- A mechanical system can include either a single object (for instance, an engine’s piston) or multiple objects that are connected (the entire engine).
When the physical dimensions are unimportant with respect to calculating forces, the object is called a particle.
This concept idealizes the system as being concentrated at a single point rather than being distributed over an extended area or volume.
Particle analysis will only have to consider the forces acting on the body and translational motion because rotation is not considered for particles.
For the purposes of solving problems, a particle can be treated as having negligible dimensions.
On the other hand, if the length, width, and breadth of an object are important for the problem at hand, it is called a rigid body.
As an example, when looking at the motion of a communications satellite as it orbits the Earth, the spacecraft can be regarded as a particle because its dimensions are small compared to the size of the orbit.
However, when the satellite is being launched and engineers are interested in the aerodynamics and flight characteristics of the rocket, the launch vehicle would be modelled as a rigid body.
Figure illustrates the conceptual distinction between forces applied to a particle and to a rigid body. You can see how a force imbalance could cause the rigid body, but not the particle, to rotate.