What is with the law of conservation of momentum?

[siddharth5129]

Consider a scenario where someone stands on the surface of the Earth and throws punches into the air. Now all the forces(which i am taking to be the tension in the muscle fibres of the arm ) acting on the system of man and Earth are internal ( internal because they are exerted by one component within the system on another component within the system.) , but the momentum of this dude's arm is constantly changing. Anyone else see a violation of the law of conservation of momentum. What am i missing here?

 

[Doc Al]

but the momentum of this dude's arm is constantly changing. Anyone else see a violation of the law of conservation of momentum. What am i missing here?

You're forgetting about the momentum of the rest of his body and the Earth itself. Total momentum of the person-earth system is conserved, not the momentum of his arm alone.

 

[siddharth5129]

Yes but the rest of his body and the Earth are at rest , aren't they?

 

[Doc Al]

Yes but the rest of his body and the Earth are at rest , aren't they?

No. The Earth moves, ever so slightly.

 

[siddharth5129]

How exactly , there aren't any forces on the earth.

 

[Doc Al]

How exactly , there aren't any forces on the earth.

Sure there are. If you push on the arm to give it momentum, the arm pushes back on you (and the earth) giving you and the Earth equal and opposite momentum. That's Newton's 3rd law.

 

[russ_watters]

Try the same thing while standing on a skateboard and you'll see the motion.