Does Newton's Third Law Hold in the Presence of Gravitational Fields?

[Danger]

Moonbear can explain it more accurately if she becomes aware of this thread, but muscles do indeed 'ratchet'. I can't remember the specifics, but it's almost as if they are barbed, with one strand pulling the next along. This is from a SciAm article a year or two back that dealt with muscle regeneration, specifically in regard to how they grow in response to exercise. There was a reference to the same thing in an article somewhere about the development of artificial muscles for robotics and prosthetics.

 

[castaway]

there is no paradox for a hypothetical situation that is impossible. why waste your time on it? i can also dream up a pile of other paradoxes based on fantasy. here's one:

If Gandalf can summon giant eagles to fight the Nasgool in an air battle at the end, why couldn't he summon them earlier when needed in battle?

then the 3rd movie won't have won so many oscars lol

 

[Skhandelwal]

Ok, Imagine yourself pushing against a air(as if there a wall), after couple of hours, you will still be sweaty, the only reason not as much is b/c you have problem balancing.

 

[Tomsk]

Specifically, that is known as normal force. I know about that. What I was wondering is taht if the energy is not transfered, meaning the object didn't move, then do I get my force back as appied it? Sort of like how you strech a rubberband, the rubberband gets back to its normal shape b/c it is the reaction, simply rebounding my force, however, if I apply too much force, then I break it and then, the force doesn't rebound, it transfers and never comes back. I don't know why I am having trouble understanding such a simple concept. I just think that when you push something, your energy still moves(as a force), but since the object doesn't move, it comes back to you. B/c if force is not energy than what is force? It has to be something, and if it is something then isn't it energy? I mean you gave me the formula for force by which I understood why force is zero but what is force made up of?

When you stretch a rubber band, you do work on it, as the atoms in it have moved. It gains potential energy. Then when you release it, the rubber band converts the PE you put into it back into kinetic energy and some heat, returning it to its original state. If you put so much energy in it snaps, it releases the PE you put into it in the form of sound and KE.

Also, if you smack a brick by the right technique, when you break it, your hand doesn't hurt(as in martial arts) but where is the action=reaction force there? However, if you are unable to break the brick, then your hand does hurt. It is like when you apply enough force, breaking something, there is no reaction back at you. B/c if there was, your hand was suppose to get hurt.

The reaction force exists as the brick applying a force on your hand, but this doesn't mean you'll get hurt! Your hand will slow down while it is in contact with the brick. If you don't break it, the energy from your hand goes into the brick, and becomes sound or heat, but you haven't put enough into break it.

btw what does this statement mean, "In the modern view mass is not equivalent to energy. It is just that part of the energy of a body which is not kinetic energy" Does this mean that mass only acounts for potential energy since the kinetic is already being used?

This is quite hard to picture if you've not done relativity, it's best to think of mass and energy as different things. This is referring to the equation KE=mc^2(\gamma-1), with E=\gamma mc^2. You can see if the kinetic energy is zero, then the particle in question still has some energy! It's not potential energy, as PE depends on the particles' position relative to another particle, but a particle has E=mc^2 even if it's completely on its own.

I think it is more appropriate to think of Newtons Laws as the laws of kinematic interactions [motion], not field interactions. It also important to keep in mind that a conservative force can NEVER do any [not even a single quanta] amount of work - much to the chagrin of aspiring perpetual motion machine inventors.

I think I've missed something here. Why is that, exactly? I know that curl(F)=0 is the requirement for a conservative force, but how does this lead to it being unable to do work? If you integrate up surely you get KE+PE=E, E being constant. So if there's a change in potential, work is done. What have I missed?

 

[russ_watters]

Ok, Imagine yourself pushing against a air(as if there a wall), after couple of hours, you will still be sweaty, the only reason not as much is b/c you have problem balancing.

Huh? That's not even a complete sentence.

