Momentum
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Momentum is a conserved quantity in physics which is the product of the mass m and velocity v of an object. It is a vector which is usually represented by the symbol p. In Newtonian physics, it is given by the equation p = mv. This equation also applies in special relativity, but with m being the relativistic mass of the object. Momentum is the product of the mass and the velocity of an object. This means that both mass and velocity affect the momentum of an object. For instance, an object with a low mass and a high velocity can have the same momentum as an object with high mass and low velocity.
Momentum is a vector. This means that both the size (numerical value) of the momentum and the direction of the momentum are important.
Momentum is a conserved quantity. This means that the total amount of momentum in a closed system never changes. The momentum is "conserved" or transferred from one object to another. This happens in such a way that the total momentum of the system remains constant.
In understanding conservation of momentum, the direction of the momentum is important. Momentum in a system is added up using vector addition. Under the rules of vector addition, adding a certain amount of momentum together with the same amount of momentum going in the opposite direction gives a total momentum of zero.
For instance, when a gun is fired, a small mass (the bullet) moves at a high speed in one direction. A larger mass (the gun) moves in the opposite direction at a much slower speed. The momentum of the bullet and the momentum of the gun are exactly equal in size but opposite in direction. Using vector addition to add the momentum of the bullet to the momentum of the gun (equal in size but opposite in direction) gives a total system momentum of zero. The momentum of the gun-bullet system has been conserved.