Why does a rocket or gun recoil if internal forces cancel out?

[HallsofIvy]

The "parts" inside an isolated system are not isolated!

 

[A.T.]

If forces will be acting on the same object then there will be no motion,but in gun or rocket case forces are not acting on the same object,no?

Depends on how you define the objects. You can consider Münchausen as arm & body to have two objects as well. Then analyze the forces between them to see what is actually different compared to gun & bullet.

 

[Kenan]

OK.An object will accelerate (begin to move) if there is an unbalanced force acting on it. It the case of the bullet fired from a gun, there is an unbalanced force acting on the bullet as it begins moving.His hair won't move since it is attached to his head! (If he really pulls it, it could tear off though.) And his body won't move because whatever force he pulls up with (on his hair) is balanced by the equal and opposite force that the hair pulls down with.

Always ask yourself: Is there a net force on an object? If so, it will accelerate. If not, it won't.

Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?

 

[Kenan]

Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?

I hope I'm right now,correct me if I'm wrong.
Thanks again.

 

[Doc Al]

Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?

Close enough.

Have you read this page from the website you linked?: What's[/PLAIN] an object and what isn't?

 

[Drakkith]

Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?

Yes. The net force on each 'piece' of the system is zero in the first example and non-zero in the rocket/gun example.