An external force must be applied to an object in order to give it a velocity or to increase the velocity it already has. Whether the force begins or changes velocity, it acts over a certain distance. Force acting over a certain distance is called work. Work and all forms into which it can be changed are classified as energy. Obviously, then, energy is required to give an object velocity. The greater the energy used, the greater the velocity will be.

Disregarding friction, for an object to be brought to rest or for its motion to be slowed down, a force opposed to its motion must be applied to it. This force also acts over some distance. In this way energy is given up by the object and delivered in some form to whatever opposed its continuous motion. The moving object is therefore a means of receiving energy at one place and delivering it to another point. While it is in motion, it is said to contain this energy, as energy of motion or kinetic energy.

Since energy can never be destroyed, it follows that if friction is disregarded, the energy delivered to stop the object will exactly equal the energy that was required to increase its speed. At all times, the amount of kinetic energy possessed by an object depends on its weight and the velocity at which it is moving.

The mathematical relationship for kinetic energy is stated in the following rule: Kinetic energy, in foot-pounds, is equal to the force, in pounds, that created it, multi­ plied by the distance through which it was applied; or it is equal to the weight of the moving object, in pounds, multiplied by the square of its velocity in feet per second, and divided by 64.

The relationship between inertia forces, velocity, and kinetic energy can be illustrated by analyzing what happens when a gun fires a projectile against the armor of an enemy ship. The explosive force of the powder in the breech pushes the projectile out of the gun, giving it a high velocity. Because of its inertia, the projectile offers opposi­ tion to this sudden velocity and a reaction is set up that pushes the gun backwards. The force of the explosion acts on the projectile throughout its movement in the gun. This is force acting through a distance, producing work. This work appears as kinetic energy in the speeding projectile. The resistance of the air produces friction, which uses some of the energy and slows down the projectile. When the projectile hits the target, it tries to continue moving. The target, being relatively stationary, tends to remain stationary because of inertia. The result is that a tremendous force is set up that leads either to the penetration of the armor or to the shattering of the projectile. The projectile is simply a means to transfer energy from the gun to the enemy ship. This energy is transmitted in the form of energy in motion or kinetic energy.

A similar action takes place in a fluid power system in which the fluid takes the place of the projectile. For example, the pump in a hydraulic system imparts energy to the fluid, which overcomes the inertia of the fluid at rest and causes it to flow through the lines. The fluid flows against some type of actuator that is at rest. The fluid tends to continue flowing, overcomes the inertia of the actuator, and moves the actuator to do work. Friction uses up a portion of the energy as the fluid flows through the lines and components.


In dealing with fluids, forces are usually considered in relation to the areas over which they are applied. As previously discussed, a force acting over a unit area is a pressure, and pressure can be stated either in pounds per square inch or in terms of head, which is the vertical height of the column of fluid whose weight would produce that pressure.

All five of the factors that control the actions of fluids can be expressed either as force or in terms of equivalent pressures or head. In either situation, the different factors are referred to in the same terms.

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