Acceleration: Definition & Concept

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson, you will learn about the concept of acceleration, which is a measure of how fast your velocity or speed is changing. You will also see examples and graphs that will help you understand the concept of acceleration.

Definition

Consider the following situation. Starting from rest, you walk one step during the first second, two steps during the next second, three steps during the third second, and so on. So for each second the number of steps increases by one step. Two things are happening in this situation. First, the number of steps increases by one step per second every second (0, 1 step/second, 2 steps/second, 3 steps/second etc.). Second, let us also assume that each step equals 1 meter. Then you travel 1 meter in the first second, two meters in the next second, three meters in the third second, and so on.

The situation described above is what happens when an object is accelerating. Acceleration is defined as the rate of change of velocity or speed. So for an object to be accelerating, the object's speed has to change each second, and another consequence of acceleration is that the distance the object travels each second changes as well.

Examples of Accelerated Motion

Any time your speed changes, you are accelerating. For instance, when you drive to work in the morning, you constantly have to stop and start when you hit traffic lights or stop signs. Every time you have to start the car you accelerate, and every time you have to stop the car for a light or a stop sign you accelerate (more commonly known as deceleration, although in physics accelerating just means that your speed changes, but does not necessarily mean that you're going faster). The main feature of accelerated motion is that the speed is different every second and the distance traveled every second changes as well. The first table below illustrates motion that is NOT accelerated so you can compare it to the second table showing accelerated motion.

Time (in seconds) Speed ( in meters/second) Distance ( in meters)
1 2.0 2.0
2 2.0 4.0
3 2.0 6.0
4 2.0 8.0
5 2.0 10.0

Notice that the speed is the same every second and the distance traveled increases by the same amount each second; in this case, the distance increases by 2.0 meters every second. Contrast this table with the table below.

Time (in seconds) Speed ( in meters/second) Distance ( in meters)
1 1.0 0.5
2 2.0 2.0
3 3.0 4.5
4 4.0 8.0
5 5.0 12.5

Notice that the speed increases by 1 meter per second every second and that the distance traveled increases from second to second. The second table is an example of accelerated motion. To further illustrate acceleration, two sets of graphs will be shown. The first set of graphs shows the speed versus time and the position versus time graphs of non-accelerated motion, and the second set of graphs shows accelerated motion.

Non-Accelerated Distance and Speed versus Time Graphs
Uniform

Notice that the distance graph is a straight line while the speed graph is a horizontal line, indicating that there is no acceleration in this case.

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