water.  The weight

of that water is  (90) x (9.8) = 882 newtons (about 198.4 pounds),

and that force tries to hold the block up, against gravity.

So while it's in the water, the block seems to weigh

      (3,528  -  882) = 2,646 newtons  (about 595.2 pounds) .

But again ... it's not correct to call that the "force of gravity acting

on the block in water".  The force of gravity doesn't change, but

there's another force, working against gravity, in the water.