Final answer:
Acceleration is the change in velocity of an object and can be a change in speed or direction. It is a vector with magnitude and direction, and its SI unit is m/s². Forces cause acceleration when they produce a change in an object's motion.
Step-by-step explanation:
Definition of Acceleration and its Unit
Acceleration is defined as the rate at which the velocity of an object changes over time. This includes both changes in speed and direction. According to Newton's first law, only a non-zero net external force can cause a change in motion, which includes acceleration. It is important to note that acceleration is a vector, meaning it has both magnitude and direction. The SI unit for measuring acceleration is meters per second squared (m/s²). Acceleration can occur as a result of changes in speed (either speeding up or slowing down, the latter often referred to as 'deceleration') or a change in the direction of motion.