Is Constant Jerk a Real Physical Situation?

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Discussion Overview

The discussion revolves around the concept of constant jerk, defined as the third derivative of displacement with respect to time, and whether there are real physical situations that exemplify this phenomenon. Participants explore the implications of constant forces and acceleration, as well as the conditions under which forces can change linearly over time.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant inquires about the existence of a physical situation where a force increases constantly and linearly over a non-infinitesimal time interval.
  • Another participant suggests that stretching a spring at a constant rate results in a linearly increasing force, but this is clarified as not being the intended meaning of the original question.
  • There is a distinction made between constant forces causing constant acceleration and constantly changing forces, with some participants expressing confusion over the definitions and examples provided.
  • A proposed example involves a mass on a frictionless surface connected to a pulley with a pan that collects water, leading to a linearly increasing force over time.
  • Participants express appreciation for the examples and clarify their understanding of the concepts discussed.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definition of constant jerk or the examples that illustrate it. There are competing interpretations of what constitutes a constant force versus a constantly changing net force.

Contextual Notes

Some participants express uncertainty about the distinctions between constant forces and constantly changing forces, indicating a need for clearer definitions. The discussion also highlights the complexity of applying theoretical concepts to physical scenarios.

Ja4Coltrane
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I asked a quesion about this a while back, but I never felt that I got a perfectly straight answer.
I want to know if there is any real physical situation where there is a constant jerk (third derivative of displacement with respect to time). I am perfectly aware of how common jerk is as well as countless higher derivatives. I actually want to know if there is something as simple as a constant force causing a constant acceleration. In other words, is there any physical situation where a force increases constantly and linearly over some non infinitesimal time interval?
 
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If you stretch a spring at a constant rate (dx/dt= constant), the force, F= kx, increases linearly at a constant rate.
 
well, that's not what I mean, I don't mean linearly with the variable of displacement, I mean time.
 
oh,I see
sorry
 
but that does not work, I need net force.
in other words, I only care whether or not a constant jerk can exist.
 
Last edited:
Ja4Coltrane said:
I actually want to know if there is something as simple as a constant force causing a constant acceleration. In other words, is there any physical situation where a force increases constantly and linearly over some non infinitesimal time interval?

Your "in other words" is a complete rewording of what came before it! On the one hand, you want to know about constant forces, and on the other hand you want to know about constantly increasing forces. Which is it? :confused: No matter, both can be addressed. But do keep in mind that they are different!

An instance of a constant force causing a constant acceleration happens whenever a constant force acts on a particle. In that case, Newton's 2nd law says that \vec{F}=m\vec{a}. In words that implies that the force is proportional to the acceleration. When one is constant, so is the other.

The example of Hooke's law provided above gives you an example of the second thing you asked for, which once again is not the same as the first.
 
edit: deleting this...
 
Last edited:
hold on
1) what is this second and what is this first, I didnt mean to ask anything about constant forces.
2) I did not mean that I just want an example of constantly changing force--I want an example of constantly changing net force. The hooke's law example does not do that.

do you see what I am saying?
 
Ja4Coltrane said:
well, that's not what I mean, I don't mean linearly with the variable of displacement, I mean time.

If you mean linearly with respect to time, then this can be rigged up with an appropriate machine. As a first approximation, consider a mass on a frictionless surface. Let a cord be attached to the mass and drawn over a pulley. Let the other end of the pulley be attached to a pan, into which flows a steady stream of water. The weight in the pan (and therefore, the horizontal force on the car) will grow linearly in time.
 
  • #10
hehe, I like that example thanks.
it is rigged, but it's cool.
thanks.
 
  • #11
interesting, thanks for your help.
 

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