SilentSky23

7 minutes ago, swansont said:

To me "If inertia resists acceleration, and therefore reduces it when a force is applied to something" implies the acceleration is somehow even smaller than a = F/m because the mass is resisting acceleration, or that it would be bigger than that if mass didn't resist acceleration. But a= F/m is what is meant by mass being resistance to acceleration. For a given force, the acceleration scales inversely with the mass.   

 

 

No, that seems right. But I think here is an example of why we use math to express what's going on, because it has far less ambiguity than the language we use for prose and poetry.

I see, I must have read sayings in the wrong places, then. But, just so it is clear to me, how is resisting acceleration different from reducing it, exactly? Just want to make sure.

7 minutes ago, Bufofrog said:

F=ma

An increase in the force means that the acceleration increased or the mass increased or both increased.

A decrease in the force means that the acceleration decreased or the mass decreased or both decreased.

 

 

Man, I forgot about those two for some reason. I guess it is what is in the formula that matters, if you know what I mean, or how the formula works. I knew there was a potential reason why my thought was possibly wrong, and I only wanted to check on that. Looks like I found the reason, thanks to what you said.

To both of you, I hope you did not mind me asking this, especially since there are some things I happened to accidentally miss.

6 minutes ago, swansont said:

Inertia is a concept: the tendency for an object to be at rest, or motion to be uniform, in the absence of a force. This can also be thought if as resistance to acceleration in a lot of situations. Thus it can be mass or it can be momentum (since F = dp/dt)

To say inertia reduces the acceleration of an object isn't right. Inertia isn't a force. The acceleration is what it is, according to F = ma (for a system with constant mass). There's no second term; we don't have a variable for inertia in our equations. To say that this reduces the acceleration is sort of double-counting.

 

 

 

I see. I kinda meant by inertia reducing acceleration, I meant by resisting it. What do you mean by double counting, anyway?

Either way, I figured there was something wrong with what I thought. Perhaps I didn't think hard enough on this, or just focused on one or a few things alone. Thanks so much.

Though to clarify, I only said reducing because of the equation, force equals mass times acceleration. As I recall, when force increases, acceleration increases, but when mass increases, the acceleration decreases due to resistance, and the more force is needed to accelerate something. Maybe I had something wrong there, too?

Well, I just had this thought some time ago, and I am curious about it. I would like to know if it is actually true or not.

So, we all know about inertia, right? The resistance to acceleration, or change in motion. Well, there is also a concept about derivatives of acceleration, mainly jerk and yank. If you don't know, jerk is said to be the rate of change in acceleration, and yank a rate of change in force. Now, about inertia, here is the thought in question.

If inertia resists acceleration, and therefore reduces it when a force is applied to something, would inertia not actually act as a yank, and thus jerk to acceleration, and thus reduce it when a force is applied? I mean, I know I could be wrong, which is why I even ask. I know that inertia is a property of matter, which is equal to mass, not a force in itself,  and by extension, not even a yank. Maybe it is a property that provides yank, and thus jerk to acceleration somehow as the resistance to change in motion? Kinda like how friction, even though it is a force unlike inertia, resists motion and slows it down? Also, inertia is equal to mass, which is a measure of how much matter there is in something, and if I recall correctly, mass is needed to exert a force on things.

I am not saying I am right on my thought, which is why this is more of a question if anything. So correct me if I am wrong, but would inertia act as or provide a yank, and thus a jerk to acceleration when  a force is applied to something?

1 minute ago, MigL said:

Relativistic mass is essentially kinetic energy, it is a frame dependent phenomenon, and not actual mass.
The ionized particles are essentially in distant orbits about the pulsar which has an extremely strong gravitational well, and are whipped about by the equally strong magnetic field, although I would have thought the magnetic field to be tightly localized ( apparently not, as per your link )
This is not a surface phenomenon as we would normally understand 'wind', but more like the solar wind, and its interaction with Earth's magnetic field.
Additionally, because of the strength of the magnetic field of the pulsar, any ionized particles that would be considered an 'atmosphere' about the pulsar ( close to the surface ), would have a preferred direction of travel; Easy along the magnetic field lines, but extremely difficult crossing them.

I think I do see what you mean. Thanks.

Well, I read here,

"A pulsar also has a wind, and charged particles, sometimes accelerated to near the speed of light, form a nebula around the pulsar: a pulsar wind nebula."

https://www.cfa.harvard.edu/news/su201643

Either way, let us go by wind as we commonly understand them for a moment.

If a wind was moving at relativistic speeds, and had its mass increased accordingly, how strong would it be? Would it be devastating to something or anything on Earth, or to any celestial bodies like planets or comets for that matter?

So, based on what I read, pulsar winds can move at speeds near that of light. Now, we all know with relativistic speeds, mass increases as it gets closer to light. Does that make such winds strong? If so, how strong are they? Do I need to add any more details? If so, what kind of details?

2 hours ago, J.C.MacSwell said:

You can break down a simple spin into vectors, say x,y, and z axis components. I don't think that will give you much of an impression of spin about those axes.

But you could recognize rotations of sub systems (body parts) and sum them to find the net rotation of the system (body)

So it is mainly just rearranging body parts to give that impression of rotating in more axes than one?

8 hours ago, J.C.MacSwell said:

There are an infinite number of ways to break down the rotation into vectors...a number of which could be natural and/or useful...as long as the net rotation is constant unless you have external torques. So the axes of rotations you believe you are seeing are not unique in that respect. (so that can be correct if broken down properly)

So, you are saying it is both breaking down rotation into vectors and rearranging body parts to give the impression of rotation about other axes?

50 minutes ago, J.C.MacSwell said:

One could say that.

