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[A] The motion of a rigid body

When a rigid body is thrown into the air, we notice something simple in its apparently complex motion: one special point in the object moves as though
  • all the mass were concentrated at that point
  • the external force (gravity in this case) acts only at that point
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This special point, the center of mass, forms the starting point for our study of the motion of collections (or systems) of particles, of which a rigid body is a special case.

By analysing the dynamics of the center of mass we discover a new fundamental law of nature: the law of conservation of momentum. We use this new law to analyse collisions between different bodies, which can be elastic or inelastic.

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Demo

(Demonstration : As shown in the figure, a juggling club provides a good illustration of the movement of the center of mass).