what would be interesting is to try to obtain the max time as a function of acceleration

but no clue if you can do that analytically

@ChrisWhite Ok, thanks !

If I apply an upwards force on an object greater than that of gravity, it will accelerate upwards. Once it reaches a given velocity, I reduce the applied force to that of gravity and the object maintains that upwards velocity, correct?

And once the velocity is constant is work still being done on the object? I'm sure there is still work being done, but am not quite sure how to reconcile this with the no change in kinetic energy (unless the answer is that the rising object is pulling on the earth which changes it's kinetic energy)

@Simplex : when you throw a brick into the air you give it kinetic energy. You do work on the brick when you do this. Gravity doesn't. It merely converts that kinetic energy into potential energy. At the top of its arc, all the KE has been converted into PE, the brick is at rest, and its mass is slightly greater than it was when you picked it up. Then

gravity converts some of that mass-energy aka potential energy back into kinetic energy, and the brick falls. After the brick has hit the ground and the kinetic energy is dissipated, the mass of the brick is reduced to is original value. Check out the mass deficit.

(removed)

@Simplex There's no change in kinetic energy, but you are constantly increasing its gravitational potential energy (linearly with height, on the scale where gravity is constant).

isn't it equally correct to say that work is delta potential energy too?

@obe How's that different from what I said?

it isn't?

I was typing it while you said that.

Ah, okay then

Since it appeared after what I wrote I thought it was a reply

@ACuriousMind so in that case, work = change in KE doesn't apply, as you also must consider the potential energy?

@Simplex Indeed. Work is change in energy, regardless of what "form" it is in.

@JohnDuffield John, that's more wrong than right. Gravity does do work on on the brick. The fact that the brick changes speed shows that there has been net work (work-energy theorem) and while the brick is in free flight gravity is the only force so net work is gravitational work.

Further the potential energy belongs not to the brick itself but to the system of brick-and-planet. That system is heavier when the brick is up high, but the brick itself is not. It's just slower.

@Simplex No. You need to pay attention to exact what is stated in the work-energy theorem. Net work equals change in kinetic energy.

When you lift the brick slowly the net work consists of the work you do (positive) and the work gravity does (negative).

When you throw the brick upward there are two (or three) phases.

First, while in contact with your hand you do a lot of positive work and gravity does a little negative work. The brick ends going faster than it started.

Second, while the brick flies upward as a projectile gravity does negative work. The brick slows to a stop.

Third, while the brick falls as a projectile, gravity does positive work. The brick ends going faster than it started

Hello, everyone.

Gravitational potential energy is arrived at by factoring the work done by gravity out of the net work and moving it to the other side.

Then you get an expression like $\Delta T + \Delta U = W_\text{everything but gravity}$.

Here I write $T$ for kinetic energy and $U$ for gravitational potential energy.

May I ask a simple question?

YES

You can keep factoring out other conservative forces and turning them into more potential terms on the LHS until you get $\Delta E = W_\text{non-conservative}$, or examine cases where no external forces but gravity are applicable and get $E_f = E_i$.

There is no such thing like centrifugal acceleration (nor force, this way), right?

there is.

They're all inertia

Oh.

@JohnDuffield Feel free to answer.

@LucasHenrique Depends on what you mean by "is" in that sentence.

How far does gravity extend?

@ChrisWhite The only case where I know that a mod was asked to step down and I know part of the inside story involved the mod getting into a very public fight with another user on the site.

@ACuriousMind what do you mean?

(according to modern theories)

anyone know what Richard did

@LucasHenrique Well, not in an inertial frame of references (ones where Newton's laws work as advertised).

Where do they exist then?

If you want to work in a rotating frame of reference then a term appears in the equations of motion (in those coordinates) that we identify as the centrifugal (pseudo-)force.

But those terms are related to the way the coordinate changes interact with the objects inertia. In a inertial frame the object still obeys Newton's 1st and 2nd law.

@LucasHenrique In an inertial frame, i.e. a frame where object go in straight lines unless acted upon, you don't have a centrifugal force. But for someone who is rotating with a body, there seems to be a centrifugal force in that freely moving objects don't seem to travel in straight lines for them.

@0celo7 First I've heard of it.

the ban hammer and all

how could you not as a moderator

@punkerplunk Infinitely far, just like electromagnetism.

@ACuriousMind allegedly

entanglement as well

allegedly

@punkerplunk It doesn't make sense to talk about the "range" of entanglement, since the basic phenomenon doesn't depend on any spatial distance at all.

that is to say, in a way, entangled particles cannot be seperated

@Huy All my students who try to study it that way do poorly. You have to understand those formulas in the context of a structured theory or you're sunk.

@punkerplunk What?

@dmckee then (when it exists) can I calculate it?

@dmckee I was helping someone out with Diff Eqs and she said she just needs to see examples so she can memorize the steps

rather, our perception of distance.

@0celo7 I used to let students bring hand-written cheat sheets to exams (these days I'm providing the cheat sheet). And I made them turn them in mostly to check the "hand written" requirement.

But I also correlated what kind of sheet they wrote with how they did.

that's what our physics profs do

People who tried to assemble a cookbook invariable struggled (mostly below median even when I knew the student to be smart in other fields).

I wrote the range equation on mine because meh

then I copied it wrong on the test ;_;

People who assembled principles and relationships had a fighting chance to do well.

does gravity propagate in a similar fashion as EM forces?

