In physics, momentum is a physical quantity related to the velocity and mass of an object.

Momentum in classical mechanics
In classical mechanics, momentum (traditionally written as p) is defined as the product of mass and velocity. It is thus a vector quantity.
The SI unit of momentum is newtonseconds, which can alternatively be expressed with the units kg.m/s.
An impulse changes the momentum of an object. An impulse is calculated as the integral of force with respect to duration.
Momentum in relativistic mechanics
It is commonly believed that the physical laws should be invariant under translations. Thus, the definition of momentum was changed when Einstein formulated Special relativity so that its magnitude would remain invariant under relativistic transformations. See physical conservation law. We now define a vector, called the 4momentum thus:
where E is the total energy of the system, and p is called the "relativistic momentum" defined thus:
The "length" of the vector that remains constant is defined thus:
Massless objects such as photons also carry momentum; the formula is p=E/c, where E is the energy the photon carries and c is the speed of light.
In quantum mechanics momentum is defined as an operator on the wave function. The Heisenberg uncertainty principle defines limits on how accurately the momentum and position of a single observable system can be known at once.
A process may be said to gain momentum. The terminology implies that it requires effort to start such a process, but that it is relatively easy to keep it going.
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