SilentSky23

1 hour ago, joigus said:

Yeah, I was pigeonholed in my theoretical mind. Couldn't think about fluids and the like. The OP I think was talking about sth. like biomechanics.

Sorry on my part: Yes! Everything rotation-based in physics changes when you displace the centre of mass, or the centre of gravity if you rotate about the same axis.

The best example I can think of is a pole dancer. These girls must really get some joint lesions from the stress of rotating out of centre.

No need to say sorry. I say I need to say that for not articulating the question well enough in the first place.

1 minute ago, studiot said:

So can you articulate a complete question now?

One further question.
Are you familiar with the idea of stable unstable and metastable equilibrium ?

I will try next time.

And no.

1 minute ago, studiot said:

Gravity plays a huge part in the activity of gymnasts, skaters, swimmers and many other athletes.
One of the most important issues is the effect of centre of gravity on balance.
 

Is this the sort of thing you want to talk about?

That should do, and if it didn’t, close enough. Thanks.

2 minutes ago, studiot said:

I have been trying to play down the mathematics since I know you are not big on that.

But once you are talking about rotation and  deformable bodies you are into the mechanics of materials ie shear stress, which is a rotation.
Such rotations are not connected with the COG of the body.

Further rotational mechanics arises in the dynamics of fluids, again the centre of gravity does not play a large part in this.

So some examples from you of the situations you wish to examine would be useful.

 

You continue to use the term centre of gravity and I tried to explain the difference between the centres of mass and gravity.

I hope you understood it. COGs are only important in a gravitational field, and particularly when gravity is the main force acting.

Ask if you don't.

Sorry. I forgot to use center of gravity, and I did not understand how complex this was.

For examples, what about a gymnast doing, well,  a back flip or another gymnastics routine if that counts? That was mainly what I was going for, anyway.

Also, forget center of gravity. Let's use center of mass for the question I was asking, instead.

3 hours ago, J.C.MacSwell said:

It's common to take the moment of inertia of a body about an axis running through the center of gravity. It represents the inertial resistance to accelerating the body rotationally about that particular axis, and can depend on the choice of axis.

I'm not sure that is what you are after though.

What I was after was if anything rotation based in physics, like moment of inertia and such, is connected to center of gravity, to the point that anything rotation based changes when the center of mass changes. Like if the center of gravity is outside the body, mainly a non-rigid body, does anything like moment of inertia change as in become easier or harder.

Why would it be hard to understand?

2 minutes ago, MigL said:

Right; the external force does not need to act through any axis.
A 'torque' can be offset from the center of mass, and impart rotation.

A top can spin about the axis through its center of mass, but it might be at an angle to the vertical, so it isn't spinning through its center of gravity.
( rotations that don't pass through the CoM, lead to imbalances, and that 'torque' will tend realign the rotation, or in extreme cases, break apart the rotating object  ). So make sure your wheels/tires are properly balanced.

This is one of those questions where framing the question correctly gives you one right answer, but not being specific enough gives you many possible right answers

My bad, I should have known that not all rotations act through the center of mass, something I learned by looking up before you made this post. I should have thought this more carefully before I made this topic. Maybe I was not talking about axis of rotation, but maybe moment of inertia? If moment of inertia has anything to do with center of gravity, does moment of inertia change when the center of gravity changes?

Either way, I am interested in the part of your post I bolded. Can you please tell me more, if you can?

2 minutes ago, MigL said:

Studiot is correct in the case where an external force is actively causing rotation.

Maybe you mean the case where an initial force causes rotation of a system of 'loose' objects, and then through collapse, and angular momentum conservation, picks up rotational speed.
it would tend to rotation about the axis passing through its center of mass ( which might not necessarily be its center of gravity ).

X-posted with Strange and SilentSky23

Isn't the external force just the cause of rotation, though? Rather than the line acting through a point in the body? I thought those were two different things. The line part was what I meant by axis of rotation.

Just now, Strange said:

I'm not sure I would call that "rotation". But perhaps the OP (and me) is thinking of something with no external forces applied. In which case, they would be correct, no?

Yeah, I wasn't really talking about external forces here.

I only said the through the center of gravity part because I kept reading that online and in books. At least for the human body int he pelvic area. Maybe I phrased it wrong, and meant intersect at the center of gravity? If that means anything? Or is the line through the center of gravity?

Now, if I am not mistaken, from what I know, the axis of rotation of a body usually goes through the center of gravity of a body. Also, the position of the center of gravity, if I recall correctly, does change when the position of the body of the living being or object in question changes. Well, just to make sure, I must ask: would the location of the axis of rotation change if the position of the center of gravity changes as well?

10 minutes ago, MigL said:

Even if you gave me numbers I couldn't give you a numerical answer ( time ), but I would be able to tell you which would stop first.
If the problem is not laid out specifically you can't expect a specific answer.

To be honest, being told which would stop first; that is actually all I need.

7 minutes ago, MigL said:

That doesn't specify anything...
Especially since the 'retarding' force is also active during the initial force application that caused the velocities.

But everything else being equal. They both feel equal retarding force ;(highly unlikely ) and if the retarding force is trivial compared to the initiating  force, the lighter mass will experience greater negative acceleration, and, since it started with a proportionately greater velocity, they should come to a stop about the same time.

So, are you expecting numbers? I am kinda having trouble understanding what you want from me when talking about this situation. If surface level and air resistance and such aren't going to be enough here as it may seem, tell me, what should I tell you, especially for next time when I do ask something like this, just to be safe?

4 minutes ago, MigL said:

The relationship is F=ma where

F is directional force
a is directional acceleration
m is mass

Only forces will change velocity of a mass by accelerating it; if no force is acting, masses will continue at their current velocity.
Obviously for the same force, the lesser mass will experience the greater acceleration.

