El Filibusterismo 10 Kabanata
1 Velocity, Speed, Acceleration, and Deceleration
The goal for today is to better understand what we mean by terms such as velocity, speed, acceleration, and deceleration. Let’s start with an example, namely the motion of a ball thrown upward and then acted upon by gravity.
A major source of confusion in problems of this sort has to do with blurring the distinction between speed and velocity. The speed s is, by definition, the magnitude of the velocity vector: s := |v|. Note the contrast: – velocity – | | – speed – |
The change in velocity is uniformly downward. | | The speed is decreasing during the upward trajectory, and increasing during the subsequent downward trajectory. |
The laws of physics are most simply written in terms of velocity, not speed.
Physics uses a technical definition of acceleration that conflicts with ordinary vernacular use of the words “acceleration” and “deceleration”. That’s tough. You’ll have to get used to it if you want to do physics. In physics, acceleration refers to a change in velocity, not speed. If you want to be really explicit, you can call this the vector acceleration. | | In the vernacular, “acceleration” commonly means speeding up, i.e. an increase in speed. If you insist on using the word in this sense, you can remove the ambiguity by calling it the scalar acceleration. |
| | The scalar acceleration can be considered one component of the vector acceleration, namely the projection in the “forward” direction (although this is undefined if the object is at rest). |
In physics, the word “deceleration” is not much used. In particular, it is not the opposite of acceleration, or the negative of acceleration. Any change in velocity is called an acceleration. | | In the vernacular, “deceleration” commonly means slowing down, i.e. a decrease in speed. |
Do not confuse the vector acceleration with the scalar acceleration. |
In physics, acceleration does not mean speeding up. | | | |
The goal for today is to better understand what we mean by terms such as velocity, speed, acceleration, and deceleration. Let’s start with an example, namely the motion of a ball thrown upward and then acted upon by gravity.
A major source of confusion in problems of this sort has to do with blurring the distinction between speed and velocity. The speed s is, by definition, the magnitude of the velocity vector: s := |v|. Note the contrast: – velocity – | | – speed – |
The change in velocity is uniformly downward. | | The speed is decreasing during the upward trajectory, and increasing during the subsequent downward trajectory. |
The laws of physics are most simply written in terms of velocity, not speed.
Physics uses a technical definition of acceleration that conflicts with ordinary vernacular use of the words “acceleration” and “deceleration”. That’s tough. You’ll have to get used to it if you want to do physics. In physics, acceleration refers to a change in velocity, not speed. If you want to be really explicit, you can call this the vector acceleration. | | In the vernacular, “acceleration” commonly means speeding up, i.e. an increase in speed. If you insist on using the word in this sense, you can remove the ambiguity by calling it the scalar acceleration. |
| | The scalar acceleration can be considered one component of the vector acceleration, namely the projection in the “forward” direction (although this is undefined if the object is at rest). |
In physics, the word “deceleration” is not much used. In particular, it is not the opposite of acceleration, or the negative of acceleration. Any change in velocity is called an acceleration. | | In the vernacular, “deceleration” commonly means slowing down, i.e. a decrease in speed. |
Do not confuse the vector acceleration with the scalar acceleration. |
In physics, acceleration does not mean speeding up. | | | |