Are you sure it is not a combination of vectors/rotation, or net rotation? If you are confused, here is what I am talking about.

https://www.physicsclassroom.com/class/vectors/Lesson-1/Vector-Addition

8 minutes ago, J.C.MacSwell said:

First off +1 for the video (and incredible that the diver that hit his head still managed to hit the water without seriously maiming or killing himself)

They are "rearranging" body parts about the only axis of rotation (air/wind affects aside) that they have after their feet leave the platform. This gives the "impression" of rotation about other axes. Note that any body parts can have different axes of rotation from the "system" (the body), and they can change as they are not isolated where the "system" is isolated (minor air affects aside).

So they are just moving their body parts around to make it look like they are rotating in other axes of rotation?

On 7/27/2018 at 2:30 AM, studiot said:

https://www.scienceforums.net/topic/112977-question-about-rotation/?tab=comments#comment-1034383

 

 

 

Two things SilentSky

Firstly it is not a good idea to name both your threads the same. Folks get confused.

Secondly I asked there and ask again for more detail for those of us who are gymnastically challenged.

Can you not post some diagrams of the manouveres you are referring to please?

Fine, I will show.

What I am talking about is that some of the divers that rotate in this video seem to be rotating around two axes, specifically, somersaulting and twisting at the same time. To me, anyways. What exactly are they doing when they are appearing to do that?

1 hour ago, J.C.MacSwell said:

 

Assuming a solid non-deforming body:

One axis of rotation at a time (instance) though the axis of rotation can be changing continuously. When something appears to rotate about 2 axis at the same time it would be rotating about nether, but instead about a continuously changing axis.

Of course gymnasts are not non-deforming, and defining axis of rotation can get quite complicated.

 

 

 

 

Can you explain the gymnast part a bit more, please?

I have some questions about torque and rotation, mainly gymnastic rotations, or possibly anything. I hope this is the right subforum for this. So, the thing is, is it possible for something like a gymnast to rotate on two axes of rotation at the same time? Maybe three axes of rotation? Also, is it possible to tilt while rotating so something is rotating at a tilted angle different from being upright? And if the answer is yes to any of these questions, why is this possible?

20 hours ago, Mordred said:

Where is the fun of learning in that type of answer 

If you say so. Thanks for pointing that out, though. It was good to learn that.

57 minutes ago, J.C.MacSwell said:

Using a frame with fixed axes in space, you remain spinning on the same axis the whole time. Using your example with the aeroplane (which has a commonly defined frame that is not generally inertial) say you had something massive inside that could contort like a cat, and you were isolated in space. You could realign the plane so that the axis of spin was along any alignment wrt the aeroplane. Most alignments would not be stable and they would tend to misalign, but at all times the axis of spin would remain fixed in space (it might not seem it at times, and the massive "cat" on board would have to be included)

I hope that makes sense. Doesn't seem as clear as I hoped it would sound.

I think I got it, but a simple yes or no, or not exactly would have sufficed.

8 hours ago, J.C.MacSwell said:

Assumptions being no external forces and you jettison nothing (no cheating by spitting etc and you are isolated in space):

You can reorient yourself with respect to your axis of rotation which is fixed in your inertial frame, always about your center of mass which is also fixed in that frame. You can do this by waving arms about, contorting etc...or if you are a "little eccentric" you can do it with no effort...

Note in the video how the axis of rotation remains fixed

 

So, you can reorient yourself to spin on the X axis while positioned on the Z or Y Axis (upside down or lying on the side) while still spinning as if on the X axis?

17 hours ago, swansont said:

If you were tumbling (e.g. a somersault), I mean twisting your body so that you would be cartwheeling or spinning like a skater. The axis of rotation would not change relative to fixed space.

Could you explain that a bit more, please?

28 minutes ago, swansont said:

No. Angular momentum is conserved. What they might be able to do is re-orient themselves so that they are aligned along a different axis. But the angular momentum about the x-axis will not change.

One more thing, why is angular momentum conserved?

EDIT: And by realigning along a different axis, you mean by tilting while cartwheeling or flipping so you are cartwheeling or flipping or twisting 90 degrees or so in another axis/angle?

3 minutes ago, studiot said:

Don't want much do you?

And what on earth do you mean by spinning in the x axis?

If they are spinning about the x axis, the x coordinate is the one that will not change.

And you mean what by the last part? Does it have to do with rotation?

Let me try a better example.

As show here, there are three axis of rotation for a plane, the Yaw axis, which is twisting like, the Roll Axis, which is cartwheeling like, and the Pitch axis, which is flipping like. Now, let us not apply these rotations to the plane, but to a body like the human body. The say the human body rotates around the "yaw axis", but then suddenly changes to spinning in the roll axis or pitch axis, without slowing down or stopping. How hard would that be if it were possible, if it is possible at all?

And by the X, Y and Z axes, I meant this.

Okay, I think I get it somewhat. Still, say a person is spinning in the X axis, but wants to change their axis of rotation to the Y axis or Z axis while still spinning at full speed without slowing down or stopping to do so. Would internal motions of the human body allow for that as well?

So it is possible. Can anyone explain how and why it is possible, in more detail if it has been done already?

4 minutes ago, zapatos said:

Do you mean like Olympic divers do?

Depends. Do they go from twisting motions to other forms of rotations such as flips/somersaults and/or cartwheel like motions?

This should be in relation to rotational physics. Now, say you have a rotating person. Would it be possible to change axis of rotation/change direction of rotation while rotating? For example, say the person is doing a head-to-toe twist, but goes from that to a cartwheel or a flipping rotation. Would that be possible? Why or why not?