Definition: Newton's laws of motion (Sir I. Newton):
- Newton's first law of motion: A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
- Newton's second law of motion: For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
- Newton's third law of motion: In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction