The product of mass and velocity.
Explanation
Momentum is a fundamental physical quantity that describes the "quantity of motion" possessed by a moving body. It is a vector quantity, meaning it has both magnitude and direction.
Analysis of Options:
- The product of force and time. is incorrect: The product of force and time is known as Impulse. Impulse represents the change in momentum of an object when a force acts upon it for a specific duration.
- The product of mass and acceleration. is incorrect: The product of mass and acceleration is defined as Force, according to Newton's Second Law of Motion ($F = ma$).
- The product of mass and velocity. is correct: Momentum ($p$) is mathematically defined as the product of an object's mass ($m$) and its velocity ($v$). The formula is given by $p = mv$. Since velocity is a vector, momentum is also a vector quantity directed along the velocity.
- The rate of change of velocity. is incorrect: The rate of change of velocity is defined as Acceleration.
Key Takeaway:
Linear momentum is the product of mass and velocity ($p = mv$). It is a vector quantity and is conserved in an isolated system where no external forces act.