# External and internal forces

1. Apr 21, 2015

Can external forces be considered as the forces acting from a distance and internal forces are the contact forces(except frictional forces)?
since we know that according to Newton 3rd law every object making a force on a second object the second object will make the same force on the first object but in opposite direction.
But in this case if we take for example a certain object if all the forces acting on this object are contact forces(except frictional) then the net force acting on this object is the sum of the internal forces.

2. Apr 21, 2015

### Staff: Mentor

Contact forces may be external or internal as may non contact forces. That is not the distinction between internal and external forces.

Forces represent interactions in Newtonian physics. An external force is an interaction between the system and something outside the system. An internal force is an interaction between two different parts of the system.

So internal or external depends on how you draw your system boundaries, which is arbitrary.

3. Apr 21, 2015

### Orodruin

Staff Emeritus
No, external forces are forces from objects outside your system that act on constituents of your system. Whether a force is internal or external depends on what you define as your system.

4. Apr 21, 2015

OK.
If the system is the whole universe then all the forces will be internal forces. Does this mean that the net force will be zero as all the internal forces are action-reaction forces?

5. Apr 21, 2015

### Orodruin

Staff Emeritus
In a universe which is well described by Newtonian mechanics, yes.

6. Apr 21, 2015