Page 330 - NS-2 Textbook

PHYSICAL SCIENCE 325

Buoyant Force = 4,900 N
A. B.

W= 9,800N W= 9,800N

Apparent Weight = 9,800 - 4,900
= 4,900 N

A stone with a volume of half a cubic meter that weighs 9,800 N in air (AJ has an apparent weight of 4,900 N in water (8) because of the up-
ward buoyant force on it equal to the weight of the water displaced.

I I marine dives. To level off at some desired depth, the sub-
Buoyant Force = 9,800 N
marine adjusts the amOlmt of water in the ballast tanks
Imtil the downward weight and upward buoyant force
are roughly in balance. The propulsion system of the sub-
marine plus its diving planes can now keep the subma-
rine at the desired depth, like an airplane flying at some
level in the atmosphere. If the weight and buoyant force
W=9,800N were exactly matched, the submarine would be able to
hover at the desired depth without any forward propul-
A hollow boat weighing 9,800 N will sink into the water until it dis-
places an equal weight of water, whereupon it floats at that level. sion, much like the balloon hovering in air.
To surface from a submerged depth, the submarine
forces water out of the ballast tanks with compressed air
to the point that it equals the downward weight, and the until the upward buoyant force is once again greater
balloon floats at this altihlde. than the downward weight. At this point the submarine
will surface, aided by its propulsion system. Thus, a sub-
THE SUBMARINE marine in V\Tater acts much like a balloon in air, even
though the densities of the media in which they operate
When cruising on the surface of a bod y of water, a sub- are much different.
marine acts just like the boat model described above. It
will sink only partially into the watel; to a depth at which SHIP STABILITY
its weight is balanced by the upward buoyant force of the
water it displaces. For most submarines this happens One of the considerations in ship design is that it should
when the hull is about hvo-thirds submerged. be stable in a wide variety of sea conditions and if dam-
Now suppose that the submarine wants to submerge aged. The stability of a ship is dependent on the location
completely. To do this, it needs more weight to compen- of its center of gravity and its center of buoyancy at var-
sate for the upward buoyant force exerted on it by the ious angles of inclination or roll. The cell tel' of gravity is
weight of the water displaced by its totally submerged defined as the center of mass of the ship, around which
hull. To provide the weight, submarines are fitted with fil- the ship seems to move. The center of gravity does not
lable water tanks inside their structure called ballast tanks. change position as the ship moves. The cfllter ofblloyancy
When water is pumped into these tanks, their weight plus is the geometric center of the portion of the ship's hull
the structural weight of the submarine now combine to that is tmderwater. It tends to move in an arc as the ship
weigh more than the upward buoyant force, so the sub- rolls. For good stability a ship should have its center of