Final answer:
Acceleration occurs when the velocity of a body changes, which includes changes in magnitude (speed) or direction. It is a vector quantity with both magnitude and direction, measured in m/s².
Step-by-step explanation:
Acceleration occurs when the velocity of a body changes. This can be due to a change in the magnitude of the velocity, which refers to speeding up or slowing down, or a change in its direction, such as when turning a corner in a car. In physical terms, acceleration is the rate at which velocity changes over time. The SI unit for acceleration is meters per second squared (m/s²). When discussing motion, it's essential to remember Newton's First Law, which tells us that a nonzero net external force is needed to cause a change in motion, thus leading to acceleration.
Moreover, acceleration does not always imply speeding up; it can also mean slowing down, which is sometimes referred to as deceleration. The critical point is that acceleration is a vector quantity, possessing both magnitude and direction. Whether you are dealing with uniform circular motion, where the direction of velocity changes while its magnitude remains constant, or linear motion where velocity changes in magnitude, there is always an acceleration involved if the velocity changes.