Changes

Classically, the momentum of an object is given by <math> \vec p = m \vec v</math> and [[acceleration]] relates to force via [[Classical Physics|Newton's Second Law]] as <math> \vec F = m \vec a </math> when mass can be assumed to be constant. More generally it is prescribed as <math> \vec F = \frac{d \vec p}{dt} </math>.<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref> In these expressions, ''F'' stands for the total vector sum of all forces, ''m'' for the mass of the object, ''a'' for its [[acceleration]] expressed as a vector, ''p'' stands for momentum vector  and ''v'' for velocity vector.  In [[Theory of Relativity|special relativity]], these presciptions must be modified so as to be [[Lorentz invariant]], which among other things, means that all [[inertial reference frame]]s stand on equal footing and have the same prescription for all physical and dynamical quantities, though observers in different [[inertial reference frame]]s  will measure different values for many of them, each observer being correct for his own frame(!).

Classically, the momentum of an object is given by <math> \vec p = m \vec v</math> and [[acceleration]] relates to force via [[Classical Physics|Newton's Second Law]] as <math> \vec F = m \vec a </math> when mass can be assumed to be constant. More generally it is prescribed as <math> \vec F = \frac{d \vec p}{dt} </math>.<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref> In these expressions, ''F'' stands for the total vector sum of all forces, ''m'' for the mass of the object, ''a'' for its [[acceleration]] expressed as a vector, ''p'' stands for momentum vector  and ''v'' for velocity vector.  In [[Theory of Relativity|special relativity]], these presciptions must be modified so as to be [[Lorentz invariant]], which among other things, means that all [[inertial reference frame]]s stand on equal footing and have the same prescription for all physical and dynamical quantities, though observers in different [[inertial reference frame]]s  will measure different values for many of them, each observer being correct for his own frame(!).

There are four known fundamental types of forces occurring in nature<ref>Lewis H. Ryder, ''Quantum Field Theory'', 2nd ed., Cambridge University Press, Cambridge (UK), 1996</ref>:

There are four known fundamental types of forces occurring in nature:<ref>Lewis H. Ryder, ''Quantum Field Theory'', 2nd ed., Cambridge University Press, Cambridge (UK), 1996</ref>

;[[Electromagnetism|Electromagnetic]] force

;[[Electromagnetism|Electromagnetic]] force