Here it is Linear momentum , which is explained below.

Principle of Conservation of momentum states that in a collision of bodies, the total momentum before and after the collision remains same. This means

Total momentum before impact= total momentum after impact.

Momentum is mass times velocity. Hence if a body of mass m₁ is moving in a certain direction, with velocity u₁, its momentum is m₁u₁. It is a vector quantity, its direction is same as the direction in which body is moving.

Suppose another body of mass m₂ moving with a velocity u2 in a direction exactly opposite to that of body m₁ collides with it, the principal of conservation of momentum would apply as follows

Momentum of the two bodies before collision is m₁u₁-m₂u₂ ( negative sign appears because of the opposite direction of u₂). If after collision, both the bodies combine and move in the same direction in which body m₁ was moving, with same velocity v, then

Momentum after collision will be (m₁ +m₂)v and

m₁u₁ –m₂u₂ = (m₁+m₂)v

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