Conservation of Angular Momentum is Dumbfounding

[Buckethead]

This is not correct. Both marbles are being fired in the same angular direction, so they have a combined non-zero angular momentum

Could you clarify this. Aren't the marbles going in opposite directions: i.e. 180 degrees apart? Why are you saying they have the same angular direction?

Are you saying that the momentum of the marbles is not zero because their trajectories draw parallel lines and not a single line (if the trajectory lines are extended in both directions)? Sorry for my thick brain on this one.

 

[russ_watters]

Could you clarify this. Aren't the marbles going in opposite directions: i.e. 180 degrees apart? Why are you saying they have the same angular direction?

If the stick is vertical and you fire a marble to the right from the top end and to the left from the bottom end, both are being fired clockwise (albeit on tangents), pushing the stick counterclockwise.

 

[Buckethead]

If the stick is vertical and you fire a marble to the right from the top end and to the left from the bottom end, both are being fired clockwise (albeit on tangents), pushing the stick counterclockwise.

I see. Interesting. So does this mean that if you have two marbles passing each other in space (parallel lines, not the same line) their total momentum is not zero?

 

[jbriggs444]

I see. Interesting. So does this mean that if you have two marbles passing each other in space (parallel lines, not the same line) their total momentum is not zero?

Total linear momentum can be zero. If it is zero (e.g. if you use a center-of-momentum reference frame) then the total angular momentum is guaranteed to be non-zero.

 

[Buckethead]

Total linear momentum can be zero. If it is zero (e.g. if you use a center-of-momentum reference frame) then the total angular momentum is guaranteed to be non-zero.

Got it! Thank you. I can see this as well if a line is drawn between two passing masses, the angle of the line changes as the distance between the masses increases.

 

[Brendan Graham]

Regarding (a), "if I applied no external forces or torques to my system, and now the thing is spinning...", of the original post; sometime before T=1s, the experimenter compressed each of the springs with a required force, thus clearly, an external vector force (torque about the fulcrum) was indeed applied. Then at T=1s, the energy stored in the previously compressed springs were dissipated producing an equal and opposite torque about said fulcrum.

 

[jbriggs444]

sometime before T=1s, the experimenter compressed each of the springs with a required force, thus clearly, an external vector force (torque about the fulcrum) was indeed applied.

Nonsense. One can compress a spring by pushing on both ends equally for zero net torque and zero net linear momentum imparted. In any case, the initial conditions for the exercise are what they are and include zero total angular momentum.