Changes

In [[classical physics]], '''force''' is defined as the product of a body's [[mass]] and [[acceleration]]. In other words, how hard you push on something determines how rapidly you change its speed. Of course, the heavier it is, the more it resists this change. So, what you're really changing is the body's [[momentum]].

In [[classical physics]], '''force''' is defined as the product of a body's [[mass]] and [[acceleration]]. In other words, how hard you push on something determines how rapidly you change its speed. Of course, the heavier it is, the more it resists this change. So, what you're really changing is the body's [[momentum]].

Using advanced mathematics, force may be defined as the time rate of change of [[momentum]] of a body <math>\vec F = {d \vec p \over dt} </math>. The [[International System of Units|SI]] unit of force is the [[newton]] and the [[US customary system]] unit is the pound.

Using advanced mathematics, force may be defined as the time rate of change of [[momentum]] of a body <math>\vec F = {d \vec p \over dt} </math>. The [[International System of Units|SI]] unit of force is the [[newton (unit)|newton]] and the [[US customary system]] unit is the pound.

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(!).