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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.
As an example of the types of forces you will encounter in statics consider the forces affecting a box on a rough surface being pulled by a cable. The loading on the box can be represented by four different types of force. The cable causes a point force, the normal and friction forces are reaction forces, and the weight is a body force.
A box is shown with a force pulling it toward the reader. It is on a rough surface. The force pulling the box is a point force (e.g., a cable). There is also a normal reaction of the surface on the bottom surface of the box (F_N is perpendicular to the surface). As the box is pulled towards the reader, a frictional force is developed that is parallel to the surface and opposite the direction of motion. The weight of the box is represented as a force going down. In reality, the weight of the box is distributed across the entire surface of contact, as is the normal force.
Figure 1.3.1. Forces on a box being pulled across a rough surface.
Some of the important terms used describe different types of forces are given below; others will be defined as needed later in the book.
A point force is a force that acts at a single point. Examples would be the push you give to open a door, the thrust of a rocket engine, or the pull of the chain suspending a wrecking ball. In reality, point forces are an idealization as all forces are distributed over some amount of area. Point forces are also called concentrated forces. Point forces are the easiest type to deal with computationally so we will learn some mathematical tools to represent other types as point forces.
Body forces are forces that are distributed throughout a three dimensional body. The most common body force is the weight of an object, but there are other body forces including buoyancy and forces caused by gravitational, electric, and magnetic fields. Weight and buoyancy will be the only body forces we consider in this book.
In many situations, these 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.
In the example above, the point force due to the cable, and the weight of the box are both called loads. The weight of an object and any forces intentionally applied to it are considered loads, while forces which hold a loaded object in equilibrium or hold parts of an object together are not.
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.
In the example above, the force of the ground on the box is a reaction force, and is distributed over the entire contact surface. The reaction force can be divided into two parts: a normal component which acts perpendicular to the surface and supports the box's weight, and a tangential friction component which acts parallel to the ground and resists the pull of the cable.
The weight, normal component, and frictional component are all examples of distributed forces since they act over a volume or area and not at a single point. For computational simplicity we usually model distributed forces with equivalent point forces. This process is discussed in Chapter 7.