Look, we've explained how energy works. You aren't listening to the answers. All of the questions you are asking go right back to the definition of work/energy already given. So it is time for us to stop answering and start making you give the answers. So: Given the definition of mechanical work/energy (w=f*d), you tell us whether energy is involved in that scenario and explain why.

 

[Skhandelwal]

thats seems like a good idea,(btw, the reason I was still asking is that you never really told me what force is made up of) Whatever you told me makes sense mathematically, but doesn't quite fit in into my mind.(may be I am just dumb) Here is what I got so far, no matter how much force you apply, if there is no motion, there is no transfer of energy. I guess the problem I am having is the concept I had that force is always energy. But know I know that force is only energy when there is acceleration. Well, when there is no acceleration, what is force?(what is it made up of?)

 

[russ_watters]

f=ma just tells you how force causes (or is caused by) acceleration of a mass. But force can also come from gravity, a stretched rubber band, a magnetic field, etc. Forces like that come from stored potential energy and generally require energy to create the situation where the force exists. But once the potential energy is stored, there is no further transfer of energy.

 

[rcgldr]

Force and energy are not the same, just like torque and power are not the same. Mass and energy are not the same, but you can convert mass to energy and energy back to mass.

Here are some definitions:

work = force x distance

if the units are pounds and feet, then

work (pound-feet) = force (pound) x distance (feet)

Energy is one of the states of an object.

For mechanical physics one type of energy is kinetic:

kinetic energy = 1/2 x mass x speed^2

If there are no losses due to friction, then work done on an object changes it kinetic energy. If you apply a force of 1 pound for 10 feet, or 10 pounds for 1 foot, the kinetic energy changes by 10 pound feet.

Assume it's a 1 slug (32.174 pound) mass, and not moving. You apply a 1035.166 pound force for 10 feet, for a 10351.66 pound feet increase in kinetic energy. Redoing the math:

Acceleration = 1035.166 pound force / 32.174 pound mass = 32.174 feet / sec^2 = 1 g of acceleration

This amount of acceleration is applied for 10 feet:

d = 1/2 a t^2
t = sqrt (2 x d / a) = sqrt(2 x 10 feet / 32.174 (feet / sec^2) = .788429 sec

v = a t = 32.174 feet / sec^2 x .788429 sec = 25.3669 feet / sec

kinetic energy = 1/2 m v^2 = 1/2 x 32.174 pound x 25.3669^2 = 10351.66 pound feet

So the increase in kinetic energy does equal the work done.

Power = a rate of work:

Power = work / time = force x (distance / time) = force x speed

Horsepower = force (pounds) x speed (mph) / 375 (conversion factor)

 

[Skhandelwal]

So when you force an object to work, do you increase it's kinetic energy or unleash it's potential?

 

[Danger]

That last question is a bit backwards. You can't force something to do work. If you try, all that you're doing is using it as a transfer medium for the work that you're doing. For example, if you force one end of a lever downward in order to lift something with the other end, you are doing the work and the lever is transfering it to the load.
If you force something into a situation where it can do work, such as winding up a rubber band, then you are increasing its potential energy.
When you let it go to do whatever it's going to do, then you're unleashing its kinetic energy.

 

[Skhandelwal]

aah, I get it, thx. But by that def.,, You can't accelerate energy b/c then you get undef. by the formula f=ma. Right?

 

[loom91]

Of course the paradoxes presented here are false paradoxes, but unless I'm mistaken N3L has been disproved, hasn't it? When you have fields propagating at finite velocities instead of instantaneous action at a distance as envisaged by Newton, N3L need not hold, either in the strong or the weak form. Examples can be found even in Classical Electrodynamics, though conservation of linear momentum would still hold with the introduction of field momentum into the mixture. In quantum mechanics of course works with energies and momenta instead of forces, so there's no question of N3L holding, though PCLM is once again there.

At least, that is what I was told (we haven't yet covered EM in detail). Is that right?