(a) Explain what a principal axis system for a rigid body is. (b) Write down the definition of the moment of inertia of a rigid body with respect to a fixed axis ñ. (c) If the principal moments of inertia of a rigid body are all equal, I₁ = I₂ = I3, does that imply that the mass-distribution p(x) in the rigid body is spherically symmetric? Give a short explanation. (d) State what a normal mode is.
Rigid Body
A rigid body is an object which does not change its shape or undergo any significant deformation due to an external force or movement. Mathematically speaking, the distance between any two points inside the body doesn't change in any situation.
Rigid Body Dynamics
Rigid bodies are defined as inelastic shapes with negligible deformation, giving them an unchanging center of mass. It is also generally assumed that the mass of a rigid body is uniformly distributed. This property of rigid bodies comes in handy when we deal with concepts like momentum, angular momentum, force and torque. The study of these properties – viz., force, torque, momentum, and angular momentum – of a rigid body, is collectively known as rigid body dynamics (RBD).
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