EXTERNAL AND INTERNAL FORCES

Forces acting on rigid bodies can be separated into two groups: (1) external forces and (2) internal forces.

1. The  external forcesrepresent the action of other bodies on the rigid body under consideration. They are entirely responsible for the external behavior of the rigid body. They will either cause it to move or ensure that it remains at rest. We shall be concerned only with external forces.

 2. The  internal forcesare the forces which hold together the particles forming the rigid body. If the rigid body is structurally composed of several parts, the forces holding the component parts together are also defined as internal forces

  

As an example of external forces, let us consider the forces acting on a disabled truck that three people are pulling forward by means of a rope attached to the front bumper ( Fig. 3.1 ). The external forces acting on the truck are shown in a free-body diagram ( Fig.  3.2 ). Let us first consider the weightof the truck. Although it embodies the effect of the earth’s pull on each of the particles forming the truck, the weight can be represented by the single force W.  The point of applicationof this force, i.e., the point at which the force acts, is defined as the center of gravityof the truck. It will be seen in Chap. 5 how centers of gravity can be determined. The weight Wt end s to make the truck move vertically downward. In fact, it would actually cause the truck to move downward, i.e., to fall, if it were not for the presence of the ground. The ground opposes the downward motion of the truck by means of the reactions R1 and   R2.  These forces are exerted bythe ground onthe truck and must therefore be included among the external forces acting on the truck. 

The people pulling on the rope exert the force F. The point of application of Fis on the front bumper. The force Ftends to make the truck move forward in a straight line and does actually make it move, since no external force opposes this motion. (Rolling resistance has been neglected here for simplicity.) This forward motion of the truck, during which each straight line keeps its original orientation (the floor of the truck remains horizontal, and the walls remain vertical), is known as a translation. Other forces might cause the truck to move differently. For example, the force exerted by a jack placed under the front axle would cause the truck to pivot about its rear axle. Such a motion is a rotation. It can be concluded, therefore, that each of the external forces acting on a rigid body can, if unopposed, impart to the rigid body a motion of translation or rotation, or both.