A particle (point object) is a system whose internal structure is not explicitly relevant to the properties described by the observer. The description of a particle consists of its position as a function of time (its trajectory) and quantities which characterise how it acts upon or is acted upon by the environment: mass, charge,... Other properties of the particle may be related to its trajectory (velocity, acceleration, ...) and mass (momentum, energy, ...).

Newton's Laws imply that the position and velocity of a particle at one time (the initial conditions), and the force exerted upon it by the environment as a function of time, are together sufficient to allow the trajectory to be obtained.

Related concepts: Observer-System Relationship, Newton's LawsScale

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