such terms as *volume*, *density*, *weight*, *center of*

A force is a push or pull. It tends to produce motion

or a change in motion. Force is what makes something

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

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.

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:

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

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

the object will move the body in the direction of the

force.

To prevent motion or to keep the body at rest, you

must apply an equal force in the same line of action but

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