Acceleration

"In fact, any change in velocity—whether positive, negative, directional, or any combination of these—is called an **acceleration** in physics." "Acceleration is the change in velocity divided by a period of time during which the change occurs. The SI units of velocity are m/s and the SI units for time are s, so the SI units for acceleration are $\text{m/s}^2$." "Average acceleration is distinguished from **instantaneous acceleration**, which is acceleration at a specific instant in time." "Keep in mind that although acceleration points in the same direction as the *change* in velocity, it is not always in the direction of the velocity itself. When an object slows down, its acceleration is opposite to the direction of its velocity. In everyday language, this is called deceleration; but in physics, it is acceleration—whose direction happens to be opposite that of the velocity." "It is easier to get plus and minus signs correct if you always assume that motion is away from zero and toward positive values on the *x*-axis. ... If speed is increasing, then acceleration is positive and also points to the right. If speed is decreasing, then acceleration is negative and points to the left."