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In introductory mechanics, the momentum of a particle is its mass times its velocity. In electrodynamics, the momentum of a field is proportional to the cross-product of the electric field with the magnetic field. In special relativity, momentum is generalized to four-momentum.

Momentum is important in Physics because it describes the relationship between speed, mass and direction. It also describes the force needed to stop objects and to keep them in motion.
In Newtonian mechanics, linear momentum, translational momentum, or simply momentum is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction.
If $m$ is an object's mass and $\mathbf v$ is its velocity (also a vector quantity), then the object's momentum is: $${\displaystyle \mathbf {p} =m\mathbf {v} ~.}$$ In SI units, momentum is measured in kilogram meters per second (kg⋅m/s).

Newton’s second law of motion states that the time rate of change of momentum is equal to the force acting on the particle i.e., if a constant force acts on a particle for a given time, the product of force and the time interval (the impulse) is equal to the change in the momentum. Conversely, the momentum of a particle is a measure of the time required for a constant force to bring it to rest.

From the definition of momentum, it becomes obvious that an object has a large momentum if both its mass and its velocity are large. Both variables are of equal importance in determining the momentum of an object.