Acceleration: Change in velocity.

When an object’s velocity changes, it accelerates. Acceleration measures the rate at which an object speeds up, slows down or changes direction. Any of these variations constitutes a change in velocity. The letter a represents acceleration.

Acceleration is a popular topic in sports car commercials. In the commercials, acceleration is often expressed as how fast a car can go from zero to 60 miles per hour (97 km/h, or 27 m/s). For example, a current model Corvette^® automobile can reach 60 mi/h in 4.9 seconds. There are even hotter cars than this in production.

To calculate average acceleration, divide the change in instantaneous velocity by the elapsed time, as shown in Equation 1. To calculate the acceleration of the Corvette, divide its change in velocity, from 0 to 27 m/s, by the elapsed time of 4.9 seconds. The car accelerates at an average rate of 5.5 m/s per second. We typically express this as 5.5 meters per second squared, or 5.5 m/s². (This equals 18 ft/s², and with this observation we will cease stating values in both measurement systems, in order to simplify the expression of numbers.) Acceleration is measured in units of length divided by time squared. Meters per second squared (m/s²) express acceleration in SI units.

Let’s assume the car accelerates at a constant rate; this means that each second the Corvette moves 5.5 m/s faster. At one second, it is moving at 5.5 m/s; at two seconds, 11 m/s; at three seconds, 16.5 m/s; and so forth. The car’s velocity increases by 5.5 m/s every second.

Since acceleration measures the change in velocity, an object can accelerate even while it is moving at a constant speed. For instance, consider a car moving around a curve. Even if the car’s speed remains constant, it accelerates because the change in the car’s direction means its velocity (speed plus direction) is changing.

Acceleration can be positive or negative. If the Corvette uses its brakes to go from +60 to 0 mi/h in 4.9 seconds, its velocity is decreasing just as fast as it was increasing before. This is an example of negative acceleration.

You may want to think of negative acceleration as “slowing down,” but be careful! Let’s say a train has an initial velocity of negative 25 m/s and that changes to negative 50 m/s. The train is moving at a faster rate (speeding up) but it has negative acceleration. To be precise, its negative acceleration causes an increasingly negative velocity.

Velocity and acceleration are related but distinct values for an object. For example, an object can have positive velocity and negative acceleration. In this case, it is slowing down. An object can have zero velocity, yet be accelerating. For example, when a ball bounces off the ground, it experiences a moment of zero velocity as its velocity changes from negative to positive, yet it is accelerating at this moment since its velocity is changing.