0A free body diagram is a diagram showing all the forces and moments of forces acting on an object. We distinguish between two types of objects:
1. Particles that have no spatial extent and thus have no moment arm (d). An example of this would be a satellite orbiting the earth because the spatial extent of the satellite is very small compared to the distance from the earth to the satellite or the radius of the earth. Particles do not have moments of forces and thus do not rotate in response to a force.
2. Rigid bodies that have a finite dimension and thus has a moment arm (d) associated with each applied force. Rigid bodies have moments of forces and thus can rotate in response to a force.
