This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
1. Force. The student, no doubt, has a reasonably clear idea as to what is meant by force, yet it may be well to repeat here a few definitions relative to it. By force is meant simply a push or pull. Every force has magnitude, and to express the magnitude of a given force we state how many times greater it is than some standard force. Convenient standards are those of weight and these are almost always used in this connection. Thus when we speak of a force of 100 pounds we mean a force equal to the weight of 100 pounds.
We say that a force has direction, and we mean by this the direction in which the force would move the body upon which it acts if it acted alone. Thus, Fig. 1 represents a body being pulled to the right by means of a cord; the direction of the force exerted upon the body is horizontal and to the right. The direction may be indicated by any line drawn in the figure parallel to the cord with an arrow on it pointing to the right.
We say also that a force has a place of application, and we mean by that the part or place on the body to which the force is applied. When the place of application is small so that it may he regarded as a point, it is called the "point of application." Thus the place of application of the pressure (push or force) which a locomotive wheel exerts on the rail is the part of the surface of the rail in contact with the wheel. For practically all purposes this pressure may be considered as applied at a point (the center of the surface of contact), and it is called the point of application of the force exerted by the wheel on the rail.
A force which has a point of application is said to have a line of action, and by this term is meant the line through the point of application of the force parallel to its direction. Thus, in the Fig. 1, the line of action of the force exerted on the body is the line representing the string. Notice clearly the distinction between the direction and line of action of the force; the direction of the force in the illustration could be represented by any horizontal line in the figure with an arrowhead upon it pointing toward the right, but the line of action can be represented only by the line representing the string, indefinite as to length, but definite in position.
That part of the direction of a force which is indicated by means of the arrowhead on a line is called the sense of the force. Thus the sense of the force of the preceding illustration is toward the right and not toward the left.
2. Specification and Graphic Representation of a Force. For the purposes of statics, a force is completely specified or described if its
(1) magnitude, (2) line of action, and (3) sense are known or given.
These three elements of a force can be represented graphically, that is by a drawing. Thus, as already explained, the straight line (Fig. 1) represents the line of action of the force exerted upon the body; an arrowhead placed on the line pointing toward the right gives the sense of the force; and a definite length marked off on the line represents to some scale the magnitude of the force. For example, if the magnitude is 50 pounds, then to a scale of 100 pounds to the inch, one-half of an inch represents the magnitude of the force.
HIGH SCHOOL AT THREE RIVERS, MICH.
J. C. Llewellyn, Architect, Chicago, 111.
HIGH SCHOOL AT THREE RIVERS, MICH.
J. C. Llewellyn, Architect, Chicago, 111.
Story, See Page 154.
It is often convenient, especially when many forces are concerned in a single problem, to use two lines instead of one to represent a force - one to represent the magnitude and one the line of action, the arrowhead being placed on either. Thus Fig. 2 also represents the force of the preceding example, AB (one-half inch long) representing the magnitude of the force and ab its line of action. The line AB might have been drawn anywhere in the figure, but its length is definite, being fixed by the scale.
The part of a drawing in which the body upon which forces act is represented, and in which the lines of action of the forces are drawn, is called the space diagram (Fig. 2a). If the body were drawn to scale, the scale would be a certain number of inches or feet to the inch. The part of a drawing in which the force magnitudes are laid off (Fig. 2b) is called by various names; let us call it the force diagram. The scale of a force diagram is always a certain number of pounds or tons to the inch.
3. Notation. When forces are represented in two separate diagrams, it is convenient to use a special notation, namely: a capital letter at each end of the line representing the magnitude of the force, and the same small letters on opposite sides of the line representing the action line of the force (see Fig. 2). When we wish to refer to a force, we shall state the capital letters used in the notation of that force; thus "force AB" means the force whose magnitude, action line, and sense are represented by the lines AB and ab.
In the algebraic work we shall usually denote a force by the letter F.
4. Scales. In this subject, scales will always be expressed in feet or pounds to an inch, or thus, 1 inch = 10 feet, 1 inch = 100 pounds, etc. The number of feet or pounds represented by one inch on the drawing is called the scale number.
To find the length of the line to represent a certain distance or force, divide the distance or force by the scale number; the quotient is the length to be laid off in the drawing. To find the magnitude of a distance or a force represented by a certain line in a drawing, multiply the length of the line by the scale number; the product is the magnitude of the distance or force, as the case may be.
The scale to be used in making drawings depends, of course, upon how large the drawing is to be, and upon the size of the quantities which must be represented. In any case, it is convenient to select the scale number so that the quotients obtained by dividing the quantities to be represented may be easily laid off by means of the divided scale which is at hand.
Examples. 1. If one has a scale divided into 32nds, what is the convenient scale for representing 40 pounds, 32 pounds, 56 pounds, and 70 pounds?
According to the scale, 1 inch = 32 pounds, the lengths representing the forces are respectively:
40/32 = 1¼; 32/32 = 1; 56/32 = 1¾; 70/32 = 2 3/16 lnches.
Since all of these distances can be easily laid off by means of the "sixteenths scale," 1 inch = 32 pounds is convenient.
2. What are the forces represented by three lines, 1.20, 2.11, and 0.75 inches long, the scale being 1 inch = 200 pounds?
According to the rule given in the foregoing, we multiply each of the lengths by 200, thus :
1. To a scale of 1 inch = 500 pounds, how long are the lines to represent forces of 1,250, 675, and 900 pounds?
Ans. 2.5, 1.35, and 1.8 inches
2. To a scale of 1 inch = 80 pounds, how large are the forces represented by 1¼ and 1.6 inches?
Ans. 100 and 128 pounds.
5. Concurrent and Non=concurrent Forces. If the lines of action of several forces intersect in a point they are called concurrent forces, or a concurrent system, and the point of intersection is called the point of concurrence of the forces. If the lines of action of several forces do not intersect in the same point, they are called non-concurrent, or a non-concurrent system.
We shall deal only with forces whose lines of action lie in the same plane. It is true that one meets with problems in which there are forces whose lines of action do not lie in a plane, but such problems can usually be solved by means of the principles herein explained.
6. Equilibrium and Equilibrant. When a number of forces act upon a body which is at rest, each tends to move it ; but the effects of all the forces acting upon that body may counteract or neutralize one another, and the forces are said to be balanced or in equilibrium. Any one of the forces of a system in equilibrium balances all the others. A single force which balances a number of forces is called the equilibrant of those forces.
7. Resultant and Composition. Any force which would produce the same effect (so far as balancing other forces is concerned) as that of any system, is called the resultant of that system. Evidently the resultant and the equilibrant of a system of forces must be equal in magnitude, opposite in sense, and act along the same line.
The process of determining the resultant of a system of forces is called composition.
8. Components and Resolution. Any number of forces whose combined effect is the same as that of a single force are called components of that force. The process of determining the components of a force is called resolution. The most important case of this is the resolution of a force into two components.