Negative acceleration indicates that the velocity of an object is changing in a negative direction, which can mean that it is traveling either slower or faster. The term “acceleration” is used in physics to describe a change in velocity for an object, and it is a vector quantity, which means that it indicates both magnitude and direction. Something that is traveling with positive velocity, velocity is also a vector, and experiencing negative acceleration is slowing down. In contrast to this, an object that has negative acceleration as well as negative velocity is actually moving faster.
It is often easiest for someone to understand acceleration in general, and negative acceleration specifically, by first understanding velocity. Although the terms “speed,” “velocity,” and “acceleration” are sometimes used synonymously in common conversation, these three words have very different meanings in reference to physics. “Speed” is the measurement of how far an object travels over a certain period of time, and is often expressed in miles per hour (mph) or meters per second (m/s).
“Velocity” is similar to speed, but indicates not only the actual speed itself but also the direction an object is moving in, which makes it a vector quantity. This means that an object traveling with a velocity of 20 m/s and another traveling at -20 m/s are both moving at the same speed but in opposite directions. “Acceleration” is a measurement regarding the change in an object’s velocity, and is often expressed in terms of meters per second, per second (m/s/s or m/s^{2}). An object moving at rest one second, then moving at 10 m/s the next second, 20 m/s the next, and 30 m/s the following second has an acceleration of 10 m/s^{2} as this is the change in velocity.
One of the simplest ways for a person to see negative acceleration is by throwing something into the air. When the thrown object leaves his or her hand, it is moving at a certain velocity, for example 40 m/s. The force of gravity pulls downward on the object as it travels upwards in a positive direction, giving it an acceleration of about -10 m/s^{2}. After one second the object is still moving at 30 m/s, another second later and it is moving at 20 m/s, and after two more seconds it comes to a brief stop before continuing the acceleration downward.
Negative acceleration simply indicates the direction of the acceleration, and can mean that an object is either increasing or decreasing in velocity. An object moving with a positive velocity, which has a negative acceleration, is slowing down. On the other hand, an object that is moving with a negative acceleration, and with a negative velocity, is actually moving faster. This same basic principle is also true for positive acceleration; when the direction of velocity and acceleration are the same the object is moving faster, and when they are the opposite the object is slowing down.