Definitions Of Force, Weight, And Mass

Force, Weight, And Mass

Fig. 1-1. Force in hydraulic system

A FORCE is a push or pull that is exerted on an object in order to change its position or direction of movement. This includes starting, stopping, change in speed and direction of movement. In a hydraulic system, force must be present at all times in order for the system to function. As shown in figure 1-1, a pump exerts a force on a stream of hydraulic fluid. This force must be sufficient both to overcome the resistance to the fluid flow by the tubing and to do the work of the system. The more work the system must do, the more force required.

An object, or substance, has WEIGHT as a result of the gravitational force, or pull, on the object. Weight is always a downward force. In a hydraulic system, the fluid in the reservoir, the lines, or in any of the components, has weight. This is true whether the fluid is standing still or moving.

All objects or substances also have MASS. The mass represents the amount of matter in an object an its inertia, or resistance to movement. The mass of the object determines its weight on the earth, or in any other gravitational field. The inertia determines how much force is required to start, stop, or cause a change in the movement of an object. The greater its mass, the more force required to overcome its inertia.

The density or mass of liquid is given as its SPECIFIC GRAVITY. The specific gravity (sg) of a liquid is also its weight compared to the weight of water in the same amount and at the same temperature. The sg of water is 1.0, while petroleum oil has a sg of 0.78 at 49^oC (120^oF), and chlorinated hydrocarbon fluids (hydraulic fluids) have a sg of 1.42 at 120^oC. Although th sg of a fluid is usually not functionally important, it can be used to help determine the type of hydraulic fluid that is present in a system.