# Newton's Three Laws of Motion

1. Oct 12, 2007

### MetricBrian

Are Newton's three laws of motion correct as Netwon stated them?

1. The first law is the principle of inertia. It states that an object in motion will continue to move unless acted on by an external force. And that an object at rest will remain at rest unless acted on by an external force.
2. The second law states that the change in momentum is proportional to the external force.
3. The third law is that for every action there is an equal and opposite reaction.

2. Oct 12, 2007

### t-money

On the third law the direction is also opposite F(1,2)=-F(2,1)

3. Oct 12, 2007

### MetricBrian

Yes, but are all these laws still considered correct today?

4. Oct 12, 2007

### brewnog

Absolutely, in their context. They're still the basis for every piece of machinery, every building; pretty much everything designed by man.

5. Oct 12, 2007

### Staff: Mentor

Except insofar as they break down at very high speeds where relativity takes over, yes.

6. Oct 12, 2007

### MetricBrian

Yes, but the three laws are general ideas about motion not equations and as general ideas are they not correct?

7. Oct 12, 2007

### Loren Booda

russ_watters
Or, as I understand it, in very dense gravitational fields (general relativity) or on very small scales (quantum mechanics).

8. Oct 12, 2007

### MetricBrian

But even in relativity its correct to say that the change in momentum is proportional to the external force.

9. Oct 13, 2007

### Hootenanny

Staff Emeritus
Yes, but this force need not be in the same direction as the change in momentum.

10. Oct 13, 2007

### Staff: Mentor

The equations equations are connected to their verbal descriptions. Newton's first law, as you stated it, is basically a verbal description of f=ma.

And yes, again, they are correct in their domain.

11. Oct 13, 2007

### Artaxerxes

The first law (a = 0 if f = 0) is a special case of the second law (f = ma).

12. Oct 13, 2007

### MetricBrian

The idea that: the change in momentum is proportional to the external force can
also be expressed as F = dp/dt which was the original Newtonian notation

Last edited: Oct 13, 2007
13. Oct 14, 2007

### keltix

or in quantum physics