High School Conservation of Angular Momentum - Problem understanding this scenario

[jonhswon]

Hello,

As far I know, in a closed system both, linear and angular monentums, are conserved.

İmagine such a scenario: everything is motionless, both momentums zero initially, then from a disk are fired (compressed spring push) two equal mass balls at same speed but opposite direction. Now balls fly away and disk is spinning. Linear momentum after firing is still zero, but angular momentum is not? What is happening?

(All usual assumptions in place, inertial reference, massless springs, etc..)

Thnaks in advance.

 

[Ibix]

angular momentum is not

Have you taken into account the angular momentum of the balls?

 

[PeroK]

... an object moving in a straight line at constant velocity has angular momentum about any point not on the line of motion.

Note also that angular momentum is always measured relative to some point.

 

[jonhswon]

OMG I was so blind. Thanks a lot !

 

[Ibix]

I first had this discussion the other way round when a classmate at university lobbed a shoe at the door to shut it. It's quite neat how the changing tangential component of linear velocity cancels with the changing radial distance to produce a constant angular momentum for an object in linear motion.