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

[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?