Im betting that there will be some folks out there that will argue that its not quite right. i would love to hear some of the theories people come up with....


The statement is as follows:

If no forces are acting on a body then it remains at rest, or moves uniformly in a straight line, and if a force is acting on a body then the acceleration is proportional to the force that is acting on it.

I can tell you a couple things why I dont think part of that is true. Say when fireing a gun, the bullet travels at whatever force that the gunpowder emits, no? Well, then if that is correct, a 50. cal should sent its shooter backwards at an equavalent distance of the bullet. Same with cars, if that thory was also true, then when a car sends something it hits flying, shouldn't the car bounce backwards the same rate? Although I guess that it can be argued in terms of mass. If two things consisting of he same mass and acceleration collide with eachother, then they should just "stop". But the exact oposite happens, the objects are propelled backwards at the same or even less than the rate of the initial collision.


Any thoughts?

Sorry but your wrong...I hope AIguardian posts a reply to your answer...he will also confirm that the statement i made is 100% true...its basic newtonion physics.

Tell that to the people on the Metaphysics thread then if you want an argument. I would state that I have some dealings with the metaphysical, but you would shrug it off as a lie. And why do you need AIgaurdian to answer your question for you? Do you even have the facts?

That doesn't happen because the person has a much greater mass than the bullet.



The kinetic energy has to go somewhere.

Good points though.



Yeah, that statement is correct (if the body moving in a straight line had a force act upon it initially). But what's the point of this thread?

The point to this thread is simple.

I have stated the truth, and inspite of that there are people who argue a case against it. i just wanted to see what kind of theories people have that are abstract from the truth. i picked a simple example with newtons first two laws of motion.

that is all

Ah, got ya.

Ah, I see your point MrHamblee

You've discovered the 'hole' in your analysis (bold above).
Why should they just stop? One has X energy the other Y, the system has to conserve energy, hence after a collision the system should still show X + Y energy - there will be some loss to friction, sound & heat depending on what materials and momentums you are dealing with. It's very basic physics.

For fun, I'll take you up on this. A little disclaimer: I'm not 100% behind this but I think it's a concept that deserves some attention.

So let me suggest that Newton's second law is, as you said, "not quite right." That is to say that force and acceleration are not directly proportional but rather have a slightly more complicated relationship. In F=ma the constant of proportionality is, of course, the inertial mass but the suggestion here is that the mass term is itself a function of acceleration. Thus the true relation looks more like F=mμ(a/a0)a, where a0 is some new constant of nature. Now when a/a0>>1 then this μ(x) term approaches one and the force law approaches the one we're familiar with. On the other hand when a/a0<<1 then the μ term becomes some function of x (note I'm using x = a/a0); maybe the function is μ(x) ≈ x or perhaps it's something like μ(x) = 1-e^-x.

The end result is that on the acceleration scales we usually deal with the regular Newtonian force law works just fine. But since a0 is a very tiny acceleration scale that means when it comes to very small acceleration F=ma just doesn't cut it. So what's the point of challenging your statement and monkeying around like this?

Let's think about a galaxy (and use μ(x) = x). Using our reworking of Newton's second law above we know that the effective acceleration a mass experiences will be μ(a/a0)a = gN. Simple Newtonian physics tell us that at large galactic nuclei the acceleration a mass experiences will be gN = GM/r^2. Since those masses are moving in a circular orbit around the galactic center the centripetal acceleration (a = v^2/r) is provided by that gravitational force so a^2/a0 = v^4/a0r^2 = gN = MG/r^2. Ugly in this format, I know, but bear with it.

So we're lead by our assumption to a simple relation: v^4 = MGa0. It turns out this relation has been empirically known for some time; it's called the Tully-Fisher relation and it's used in the cosmological distance ladder. The T-F relation relates the luminosity of a galaxy to the fourth power of its rotational velocity so if you see how fast a spiral galaxy is spinning you can get an idea of how bright it is and then it's fairly simple to figure out how far away it is by noting how bright it appears to us. You'll note that the relation we derived doesn't have a luminosity term in it, instead it relates the fourth power of the rotational velocity to the mass of the galaxy. This isn't a problem because the assumption is that more massive galaxies have a proportionally greater magnitude.

Besides predicting things like the Tully-Fisher relation our tweaking of the second law can account for something else that's very noteworthy--the flatness of galactic rotation curves. Usually the faster than expected rotational velocities of stars on the outskirts of galaxies are taken to indicate that there's extra mass (and lots of it) around the galaxy: the famous dark matter. Our tweaking, however, raises the possibility that the laws we use to deduce the presence of that extra mass are in fact not quite right. So instead of dark matter influencing them perhaps the stars on the outskirts of galaxies are just following slightly different laws than we believe.

This whole idea is called Modified Newtonian Dynamics (MOND) and was first proposed in 1983 by Motti Milgrom. The idea has since grown up a bit. Two years ago Jacob Bekenstein (the guy who deserves credit along with Stephen Hawking for his work on black hole thermodynamics) unleashed a souped-up relativistic version of MOND called tensor-vector-scalar gravity that can do things the original couldn't, like account for the gravitational lensing that's usually taken to be caused by dark matter.

I've been meaning to start a thread about all this because there's a particular aspect of it that I think is potentially pretty cool. Anyway, I guess the point is keep looking at things in different ways and maybe you'll eventually approach the truth; but don't assume you're already there.

WARNING

Guy too smart for his own good above me!!!



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Did you write that all by yourself or is it pasted?

I think this was a cute topic Cute=precocious. I like it. One thing about this site is that you can find someone to argue two extremes(at least) of any proposition. Taking a universally accepted (in the normal world) proposition and opening it up to debate here is really, well... cute! One thing about your first sentence though. The object could still be moving even though no forces are now acting on it, if there had been forces in the past. I think the original statement said that an object "at rest" will remain "at rest", unless a force is applied to it. (The bit about continuing to travel in a straight line may cover this, but is not quite as clearcut as the original.)

No, it's not pasted, though the ideas can all be found in Milgrom.

Thanks