Law of conservation of momentum in a non isolated system.

[Ali Hamaiz]

Is the law of conservation of momentum valid in non isolated system . What is an isolated system? My teacher confuses me that the law is valid in an non isolated system . They even say that when we say about the isolated system (meaning they are present ) we do not consider the air resistance or frictional forces is that true? They give me the example of a gun recoiling , I am confused .

 

[BvU]

Isolated in this case means no external forces and no mass crossing the boundary. A tennis ball in the air is not an isolated system: it will fall to the ground. A gun also, but if you look in the direction of where it's aiming then the momentum in that direction is conserved.

 

[Ali Hamaiz]

@BvU what are you talking about the direction?

 

[BvU]

You need a constant force to keep the gun from dropping to the floor. But to calculate the kickback you can use conservation of momentum in the direction the bullet flies.

 

[jbriggs444]

You need a constant force to keep the gun from dropping to the floor. But to calculate the kickback you can use conservation of momentum in the direction the bullet flies.

[and make sure that you place the boundaries around the "system" so that both gun and bullet remain inside]

 

[Ali Hamaiz]

What will be the conclusion , will the law of conservation of momentum hold in an non isolated system .

 

[BvU]

No. See #2.