### Video: What is Acceleration?

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#### Acceleration

Video Transcript

Accelerating objects are changing their velocity. The velocity of an object at any given instant in time is the speed of the object with a direction. So if an accelerating object is changing its velocity, then it must either be changing its speed and/or changing its direction. Put another way, we could say that accelerating objects are either speeding up, slowing down, or turning. Constant velocity objects, no matter how fast they are moving, are not accelerating. To be accelerating means to be changing how fast or in what direction one is moving.

Accelerating objects are described as having an acceleration. The quantity acceleration is described as the rate at which the velocity changes. Acceleration is a vectory quantity. That means that it is fully described by both a numerical value (sometimes called a magnitude) and a direction. To determine the numerical value of acceleration, one needs to know the velocity change and the time over which the velocity changes. This equation can be used to calculate the acceleration.

acceleration = ∆velocity / time

If we re-write the equation again using symbols, we would write it as a = ∆v/∆t. The numerator is the change in velocity and can be calculated by subtracting the initial velocity value from the final velocity value. The denominator is the time over which the velocity changes or simply the ∆ in time associated with the ∆ in velocity. As one would reason from the equation, the units on acceleration are velocity units divided by time units. That would be meter/second divided by second, or simply meter/second squared.

I mentioned that acceleration is a vector. And because it is a vector, it has a direction. Adjectives such as rightward, leftward, northward, westward, downward, and more are used to describe the direction of an object's acceleration. Suppose an object is moving in a a straight line. To determine the direction of the object's acceleration, you need to know two things: (1) the direction the object is moving, and (2) whether the object is speeding up or slowing down. The simplest rule that I can give you for using these two bits of information to determine the direction of acceleation goes like this:

If an object is slowing down, then the direction of its acceleration is in the opposite direction that the object is moving.

So if an object is moving northward and slowing down, we would describe its acceleration direction as being southward. But what if an object is moving north and speeding up? Well then the acceleration would be northward. Because for speeding up motions, the direction of acceleration is in the same direction that the object is moving.

If you understand what accelerating objects are doing, how to calculate the quantity acceleration, and how to determine the direction of acceleration, then you're on the fast track to understand the concept of acceleration. But to be certain, check out the activities described in the Description of this video to ensure that You got this!

I'm Mr. H. Thanks for listening!

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