# Newtons Laws 2/3 Why's

1. Aug 5, 2007

### caybo

Why do we accept newtons laws?
For every action, there is an equal and opposite reaction. why?
An object in motion tends to stay in motion. Why does it need a force to stop it?

2. Aug 5, 2007

### Staff: Mentor

For the same reason we accept any physical law: they are supported by massive amounts of empirical evidence.

3. Aug 5, 2007

### caybo

"empirical evidence" It is true that we have massive evidential support, but I still don't understand. We don't have any reasoning as to what causes these laws actually to be correct, right?

4. Aug 5, 2007

### ank_gl

main reason why we accept them is that we havnt had any contradictions to them so far.
u make a situation where the law ceases to exist, it wont be accepted anymore or maybe conditioned

5. Aug 5, 2007

### Gokul43201

Staff Emeritus
We don't require that there always be a deeper reasoning behind any given law though that would often be nice. In this case though, a depper reasoning does exist. Newton's Laws can be derived from an assumption of the invariance of physical laws upon a continuous translation through space, via Noether's Theorem.

6. Aug 5, 2007

### arildno

However, note that "invariance of physical laws" is NOT a metaphysical dictum, or the only logically necessary position one might conceivably hold.

Rather, it is a type of "minimalist" assumption one uses that is...abundantly supported by evidence.

If you ask philosophically as to what the FINAL physical theory would be, it would in sum say "that's just how it is, dear".

7. Aug 5, 2007

### pardesi

well it doesn't really matter if it is 'absolutely' right or wrong ..since i am able to explain with it all the things i see i am satisfied and when time comes i will modify it if necessary

8. Aug 5, 2007

### rbj

caybo, as an electrical engineer that thinks about signal processing and information theory, here is my philosophical spin on it:

a physical law (or a "law" in any other science) is really a form of data reduction. we *could* express all of this empirical evidence in their non-data-reduced form:

"on April 23rd we observed a 2 kg object launched east, with a speed of 10 m/s, off of a 20 kg cart with extremely well designed and lubricated wheels, and the 20 kg cart moved west with a speed of very nearly 1 m/s."

"on April 24th we observed a 3 kg object launched north, with a speed of 10 m/s, off of a 20 kg cart with extremely well designed and lubricated wheels, and the 20 kg cart moved south with a speed of very nearly 1.5 m/s."

"on April 25th we observed a 4 kg object launched west, with a speed of 10 m/s, off of a 20 kg cart with extremely well designed and lubricated wheels, and the 20 kg cart moved east with a speed of very nearly 2 m/s."

"on April 26th we observed a 2 kg object launched east, with a speed of 10 m/s, off of a 20 kg cart with extremely well designed and lubricated wheels, and the 20 kg cart moved west with a speed of very nearly 1 m/s."

now after you do enough of these, you might conclude that the date was irreleavant to how the experiment turned out, empirically. and that it didn't matter what direction, except that the movement of the cart was the opposite direction of the lauched object. so, after reading a zillion of these empirical reports, you decide to do a little data reduction and say:

"on any particular day or moement when we observed an M1 kg object launched in some direction, with a speed of V1 m/s, off of a M2 kg cart with extremely well designed and lubricated wheels, the M2 kg cart will move in the opposite direction with a speed of very nearly V1M1/M2 m/s."

saying that statement, instead of all of the unlimited number of statements of particular empirical observations, saves data and, if it continues to be accurate in the domain where it is said to be applicable, we call it a "law of science".

9. Aug 5, 2007

### Staff: Mentor

Everywhere in physics, if we dig deeply enough in the chain of "explanations," we always come to a level where we don't have any further explanation, and we have to accept the laws given at that level as axioms, at least for the time being.

As Gokula noted, we can explain Newton's Laws by invoking Noether's Theorem and postulating that physical laws must have a certain kind of invariance. But that immediately raises the question, "why that particular kind of invariance"? Maybe someday we'll have an answer, but then we'll have a new question!

10. Aug 5, 2007

### alvaros

There is no answer to your questions but this is what I think:
What would happen if the laws about of energy, mass, momentum were false ?
This would be a fool world: suddenly your computer rises and flies, your room gets warmer or colder, and life wouldnt be possible...