Final answer:
An object slows down in physics when its velocity decreases due to an acceleration in the opposite direction of its motion.
Step-by-step explanation:
When an object slows down in physics, it means that its velocity is decreasing over time. This decrease in velocity is caused by an acceleration that is opposite to the direction of motion. In physics, we refer to this opposite acceleration as deceleration, although it is still technically considered acceleration. For example, if an object is moving to the right along the x-axis and its velocity is positive, a negative acceleration would cause it to slow down.