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Engineering Statics: Open and Interactive

Section 1.3 Forces

Statics is a course about forces and we will have a lot to say about them. At its simplest, a force is a “push or pull,” but forces come from a variety of sources and occur in many different situations. As such we need a specialized vocabulary to talk about them. We are also interested in forces that cause rotation, and we have special terms to describe these too.
Some terms used to describe forces are given below; others will be defined as needed later in the book.
Point Forces, also called concentrated forces, are forces that act at a single point. Examples are the push you give to open a door, the thrust of a rocket engine, or the pull a the chain suspending a wrecking ball. Point forces are actually an idealization, because real forces always act over an area and not at a mathematical point. However, point forces are the easiest type to deal with computationally so we will usually represent other types of forces as equivalent concentrated forces.
Distributed forces are forces that are spread out over a line, area or volume. Steam pressure in a boiler and the weight of snow on a roof are examples of forces distributed over an area. Distributed forces are represented graphically by an array of force vectors.
Body forces are distributed forces acting over the volume of a body. The most common body force is the body’s weight, but there are others including buoyancy and forces caused by electric and magnetic fields. Weight and buoyancy will be the only body forces we consider in this book.
In many situations, body forces are small in comparison to the other forces acting on the object, and as such may be neglected. In practice, the decision to neglect forces must be made on the basis of sound engineering judgment; however, in this course you should consider the weight in your analysis if the problem statement provides enough information to determine it, otherwise you may ignore it.
Loads are the forces which an object must support in order to perform its function. Loads can be either static or dynamic, however only static loads will be considered here. Forces which hold a loaded object in equilibrium or hold parts of an object together are not considered loads.
Reaction forces or simply reactions are the forces and moments which hold or constrain an object or mechanical system in equilibrium. They are called the reactions because they react when other forces on the system change. If the load on a system increases, the reaction forces will automatically increase in response to maintain equilibrium. Reaction forces are introduced in Chapter 3 and reaction moments are introduced in Chapter 5.
Internal forces are forces which hold the parts an object or system together. Internal forces will be discussed in Chapter 8.
As an example of the various types of forces, consider a heavy crate being pulled by a rope across a rough surface.
A crate is shown with a point force pulling it toward the reader. It is on a rough surface.
(a) Pull - Concentrated force
The crate is shown with its weight represented as an array of downward force vectors distributed throughout.
(b) Weight - Body force
The crate is shown with an array of horizontal force vectors acting on its lower surface to oppose the pull.
(c) Friction - Distributed force
The crate is shown with an array of upward pointing force vectors acting on its lower surface to oppose the weight.
(d) Normal Force - Distributed force
Figure 1.3.1. Forces on a crate being pulled across a rough surface.
The pull of the rope and the weight of the crate are loads. The rope applies a force at a single point, so is a concentrated force. The force of the ground holding the crate in equilibrium is a reaction force. This force can be divided into two components: a tangential friction component which acts parallel to the ground and resists the pull of the cable, and a normal component which acts perpendicular to the bottom surface and supports the crate’s weight. The normal and tangential components are distributed forces since they act over the bottom surface area. The weight is also a distributed force, but one that acts over the entire crate so it’s considered a body force. For computational simplicity we usually model all these distributed forces as equivalent concentrated forces. This process is discussed in Chapter 7.