# Newton's third law

1. Dec 13, 2007

### DaTario

Hi all,

I would like to know in which circunstances the third law applies just in the weak form.

Thank you

DaTario

2. Dec 13, 2007

### Shooting Star

Mostly involving electromagnetic forces.

3. Dec 13, 2007

### DaTario

Could you give an example. Doesn't the Lorentz force obey Newton's third law ?

4. Dec 13, 2007

### Shooting Star

Consider two identical charges moving along the x and y axes with same speed away from the origin. The electrical forces between them is repulsive, but now just find out how the magnetic forces between them behave. You don't have to calculate -- just roughly think of the charges as currents and find the direction of the associated magnetic fields like we do for currents.

The total force of one on the other is equal to the force of the other on the former, but they are not opposite.

Ultimately, momentum will be conserved, because the fields themselves carry momentum.

5. Dec 13, 2007

### DaTario

Thank you a lot.

Best wishes

DaTario

6. Dec 13, 2007

### alvaros

Any link where this example is further explained ?
Thanks.

7. Dec 13, 2007