What is the jerk of a falling object?

[Doc Al]

Here's the thing: I happen to be a great believer in Newtonian mechanics. I think its awesome in its explanatory power. It is certainly the greatest achievement in physics, as not even Newton could calculate the positive effect it has had on the human condition.

However, I've been reading a lot lately about relativity, so I know that if I call Newton's equations into question, I am far from the first to do so. I also know that I would certainly not be the first to call Einstein in question. When I learned that the inventor of the atomic clock was one of these, it got me to thinking.

The only thing you seem to be calling into question is your understanding. Nothing at all you've stated in this thread (which is just about run out of steam) has questioned classical mechanics or relativity.

It occurred to me that the maybe the whole of motion due to gravitation is smooth and proceeds from zero, not just the velocity. So that got me to wondering what successively higher order approximations of displacement would show us.

And where did you ever get the idea that gravitation isn't smooth?

On the other hand, we are so conditioned to thinking about motion in terms of polynomials that I think we get trapped into accepting that polynomials define motion rather than describe it. What if displacement due to gravity is not polynomial but has a extremely good 4th order polynomial approximation?

We laugh when we see Wile E. Coyote momentarily suspended, then suddenly yanked to his fate. I posit that neither is he suddenly forced to his fate, but proceeds according to uniform motion.

Seems to me that you are at war with a figment of your imagination.

Well, hopefully, your experience has not been having had rockets fired at you. However, jerk is defined as a change in acceleration over time.

Even in the falling body example there is jerk because the acceleration is increasing.

Again: So?

 

[ejungkurth]

I'm not at war with my imagination, I'm using it. To tell you the truth, I didn't think this thread would go beyond the first couple of posts.

I'm not trying to mislead anybody, I'm just trying to exercise my capability to think abstractly. If I lost that capability, then I'd be stuck calling everybody I disagreed with wrong.

 

[Mathnphysics]

I agree that, if I tossed the same object straight up, it would go through zero velocity and would have no jerk. That object would always be accelerating at g.

But, if the object is held in place, it seems there should be a jerk as it begins to accelerate.

I'm trying to think of an experiment to test this. My jerkometer only works horizontally.

The object is always under the acceleration of gravity. That is, it always has a force exerted on it due to the acceleration. It only moves once you let go of the object because otherwise your hand is canceling this force. Therefore, the change in acceleration is zero because the gravitational force causing the acceleration is constant throughout.

 

[TVP45]

So, graph it: a vs t. Start at t= - 2 s. Remember that v is the area under the curve so you got to have a graph that has v=o, then v increasing.

 

[pghislain]

I have another view:

A masse suspended is a two opposed and equal forces system:

f = G - F

Where f = 0 if it is imobile.

So F = m * a and G = m * g; a = -g;

Its a bit basic, I know! A force and an acceleration do'nt have to be associated to a movement. A force to exist must be associated to a masse and an acceleration.
When you suspend a masse in order to avoid it to fall, you must produce a force and its acceleration is not null, even if it do'nt move. If you where in space, this force would project the mass Up but in this example, it just opposes the gravitation acceleration. Acceleration doesnot implies movement.

But in this case ... speed is not null. Yes as well as the accelerations, it is the resulting speed of the system that is null, as well as the resulting force is null.

When you stop suspending the mass, the system of force resumes itself to the gravity multiply by mass, which is a constant force. No jerk at all.

In all the case, for what I understand, jerk is only an analitical tool as well as jounce (4th derivation) regarding acceleration that is in fact, multiply by mass, a physical parameter of the world. Jerk only document acceleration caracteristics, it is not independent of acceleration in some physical effect to be presented, I supose.

I don't know any physical experience or fact or properties that jerk or jounce produce. Acceleration of a mass produce Force. Jerk of a mass produce ... nothing special : information regarding acceleration ! :) Jerk allow to define boundaries for the variation of the acceleration in order to produce Force that shall not indispose human being or destroy equipments. Jounce allow to get the min and max of jerk.

Do you know something of the real world that jerk or jounce manage, produce, ..., some new physical effect ?

 

[TVP45]

I don't know any physical experience or fact or properties that jerk or jounce produce. Acceleration of a mass produce Force. Jerk of a mass produce ... nothing special : information regarding acceleration ! :) Jerk allow to define boundaries for the variation of the acceleration in order to produce Force that shall not indispose human being or destroy equipments. Jounce allow to get the min and max of jerk.

Do you know something of the real world that jerk or jounce manage, produce, ..., some new physical effect ?

Well, yes. Jerk can damage cam followers. Jerk can derail trains. Jerk can (and in fact often does) cause cars to skid on turns. Jerk will cause your hot coffee to spill in your lap. And, so on. Generally, rotational jerk is a more common problem than linear jerk.