Acceleration, and displacement

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Discussion Overview

The discussion revolves around the concepts of acceleration and displacement, particularly focusing on the nature of negative values in these contexts, the definitions of acceleration, and the implications of sign conventions in vector quantities. Participants explore theoretical aspects and clarify misconceptions related to these fundamental physics concepts.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that displacement cannot be negative, questioning the implications for acceleration.
  • Others argue that the sign of a vector, including displacement and acceleration, depends on the chosen sign convention, suggesting that negative values can exist based on context.
  • One participant proposes that acceleration should be understood as the change in velocity over time (Δv/Δt) rather than as a change in displacement over time squared (Δx/Δt²).
  • A participant mentions that negative acceleration indicates the direction of the acceleration vector, using the example of a falling body to illustrate this point.
  • Another participant introduces the idea that acceleration can be classified as negative or positive based on the scalar product of acceleration and velocity, noting specific conditions such as circular motion.
  • Concerns are raised about the potential for confusion when using Δx/(Δt)², particularly in scenarios involving constant velocity.

Areas of Agreement / Disagreement

Participants express differing views on the nature of negative displacement and acceleration, with no consensus reached on whether displacement can be negative or the best way to define acceleration. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

Participants highlight the importance of sign conventions and the potential for misunderstanding when applying different definitions of acceleration. There are unresolved questions regarding the implications of using various formulas in different contexts.

jaja1990
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I think we can't have a negative displacement. Is that right?

Acceleration: the change in displacement / change in time squared
Since displacement can't be negative, and time can't be negative, acceleration also can't be negative.
But I know (it's in the books) that when we have decreasing acceleration, it's negative. How is that?
 
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jaja1990 said:
I think we can't have a negative displacement. Is that right?
Why do you think this? Realize that whether a vector is negative or not just depends on the sign convention used when specifying its components.
Acceleration: the change in displacement / change in time squared
Better to think of acceleration as Δv/Δt.
Since displacement can't be negative, and time can't be negative, acceleration also can't be negative.
Again, the sign of a vector is rather arbitrary.
But I know (it's in the books) that when we have decreasing acceleration, it's negative. How is that?
Acceleration is a vector. A negative acceleration just means that the acceleration vector points in the negative direction. For example, if you take up as positive, the acceleration of a falling body will be negative.
 
Doc Al said:
Why do you think this? Realize that whether a vector is negative or not just depends on the sign convention used when specifying its components.

Better to think of acceleration as Δv/Δt.

Again, the sign of a vector is rather arbitrary.

Acceleration is a vector. A negative acceleration just means that the acceleration vector points in the negative direction. For example, if you take up as positive, the acceleration of a falling body will be negative.
I understand now, it's the direction that is negative, not the magnitude.

Why is it better to think of acceleration as Δv/Δt?
I guess it just a matter of which is more intuitive/elegant, but maybe there is another reason; is there?
 
Acceleration is the derivative of the velocity so it's dv/dt.

The sign of the acceleration can have different criteria, I think an acceleration is negative if the scalar product (dv/dt)•v < 0 and positive if (dv/dt)•v > 0. If the scalar product it's zero then the acceleration is perpendicular to the motion. This is the case in circular motion, for example.
 
jaja1990 said:
Why is it better to think of acceleration as Δv/Δt?
I guess it just a matter of which is more intuitive/elegant, but maybe there is another reason; is there?
Well, Δv/Δt is the definition of acceleration (at least average acceleration).

Blindly using Δx/(Δt)2 can lead to silly results. Imagine something moving at a constant velocity of 10 m/s for 1 second. Δx = 10, Δt = 1. Obviously the acceleration is zero here, so that formula fails.
 

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