The engineering mechanics may be divided into Statics and Dynamics. The branch of science, which deals with the study of a body when the body is at rest, is known as Statics while the branch of science which deals with the study of a body when the body is in motion, is known as Dynamics. Dynamics is further divided into kinematics and kinetics. The study of a body in motion, when the forces which cause the motion are not considered, is called kinematics and if the forces are also considered for the body in motion, that branch of science is called kinetics.

The forces, which are having their line of actions parallel to each other, are known parallel forces. The two parallel forces will not intersect at a point. The resultant of two concurrent forces (i.e., forces intersecting at the same point) can be directly determined by the method of parallelogram of forces. This method along with other methods for finding resultant of collinear and concurrent forces, were discussed in earlier chapters.

When a solid body slides over a stationary solid body, a force is exerted at the surface of contact by the stationary body on the moving body. This force is called the force of friction and is always acting in the direction opposite to the direction of motion. The property of the bodies by virtue of which a force is exerted by a stationary body on the moving body to resist the motion of the moving body is called friction. Friction acts parallel to the surface of contact and depends upon the nature of surface of contact.

A structure made up of several bars (or members) riveted or welded together is known as frame. The isometric view of a structure made of trusses is shown in Fig. 4.1 (a) and front view is shown in Fig. 4.1 (b). If the frame is composed of such members which are just sufficient to keep the frame in equilibrium, when the frame is supporting an external load

Centre of gravity of a body is the point through which the whole weight of the body acts. A body is having only one centre of gravity for all positions of the body. It is represented by C.G. or simply G.

Kinematics is that branch of Engineering Mechanics which deals with motion of particles and bodies without consideration of forces required to produce them. Hence the term kinematic stands for the displacement, velocity and acceleration of a particle or of a body. Particle is a body whose size can be neglected whereas the rigid body is a body in which the distance between any two points remains fixed for all the time.

Kinetics of that branch of Engineering Mechanics which deals with the force system which produces acceleration and resulting motion of bodies. In the previous chapter i.e., chapter of Kinematics of Rigid Bodies, we have dealt with the motion of the bodies (i.e., displacement, velocity and acceleration of Rigid bodies) without consideration of the forces which produce these motion.

The virtual work is the work done by a force on a body due to small virtual (i.e., imaginary) displacement of the body. A virtual displacement is any arbitrary very small change in the position of a body. The displacement is only imagined, as the body is not actually displaced, for which the displacement is called virtual displacement. The product of the force and the virtual displacement in the direction of the force is called virtual work.