The Constant Speed of Free Falling Objects

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Discussion Overview

The discussion revolves around the nature of motion for free-falling objects, specifically addressing the concepts of acceleration and speed in different frames of reference. Participants explore whether free-falling objects exhibit uniform or non-uniform motion, particularly in the context of gravitational acceleration and relativistic effects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the speed of a free-falling object changes, asking if it practices uniform or non-uniform motion.
  • Another participant distinguishes between proper acceleration and coordinate acceleration, noting that an object in free-fall has a proper acceleration of 0, indicating constant speed in local inertial frames.
  • A clarification is made regarding gravitational acceleration, specifically referencing the value of 9.8 m/s².
  • A further challenge is presented regarding the assumption that coordinate acceleration is always constant for free-falling objects, using the example of a test object falling towards a black hole, where coordinate acceleration may vary and can even be negative.

Areas of Agreement / Disagreement

Participants express differing views on the nature of acceleration and speed in free-fall scenarios, with no consensus reached on the implications of these concepts in various frames of reference.

Contextual Notes

The discussion highlights the complexity of defining acceleration in different contexts, particularly in relation to general relativity and the effects of strong gravitational fields, such as those near a black hole.

isyang94
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When an object free falls, its accerleration is constant, but does its speed change? If so is it practicing uniform or non uniform motion?
 
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That depends what you mean by acceleration. In relativity there are two distinct concepts called acceleration. One is proper acceleration, which is the acceleration measured by an accelerometer. The other is coordinate acceleration, which is the second time derivative of the coordinate position.

An object in free-fall has a proper acceleration of 0, so its speed is constant in any local inertial frame.

In non-inertial frames it may have a non-zero coordinate acceleration so its speed would not be constant in that coordinate system.
 
Acceleration as in the gravity 9.8m/s/s
thanks for the reply
 
isyang94 said:
When an object free falls, its accerleration is constant, but does its speed change? If so is it practicing uniform or non uniform motion?
Why do you mistakenly conclude that a free falling object's coordinate acceleration is always constant?

Consider a radially free falling test object approaching a black hole as modeled by the Schwarzschild solution, its coordinate acceleration is not constant and may in some cases and 'locations' even be negative (e.g. directed away from the black hole).
 
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