such terms as volume, density, weight, center of
Force
gravity, force, and moments.
A force is a push or pull. It tends to produce motion
or a change in motion. Force is what makes something
Volume
start to move, speed up, slow down, or keep moving
against resistance (such as friction). A force may act on
The volume of any object is determined by the
an object without being in direct contact with it. The
number of cubic feet or cubic units contained in the
most common example of this is the pull of gravity.
object. The underwater volume of a ship is found by
Forces are usually expressed in terms of weight units,
determining the number of cubic feet in the part of the
such as pounds, tons. or ounces.
hull below the waterline.
Figure 12-4 shows the action of a force on a body.
An arrow pointing in the direction of the force is drawn
Density
to represent the force. The location and direction of the
force being applied is known as the line of action. If a
The density of any material, solid or liquid, is
number of forces act together on a body, they may be
obtained by weighing a unit volume of the material.
considered as a single combined force acting in the
For example, if you take 1 cubic foot of seawater and
same direction to produce the same overall effect. In
weigh it, the weight is 64 pounds or 1/35 of a ton
this manner you can understand that F4 in figure 12-4
is the resultant or the sum of the individual forces F1,
(1 long ton equals 2,240 pounds). Since seawater has a
F2, and F3.
density of 1/35 ton per cubic foot, 35 cubic feet of
seawater weighs 1 long ton.
Weight
F
If you know the volume of an object and the
density of the material, the weight of the object is
found by multiplying the volume by the density. The
formula for this is as follows:
W = V x D (weight = volume times density)
When an object floats in a liquid, the weight of the
F1
volume of liquid displaced by the object is equal to the
F4
F2
weight of the object. Thus, if you know the volume of
F3
the displaced liquid, the weight of the object is found
by multiplying the volume by the density of the liquid.
DCf1204
Figure 12-4. Lines indicating direction of force.
Example:
If a ship displaces 35,000 cubic feet of salt water,
Whether you consider the individual forces F1, F2,
the ship weighs 1,000 tons.
and F3, or just F4 alone, the action of these forces on
W = V x D (weight = volume times density)
the object will move the body in the direction of the
force.
W = 35,000 cubic feet x 1/35 ton per cubic foot
To prevent motion or to keep the body at rest, you
W = 1,000 tons
must apply an equal force in the same line of action but
Center of Gravity
in the opposite direction to F4. This new force and F4
will cancel each other and there will be no movement;
The center of gravity (G) is the point at which all
the resultant force is zero. An example of this is a Sailor
the weights of the unit or system are considered to be
attempting to push a truck that is too heavy for him to
concentrated and have the same effect as that of all the
move. Although the truck does not move, force is still
component parts.
being exerted.
12-3
