Linear momentum is a fundamental concept in physics that describes the motion of an object with mass in a straight line. It is defined as the product of an object's mass (m) and its velocity (v). Mathematically, linear momentum (p) can be expressed as p = m * v.

Linear momentum is a vector quantity, which means that it has both magnitude and direction. The magnitude of momentum is directly proportional to the speed of the object, while the direction of momentum is the same as the direction of the object's velocity.

The SI unit for momentum is kilogram-meter per second (kg·m/s). This unit shows that momentum is equal to the product of mass (in kilograms) and velocity (in meters per second).

The principle of conservation of linear momentum states that the total momentum of a system of objects remains constant if no external forces act on it. This means that in the absence of external forces, the initial momentum of an object or a collection of objects will be equal to the final momentum after any given time.