If that force ( and acceleration ) are limited to a certain time, then the less massive object will have a higher final velocity.

From that point different forces come into play, that tend to slow the masses down, and accelerate it in the opposing direction ( reducing its velocity ). These can be due to surface friction, air resistance or even the 'level' of the surface.
Unless all of these factors are specified, we can't even begin to answer how long it takes for the masses to come to rest.
 

I did say the type of surface, which is wooden. Not to mention smooth. As for the other things, say the air resistance is normal, or the usual amount in everyday life (nothing too strong) with no wind, and the level of the surface is, well, flat.

Anything else you need to know?

9 minutes ago, studiot said:

Just  try again and set out what you actually want to say.

Well, what I meant in my initial saying was that I believe lighter objects accelerate faster and go farther than the ones with more mass, under the same exerted force,  of course. Now, for a light object and a heavy object going at the same acceleration, under different forces acting on each of them; assuming the forces are both different in magnitude but acting in the same amount of time; with resisting forces like friction being the same on both objects in terms of magnitude, but acting for a longer time than the two forces that accelerated the objects in the first place, which one would come to a stop sooner than the other if one has the higher inertia than the other?

How is this?

EDIT: For the surface, say the surface is a wooden floor in a house.

5 minutes ago, studiot said:

You need  to be clearer  how long the original force(s)  is maintained, (when it starts and when it stops) and  the same  for the resisting forces.

Acceleration in not like velocity, which is why they are different properties.

Yeah, I know, I was going to edit that part about acceleration, but I see I can't now. Still, say that the lighter object is exerted for less than a second, and the second object has a bigger force around the same amount of time. For the negative acceleration from the resistance that eventually stops them, I am not sure. Would the times of negative acceleration be different for each of them?

7 minutes ago, Curious layman said:

Ok complete guess and I know your looking for someone more qualified, but surely the lighter one would go further due to more friction on the heavier one. 

 

But by inertia, wouldn't the lighter object stop sooner, since it has less resistance to change in its velocity?

I wanted to make sure of something here. Now, I believe it is the case when an object with lighter mass, or inertia, will travel faster and farther than one with a heavier mass or inertia under the same force. Now, consider an object that is light and one that is heavy, both going at the same acceleration, but with different forces acting, and they start moving across either the ground or the air, depending on where they are when a force acts on them. Now, with the same acceleration, which object will go farther before stopping, with either friction, air resistance, or a force exerted by something or someone that is the same force for both objects?

I have a question. Before I start, I want to say that I know negative mass itself has not been observed or proven to exist, if it does at all. That is why this is more of a what-if question if that is okay. Anyway...

Say an object has negative mass and therefore negative inertia. Now, considering inertial reference frames, or fictitious forces, objects with normal mass appear to move, such as going backwards seemingly when say, a car it is in speeds up. If a negative mass object were to exist, and were place inside the car before it speeds up, would the object appear to go the opposite direction of what a normal mass object would do when that car moves, as in that negative mass apparently going forward rather than backward when the car speeds up? As well as going backward apparently instead of forward when a car slows down, if I have inertial reference frames down correctly? All due to fictitious forces? Or would it be the same as normal, positive inertia?

I know this seems rather outlandish, possibly even for a hypothetical question in physics, but I have been thinking about it and I am rather curious. I hope nothing bad happens by asking this. So, is there anything to say about this question, if there is something?

2 minutes ago, Strange said:

That is not really related to mass. I think you would need to be more specific about the conditions to provide any sort of answer. For example, if you push on a wall, then the wall pushes back on you. So to avoid moving you need to push against something else; the floor, maybe. In that case your mass will have some effect on how much friction your shoes have on the floor.

Alright, I now get it. Thanks.

2 minutes ago, Strange said:

Do you mean weight? The force an object sitting on a table exerts on the table, for example?

More like a hand pushing against a wall, if that counts.

2 minutes ago, Strange said:

Because if you ask about something impossible, then there is no scientific or factual answer.

You might as well as, "what would happen if the sun spontaneously turned into a chicken?" There is no reasonable answer, because it can't happen.

What do you mean by "contact force"? Friction? Or something else?

Mass is only indirectly related to friction.

 

Applied or normal force, to be exact.

1 minute ago, Strange said:

None of those can be realistically evaluated for a non-physical case like this.

If you invent an impossible thought experiment, then you can invent any answers you like. 

May I ask why that is the case? Do you mean they can't or, there is just no knowledge of what happens?

Let me ask another question: Does mass, not infinite, of course, exert contact forces at all, or does it not?

1 minute ago, Strange said:

An object with infinite mass would exert infinite gravitational force because the force is proportional to the mass: F=Gm1m2r2

Whether it is moving or not makes no difference.

Luckily, nothing can have infinite mass.

What about non-gravitational forces? Like applied forces or other contact forces?

And that question is...

Would a moving object with infinite mass exert an infinite force on other objects?

If so, why? If not, why not?

3 hours ago, swansont said:

To reduce implies a change from one value to another. But that's not what's happening. Mass doesn't change the acceleration. It tells you what the acceleration will be. F = ma. Nothing has changed. Nothing has been reduced.

Changing the mass changes the acceleration.

Again, this is an issue of semantics that (largely) goes away when you look at the equation.

 

 

So you are saying it is both increasing and decreasing the acceleration, depending on how mass is changed?

I guess I misused the word "reduced", as I only talked about what happens to acceleration when mass increases. I did not include what happens when mass decreases. My bad. That said, I could go back and say inertia could provide a yank and thus jerk for acceleration, keyword, could. However, if I understand inertia being a resistance, even if I say that the resistance somehow provides a yank, I doubt that inertia being a yank is the case very much. I mean, inertia is not just a property, but a law, a tendency, is it not? Anything else I am missing?