Uniform Circular Motion and Centripetal Acceleration

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Discussion Overview

The discussion revolves around the application of the centripetal acceleration equation, a_c = v^2/r, particularly in the context of uniform versus non-uniform circular motion. Participants explore whether this equation can be applied when the speed of an object moving in a circular path is not constant, raising questions about the nature of acceleration and its dependence on instantaneous versus average values.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the restriction of the centripetal acceleration equation to constant speed, suggesting it could apply to variable speeds as well, with a_c changing accordingly.
  • Another participant references external material that discusses the general case of centripetal acceleration for variable speeds, indicating that further study is needed.
  • A suggestion is made to learn the Frenet-Serret apparatus, which relates to vector calculus and may provide deeper insights into the topic.
  • Concerns are raised about the definition of acceleration, particularly regarding instantaneous acceleration and its relationship to changes in velocity over time.
  • One participant asserts that while the centripetal component of acceleration is valid for changing speeds, a tangential component must also be considered in such cases.
  • Another participant emphasizes that to determine acceleration at a specific instant, knowledge of velocity over an interval is necessary, highlighting the importance of the derivative in this context.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the centripetal acceleration equation to variable speeds. While some acknowledge the need for additional components of acceleration when speed changes, others question the fundamental definitions and implications of acceleration itself. The discussion remains unresolved regarding the broader applicability of the centripetal acceleration equation.

Contextual Notes

Participants note limitations in understanding instantaneous acceleration and the need for definitions that encompass average rates of change. The discussion touches on the mathematical foundations of acceleration without reaching a consensus on the implications for centripetal acceleration in non-uniform motion.

Nathanael
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In introductory physics books (or at least mine) it limits the equation a_c=\frac{v^2}{r} to the sitaution where the speed around the circular path is constant. It enforces the idea that the speed is CONSTANT.

But wouldn't the equation also apply to non-constant speeds? (a_c would just change from being a constant to being a function of the speed)

It would be very counter-intuitive to me if this equation did not apply to variable speeds (because why does this instant in time care about the speed of the next instant in time?)


So my question is, can you also use this equation for variable speeds?
 
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Some parts of your question are dealt with here: http://www.sweethaven02.com/Science/PhysicsCalc/Ch0119.pdf

The machinery required to solve for the general case of centripetal acceleration for an object constrained to travel in a circle of constant radius, but with variable speed, is discussed ... you should be able to work through to the answer on your own from this point.
 
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And if you want to go further, learn the Frenet-Serret apparatus; usually taught as part of vector calculus - calc 3.
 
Nathanael said:
why does this instant in time care about the speed of the next instant in time?

Is not the definition of acceleration the rate of change of velocity over time?

Is it possible to have instantaneous acceleration? A classic definition of \overline{a} = (v_f - v_i) / t would be undefined at t = 0.
 
Impulse said:
Is not the definition of acceleration the rate of change of velocity over time?

Is it possible to have instantaneous acceleration? A classic definition of \overline{a} = (v_f - v_i) / t would be undefined at t = 0.

The mathematical definition would be the limit (if one exists) of the average rate of change (vf-vi)/(tf-ti) as tf approaches ti without actually getting there.

That is to say that acceleration is the derivative of velocity.

http://en.wikipedia.org/wiki/Derivative
 
Your equation does give the centripetal component of the acceleration even when the speed is changing. But if the speed is changing, there is also a tangential component of the acceleration.

You will probably meet this later on if your course deals with objects moving in a vertical circle, where the speed is greater at the bottom of the circle than at the top.
 
Nathanael said:
...because why does this instant in time care about the speed of the next instant in time?

The following is a general remark about acceleration. Acceleration is the rate of change of velocity. You can't determine acceleration at a given instant of time by only knowing velocity at that instant of time. You need to know it in some open interval centered on that instant of time. This is part of the basic definition of a derivative.
 

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