What are the equilibrium requirements for particles and rigid bodies?
1 Answer


  • A mechanical system can include either a single object (for instance, an engine‚Äôs 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.

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  • 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.

Rigid bodies:

  • 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.

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  • 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.

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