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 different kinds of constant motion in physics. There are two types of constant motion relevant to physics: constant speed and constant acceleration. In this lesson you will learn the definition of both motions and how they are graphically illustrated.
Definition of Constant Motion
Constant motion refers to any type of motion when either the distance traveled by the object is the same for each second, or the speed of the object changes by the same amount each second.
Imagine tapping your hand on the table once every half a second, and what that would sound like. Now think about taking one step every half a second. If you methodically take one step every half second, you are moving with a constant speed, one type of constant motion. Constant speed means that you are moving the same distance every second, so your speed does not increase at all.
Now imagine tapping the table once during the first second, twice during the next second, three times during the third second, etc. Now think about taking one step during the first second, two steps in the next second, and three steps during the third second, etc. This process describes another type of constant motion: constant acceleration. In constant acceleration, your speed increases by the same amount every second. So if you have 1 m/s (or meters per second) during the first second, and 2 m/s during the next second (an increase of 1 m/s), then you must travel 3 m/s, 4 m/s, and 5 m/s during the third, fourth, and fifth seconds, etc. Your constant acceleration is 1 m/s/s (meters per second per second or meters per second squared).
Example: Constant Speed
Now let's discuss an example of constant motion. Let's say you are driving interstate I-10 from San Antonio to El Paso, Texas. Most of the sections of this path are fairly flat and straight. You reach a cruising speed of 75 mph when you decide to engage your cruise control. The cruise control system keeps the vehicle speed at 75 mph until you hit the brakes or accelerate. While you are using the cruise control, you are moving with constant speed. This table illustrates the distance traveled during a four-hour trip on cruise control.
Based on the table, you traveled 75 miles each hour while you were on cruise control. That is a total of 300 miles for a four-hour trip on cruise control. This is an example of what constant speed data would look like.
Constant Speed Motion Diagrams
These figures illustrate constant speed using dots. Each dot shows the location of the object for each one second interval. Figure A represents a slow-moving object since the dots are closer together. Figure B represents a fast-moving object since the dots are farther apart each second. Note that the distance between each dot is the same in each figure. Therefore, the object represented by the dot is moving with constant speed in both cases. These figures make up what we call a motion diagram.
The position versus time graph below illustrates constant speed in a different way. During each second, the object moves the same distance.
Distance versus Time Graph
Constant Acceleration Motion Diagrams
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This motion diagram shows two types of constant acceleration. Figure A shows the object speeding up, which is indicated by the distance between the dots getting bigger every second, and figure B shows the object slowing down, which is noted by the distance between the dots getting smaller every second. Constant acceleration means that the speed of the object increases or decreases by the same amount each second as illustrated by this time and speed table. In the table, the speed decreases by 10 m/s every second, which means the constant acceleration is -10 m/s/s. The graph shows the data in the table plotted as speed versus time.
Velocity versus Time Graph
The most important example of constant acceleration is the acceleration due to Earth's gravity. Every object near the surface of Earth experiences the same constant acceleration due to gravity which is about 10 m/s/s. That means than any object that is dropped or thrown will increase its speed by about 10 m/s for every second that it falls.
Both of these types of motion occur when you parachute from an airplane. As soon as you jump out of the airplane, you are falling at a rate of about 10 m/s for each second of the fall. However, when the strength of the air resistance is equal to your weight, you reach what is called terminal velocity, or terminal speed. When you reach terminal speed, you are no longer accelerating and you fall with the same speed until you deploy your parachute.
In physics, constant motion refers to any type of motion when either the distance traveled by the object is the same for each second, or the speed of the object changes by the same amount each second. There are two important types of constant motion: constant speed and constant acceleration. Constant speed means the speed does not change at all for each second of the motion. Our example of driving a car on cruise control illustrates constant speed. Constant acceleration means that the speed increases by the same constant rate for each second of the motion. We illustrated this with our example of someone jumping out of an airplane. In this example, there is constant acceleration until the person reaches terminal speed. Terminal speed, or terminal velocity, occurs when the strength of the air resistance is equal to your weight.
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