Problem in understanding instantaneous velocity

  • Context: Undergrad 
  • Thread starter Thread starter Mike_bb
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Discussion Overview

The discussion revolves around the concept of instantaneous velocity and its relationship to average velocity, particularly as the time interval approaches zero. Participants explore the mathematical and intuitive aspects of this concept, including the definitions and implications of limits in calculus.

Discussion Character

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

Main Points Raised

  • Some participants suggest that instantaneous velocity can be understood as the limit of average velocity as the time interval approaches zero.
  • Others propose that the slope of the tangent line at a point represents instantaneous velocity, while the slope of secant lines represents average velocity.
  • A few participants express intuitive understandings of instantaneous velocity, likening it to the speed shown on a car's speedometer.
  • There are discussions about the mathematical definition of limits and how they relate to the concept of approaching a tangent line without ever actually becoming it.
  • Some participants challenge the interpretation of limits, arguing about the distinction between approaching and becoming in the context of secant and tangent lines.
  • There is mention of the need for a rigorous understanding of calculus to fully grasp these concepts.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of limits and the relationship between secant and tangent lines. While some agree on the general concept of limits, there is no consensus on the nuances of the terminology used in the discussion.

Contextual Notes

Some participants highlight the importance of understanding the mathematical definitions and implications of limits, while others point out that intuitive understandings can also play a role in grasping these concepts. The discussion includes varying levels of mathematical rigor and informal explanations.

  • #31
Mark44 said:
Sure you can, and speedometers do exactly this. One kind of speedometer on a motorcycle I own has a gear driven sensor on the front wheel. The sensor has a worm gear that turns when the wheel turns. The worm gear drives a cable, the other end of which causes a magnet to rotate that in turn causes a needle to sweep to a certain position that corresponds to the speed of the motorcycle (speed = magnitude of velocity). Another motorcycle I have has a cable that comes from the transmission that drives the speedometer in a similar way. These measured values are the instantaneous speeds.
The needle has mass and is damped. Undamped needles bounce. A torque that acts on such a needle will not result in an instantaneous change in the needle's position. What the speedometer shows is some sort of approximate weighted average of past velocities.
 
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  • #32
jbriggs444 said:
The needle has mass and is damped. Undamped needles bounce. A torque that acts on such a needle will not result in an instantaneous change in the needle's position. What the speedometer shows is some sort of approximate weighted average of past velocities.
I read more about speedometers and I agree with you. Only analogue speedometer with the arrow can show exact value of instantaneous speed because such speedometer is based on induction principle.
 
  • #33
jbriggs444 said:
The needle has mass and is damped.

Mike_bb said:
Only analogue speedometer with the arrow can show exact value of instantaneous speed because such speedometer is based on induction principle.
The speedometers on my old bikes are strictly analogue, and as far as I know, aren't damped.
 
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  • #34
Here's how I visually motivate the interpretation of the velocity (as the slope of the tangent line) to my students.
Assuming a nice x-vs-t graph,
at the instant T of interest, centered at (T,x(T)),
I zoom in enough so that "my graph looks like a straight-line" in the viewport.
Since the motion is practically a steady-velocity motion for a sufficiently-short time-interval,
the [instantaneous] velocity is practically equal to the slope of that [approximate] straight-line in the viewport.

In the Desmos visualization below,
I have already activated the tangent-line (which you can disable by clicking on the filled circle for its folder).
To animate the zoom, click on the play-button ⏵ of the z-slider. (You can drag the z-slider to manual control the zoom.)

www.desmos.com/calculator/ghfds0lbht
 

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