First law: a rigid body continues in a state of rest or a uniform motion in a straight line unless it is acted upon by external forces
Second law: the rate of change in the linear momentum of a rigid body is proportional to and in the same direction as the force applied. If the mass remains constant, then F = ma [i.e., the force (F) is equal to the product of mass (m) and acceleration (a)].
Third law: if one rigid body exerts a force on another, then there is an equal and opposite force (i.e., ‘for every action there is an equal and opposite’, a popular way of describing this law, but one that is open to misinterpretation).
With tongue in cheek, the science cartoonist Sydney Harris added another law to Newton’s three:
Fourth law: physicists get together regularly in unknown European towns to make a group picture and disperse afterward.
See Biomechanics, Constraint, Dynamics, Force, Inertia, Kinetics, Laws of nature, Linear dynamical systems, dynamics, Model, Newton’s law of universal gravitation, Newtonian (or classical) mechanics, Tautology