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In this Physics tutorial, you will learn:
|3.1||Motion. Types of Motion|
Suppose that you are watching an object. Apparently, it may seem not moving. However, when you watch it again after having lost your attention for a while, you notice that the object is not anymore at the place where it was before. To draw this conclusion, you compare its actual location to any object you consider as fixed (unmoveable) in the surroundings (a building, a tree etc.). Only then, you realize that the object has moved, because it has changed its location with respect to time.
In physics, Motion is the phenomenon in which an object changes its location over time. Mathematically, Motion is described in terms of displacement, distance, velocity, acceleration, speed, and time. We will discuss all these terms in the next articles of this section (chapter).
The following figure shows a cyclist photographed in very short intervals of time. The frames are merged, so you can see how its location has slightly changed in each interval of time.
In few words, anything that keeps changing its LOCATION with respect to TIME is said to be in motion. In the above figure, different locations of the object during its motion are shown using a professional camera. We can notice the change in location taking as a fixed point the tree in the photo for example.
In Physics, the Motion is classified according to 2 main criteria:
Each of the above categories contains in itself a number of sub-categories. Let's take a quick look at all of them.
In this category there are two main sub-categories included. They are:
I. Uniform motion. It takes place when an object moves at the same velocity over time. This means it does not change its moving rhythm. Mathematically, we say an object is in uniform motion when in equal time intervals it travels the same distance. Look at the figure below:
II. Non-uniform motion. This sub-category is further divided into Accelerating (Speeding up) and Decelerating (Slowing down) motion. Accelerating motion occurs when an object increases its velocity over time and Decelerating motion occurs when it decreases the velocity over time.
On the other hand, if the acceleration (or deceleration) is uniform, the object accelerates (decelerates) at the same rate. This kind of motion belongs to the uniformly accelerated (decelerated) motion. If the acceleration (deceleration) is not uniform, this is considered as a non-uniformly accelerated (decelerated) motion.
Look at the figures below for more insight in this concept.
This figure represents a uniformly accelerated motion as the velocity increases by the same rate (in the next second the object travels 2m more distance than in the previous second). If the bicycle doesn't change its moving rhythm, we can find what distance will it travel after a certain time. That's why it is a uniformly accelerated motion.
In this figure, we have a uniformly decelerated motion as the bicycle slows down at the same rate (2 m/s for each second).
In the next figure, you see that the speeding up process is not regular. You are not able to find the velocity after a given time from the data shown. Thus, although it is obvious this motion is accelerated, it is not uniformly accelerated one, as its velocity does not increase by the same rate.
On the other hand, the figure below shows a non-uniformly decelerated motion as the slowing down process does not occur at a regular rate. You cannot foresee what velocity the object will have after a certain period, based on the data provided.
If the motion is irregular, we can split in into small regular sections and making a separate study for each of them.
The scheme below shows how various types of motion based on the moving rhythm are related to each other.
This category contains a number of subcategories as well. They are:
This is a kind of motion, in which the object moves in a straight line. Look at the figure below.
The road is straight, so the car is making a rectilinear motion.
In this kind of motion, the object moves in a circular path, i.e. it revolves around a fixed point (the centre) at the same distance (radius r) from it. Look at the figure.
It describes an object moving according along a curved path. Look at the figure:
It differs from Circular Motion as here the curve may not be perfect, so it is impossible to find a fixed point whose distance from the moving object remains constant.
This is a back and forth motion around a fixed point, which is known as the "equilibrium position". An elastic spring oscillating up and down when we hang an object in it - as shown in the figure - represents a typical example of Vibratory Motion.
This kind of motion typically occurs when we throw an object at an angle θ different from 0° and 90° to the horizontal direction (θ ≠ 0° and θ ≠ 90°). The trajectory of a projectile is parabolic due to the attraction effect of gravity. Look at the figure below in which the ball has been "photographed" in 7 consecutive and equal time intervals.
This kind of motion describes and object moving according an elliptic path. All planets move in this way when they revolve around the Sun. (Remember from Maths that an ellipse has two foci. The Sun is located in one of them). Look at the figure.
If two or more of the abovementioned motions are combined, other types of trajectory (which are worth to mention) are produced. Some of them include:
This kind of motion is a combination of circular and vibratory ones. That is, the object swings around an equilibrium position but not in a straight line. Its trajectory is similar to that of circular motion. A simple pendulum represents a typical example in this regard. Look at the figure.
In this kind of motion there is a combination of rectilinear and vibrational ones. If you shake a rope, it will look like this:
If we take a certain particle in the rope, we will see that it moves only up and down around the equilibrium position. This may seem as vibratory motion. However, the disturbance in the rope spreads according a rectilinear trajectory. In this way, a wavy motion is generated. Therefore, in wavy (sinusoidal) we have a combination of two basic types of motion: vibratory and rectilinear.
We say this motion is sinusoidal because the wave it corresponds is similar to the graph of a sinusoidal (sine) function.
Describe the motion for every section shown in the figure.
In the section AB the motion is curvilinear as the car is moving along a curved path but there is not any fixed point around which it rotates.
In the section BC the motion is rectilinear because the car moves along a straight line.
In the section CD the motion is wavy (sinusoidal) as the car's trajectory is similar to that of a wave.
In the section DE there is a circular (rotatory) motion as the trajectory is a similar to a circle.
In the section EF the motion is rectilinear as the car moves along a straight line.
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