So right now I'm providing the principles and relationships in hopes that the C students will do a little better. Without much success, alas.

@punkerplunk yes

there are gravitational waves

@0celo7 allegedly ;)

BUT

well, yes

@0celo7 Well, we have strong reason to suspected that, but we haven't actually observed them yet.

but gravity has nonlinearities

but not entanglement. that does not propagate.

@dmckee That should be a "without" at the end, right?

@dmckee programming?

@dmckee you don't need to tell me

(if not, congrats!)

but then "alas" does not make sense

doesn't "alas" imply failure?

@ACuriousMind Yeah.

It's just weird to think of gravitons as 'things' which can 'traverse a distance' and than attract some body upon impact

@punkerplunk That's because it's not a force. You can't talk about it like that.

@LucasHenrique Know, the period changes we observe in compact binary systems are consistent with a theory of energy loss by gravitational radiation.

HAHA

@ACuriousMind there's a mod which makes The Lusty Argonian Maid into a one handed skill book

@punkerplunk It's weird because it isn't true, just like electromagnetism isn't one charge sending out photons and attracting another by that.

I'm so confused (-_-)

@ACuriousMind whaaaaat

This image comes from a wrong interpretation of "virtual particles" in Feynman diagrams that describe the interaction between charged / massive objects.

@0celo7 ::sigh::

(Okay, I chuckled)

@dmckee Could that actually be mass ejected from the system

@ACuriousMind for your Skyrim pleasure

is it possible for a body to gravitate with itself?

what do you mean

@ACuriousMind what is the right interpretation

The problem was: A car is at 10m/s made a full circular rotation. Then the $a_x$ was like 3m/s² and $a_z$ was like 4m/s². But I still don't know if these were influenced by centripetal or centrifugal forces. It moves only in z and x (because of gravity)

@dmckee When I lift a brick with constant velocity, there is no net work. However, the gravity is doing (negative) work.

Would it be correct to consider the work done by gravity in this case as a consequence of the difference in the motion of the brick if there were no gravitational force?

ie, the displacement of the brick would be greater if there were no gravity opposing the lifting force, thus the gravity is changing the "would be" displacement and therefore is doing work?

just detonate a planet size nuke and make the Sun wobble, measure the radiation, ..., profit Nobel

I'm not sure where you are going with that. If you applied the same force over the same distance in the absence of gravity you would be accelerating the object and would achieve a difference in kinetic energy.

@ChrisWhite Personally I'm waiting to see what they have before taking my best poorly informed guess.

okay, so when an object is under influence of the 'non' force of an object in motion gravity, are they attracted to the current location of that object, or the time-delayed location of that object/

sorry, I thought I deleted more words than I did

I'm asking if the fact that gravity is preventing the brick from accelerating due to the lifting force is the reason that gravity is doing work on the brick

@Simplex It sounds like your expecting work to be a special physical thing. It's mostly a bookkeeping tool.

We prattle on about it a lot, but ... it is just a way of keeping track of transfers of energy.

Now, energy. Energy is something that my inner Platonist believes in.

in a gravitational system, where it is assumed the contributing masses are all in motion, are the parts 'updated' instantly

Much like circles. Circles are things and so is energy.

or are they lagging based on distance

@punkerplunk No. In GR and in retarded versions of Newtonian gravity (yeah that's a thing, it gets several things right) there is not instantaneous knowledge of where the other body is "now". Just where it was $c |r|$ ago.

So the last thing I said isn't correct? In other words, If I know there is a force applied upwards and I note that the object isn't accelerating upwards, but instead moving at a constant velocity, I could then deduce there must be a force in the opposite direction preventing this acceleration, and that this "prevention of acceleration" is the work done by the opposing force (in this case, gravity).

Sorry, I'm kinda slow

@Simplex So, when you ask if what you said is right or wrong, there are two ways to answer. In one I say, I think you got the right outcome this time, so you were right. In the other I have to try to judge if that way of thinking about it will be consistently right or not.

I don't feel I understand how you are thinking well enough to answer the second version.

I just go back to the work-energy theorem and laboriously pick my way through the steps.

Mind you, I've done it so much that these days I can do it in my head.

@0celo7 I can't tell if you are joking or if you have never seen/forgotten what I say at almost every mention of "virtual particle"

@ACuriousMind mostly joking

but Zee swears up and down that there are little particles whizzing about

and from an argument of authority, he wins :O

PhD and hundreds of publications and all

@dmckee Ok, thanks for taking the time to help!

@ACuriousMind OTOH, an awesome mod for people older than 14

Please help me :/

Lucas, how!

Tfw haven't played Skyrim in over a month

@punkerplunk I want to know the general way to discover centrifugal acceleration. I've shown the problem my teacher gave me up there

centrifugal or centripetal?

@LucasHenrique Useful tip: the magnitude of the centripetal acceleration measured in an inertial frame and the centrifugal acceleration measured in a axially-mounted co-rotating frame is the same.

as i understand those are two words for the same force. the only difference is, if your observing the force from inside the car or outside the car

would 'centrifugal acceleration' be the force experienced by the passangers of the car? you'd need to know the turning radius

Well acceleration is acceleration and force is force and they are not the same thing. But they are linked by the mass of the object affected.

And yes, you need to know either the beginning and ending velocity vectors or the speed and radius of curvature to find the acceleration.